How to Hot Rod Your Fender Amp+OCR
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Jeffrey FalIa Photography by Aurora Johnson
Voyageur Press
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DEDICATION
To Daniel Falla Acknowledgments My thanks to Brock Kline, who let me use his Super Reverb reissue amp for some of the modifications in this book. Thanks as well to Robyn Orsini, Jason Farrell, and Jake Hill at Fender Musical Instruments Corporation for all oftheir kind assistance.
First published in 2011 by Voyageur Press, an imprint of MBI Publishing Company, 400 FirstAvenue North, Suite 300, Minneapolis, MN 55401 USA Copyright © 2011 by Jeffrey Falla Photography copyright © 2010 by Aurora Johnson except where noted. All rights reserved. With the exception of quoting brief passages for the purposes of review, no part of this publication may be reprod uced without prior written permission from the Publisher. The information in this book is true and complete to the best of our knowledge. All recommendations are made without any guarantee on the part of the author or Publisher, who also disclaims any liability incurred in connection with the use of this data or specific details. This publication has not been prepared, approved, or licensed by Fender Musical Instruments Corporation. FENDER ®, BAND-MASTER®, BASSMAN ®, CHAMp®, TWIN REVERB ®, SUPER REVERB ®, BANDMASTER'", TWIN AMP '", DELUXE'", HOT ROD DELUXE'", SUPER-SONIC'", STRAT®, STRATOCASTER®, TELECASTER®, TELE®, and the distinctive headstock designs commonly found on the STRAT and TELE guitars are trademarks of Fender Musical Instruments Corporation and are used herein with express written permission. All rights reserved. FMIC is not affiliated in any way with the author or publisher, nor does FMIC endorse the modifications to FMIC products discussed herein. Voyageur Press titles are also available at discounts in bulk quantity for industrial or salespromotional use. For details write to Special Sales Manager at MBI Publishing Company, 400 First Avenue North, Suite 300, Minneapolis, MN 55401 USA. To find out more about our books, visit us online atwww.voyageurpress.com. ISBN 978-0-7603-3847-6 Digital edition: 978-1-61059-765-4 Softcover edition: 978-0-7603-3847-6 Library of Congress Cataloging-in-Publication Data Falla, Jeffrey, 1958How to hot rod your Fender amp : modifying your amplifier to get magical tone / Jeffrey Falla. p. cm. Includes index. ISBN 978-0-7603-3847-6 (sb: alk. paperl 1. Guitar amplifiers. I. Title. ML1015.G9F46 2011 787.87'192--d c22 2010028256
Editors: Darwin Holmstrom and Michael Dregni Design Manager: Katie Sonmor Layout by: Greg Nettles Designed by: Greg Nettles and Simon Larkin Cover designed by: Andrew Brozyna
Printed in China
Disclaimer:Tube amplifiers contain very high voltage that can cause injury or death. These voltages can be stored in an amplifier's capacitors after turning off the power. Do not open up an amplifier, do not attemptto repair an amplifier, and do not attemptto modify an amplifier unless you are absolutely certain of what you are doing. Many ofthe procedures detailed in this book are meant for individuals with knowledge of electronics and proper safety procedures. The author and the publisher accept no responsibility for any destruction of property or for any personal injury, accident, or death. On the title page: Fender's '57 Tweed Twin Reissue amp with a 1952 Fender Telecaster Reissue .
Fender Musical Instruments Corporation On the contents page: Fender's '57 Champ with a Custom Shop Master Built 50th Anniversary 54 Stratocaster. Fender's Champion 600. Fender Musical Instruments Corporation
CONTfNTS 6
Chapter 1
Choosing an Amp
Chapter 2
Tools and Electronic Components
16
Chapter 3
Speakers, Switches, and Schematics
40
Chapter 4
Bias
52
Chapter 5
Quick, Basic, and Essential Modifications
76
Chapter 6
Overhauling the Silverface, Hot Rodding the Hot Rod, and Rebuilding the Reissue
92
Chapter 7
Reverb
120
Chapter 8
Tone Stack Modifications
132
Chapter 9
Adding Gain
146
Chapter 10
Dual Channel Modifications
162
Chapter 11
Switch Boxes
174
Resources
182
Index
182
chapter
1
CHOOSING AN AMP The history of Fender is the story of a quest for clean amp sound, culminating in four-tube 100-watt units, such as the Twin. Most modifications to Fender amps have represented attempts to restore the distortion by going back in history-"blackfacing the silverface" is a case in point. In this book we'll not only discuss these modifications, we'll go beyond them.
Fender Amplifier History The first Fender amps appeared on the market in 1946 and sported uncovered wooden cabinets. In 1948, Leo Fender introduced the famous "tweed" Fenders, so called because of the tweed-like material Fender used to cover them. These amps were primarily low-wattage, cathode-biased amps. The circuit design resulted in early tube breakup or distortion that became the legendary sound of many blues guitarists. To this day the "tweed" circuit design IS extremely popular, and many of the boutique amplifiers currently being produced are mainly clones of the tweed amps with improved components. Due to their hand-wired construction and high-quality parts, boutique amps are quite expensive, even though the basic tweed design is simple and the operating theory extremely basic. What you're mostly paying for is the care and attention to detail that goes into these amps. Indeed, the interior chassis of a decent boutique amp
proves that high-quality artisanship and skillful manufacturing are alive and well in the United States. The era of the blonde and brownface Fenders ran from 1960 to 1963, when the amplifiers were covered with blonde or brown tolex. These were innovative and transitional amplifiers with the introduction of the separate head and speaker cabinet models, the introduction of vibrato and reverb circuitry, silicon rectifiers, and a fairly unique tone stack that would evolve into the famous three-band equalizer that helped define the Fender sound in the 1960s and early 1970s. Blackface models typically date from 1963 to 1967, with slight date variations according to model. The
The original tweed Deluxe is considered by many guitarists to be the amplifier with the quintessential vintage sound. Steven Seagal Collection/Rick Gould Opposite: Fender's classic tweed Bassman was created as a bass amp but quickly became the favorite of guitarists as well. The amp has been reincarnated as the '59 Bassman Reissue, show here in the LTD "lacquered tweed" edition. Fender Musical Instruments Corporation
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The blonde tolex Fender amps mark the transition from tweed to blackface. Shown here is a 1962 Tremolux head and matching cabinet. Oliver Lieber Collection/Rick Gould
Fender "silverface" era generally ran from 1968 to 1981, again varying slightly with each model. Contrary to popular conception, the sale of Fender to CBS, Inc. in 1965 does not mark the distinction between the blackface and silverface models. In fact, blackfaces continued to be made for a couple of years after the sale. Silverface models, on the other hand, definitely are CBS-era amps and, due largely to this fact, are at times unfairly maligned and disregarded. The criticism is understandable when put in the context of the end of an era. Yet,
Fender Amp Timeline 1948: Leo Fender introduces t he famous "tweed" Fenders 1948-1960: Tweed ampl ifiers 1960-1963: Blonde and brownface amplifiers 1963-1967 : Blackface amp lifiers 1968-198 1: Si lverface amp lifiers
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no one really twisted Leo Fender's arm and forced him to sell his company. Silverfaces can be divided into early models (1968 to 1972) and late models (1973 to 1981). The tipping point between early silverfaces and later versions was the first inclusion of the master volume control. Circuits differ across the silverface era and this distinction is meant to further clarify the differences. For actual dating of a particular Fender, consult the appendix for websites that can assist in that process. Because of their fairly wide availability and still moderate price tags, the first choice for modifications is silverface-era Fenders, preferred models being Super Reverb, Pro Reverb, Deluxe Reverb, Twin, Bandmaster, Bassman, and Showman. It should be noted that the silverface Bassman is worlds away from the original tweed-era Bassman. Essentially, the different circuitry between these amps makes them similar in name only. Furthermore, silverface circuitry is fairly close to that of the blackface, which is believed to be of the best Fender circuit designs. Indeed, blackface models are excellent choices for the modifications in this book, but with the added caveat that in general they cost twice as much as silverface models. For instance, while a silverface Super Reverb costs around $700 to $800, a
Fender's great blackface reissue amps-such as these Twin Reverbs-look like the originals, but the mass-produced circuit boards make them more difficult to hot rod. Fender Musical Instruments Corporation
blackface Super Reverb will run about $1,500. Due to their vintage value, therefore, any modifications done to a blackface should be nondestructive and easily reversible, as most modifications in this book are. Blackface models are so popular that Fender has manufactured reissues of certain models over the past couple of decades. At first glance, these models might seem to be perfect for modification since they are less expensive than the originals and lack the vintage status seen by many as taboo for modifying. However, while the reissues are comparable in price to the silverface (with some actually being more expensive than comparable silverfaces), they tend to be more difficult
to modify due to their printed circuit boards and substandard jacks and potentiometers, which are attached directly to printed circuit boards. Some intermediate to advanced modifications involve changing potentiometers, for instance, and while the procedure proves fairly straightforward on a silver, it can be a nightmare on a reissue. Yet, the reissues can be hot rodded, and to that end, I've included a number of modifications for not only the blackface reissues but also the tweed Bassman reissue. Other less desirable choices for modifying include newer tube amps, such as the Hot Rod Deluxe and Deville. In addition to printed circuit boards with
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The Hot Rod Deluxe-such as this Texas Red special with extension cabinet-and its big sibling, the Hot Rod Deville, can be re-voiced without much trouble. Fender Musical Instruments Corporation
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A typical Fender silverface amplifier-such as this Super Reverb-is an excellent candidate for hot-rodding. Silverface amps were built from 1968 to 1981. This is the amp used for many of the modifications in this book.
potentiometers and jacks attached to them, the tube sockets are mounted on printed circuit boards rather than the chassis. Besides discussing the wide range of tubes that can be used on the Hot Rod models, there is a special section in Chapter 5 dedicated to a series of approaches for improving the Hot Rods overall. These include re-voicing the overdrive channel to give it more depth and warmth as well as smoothing the sometimes brittle edge of the reverb effect. In fact, with some basic capacitor and tube changes in addition to overdrive and reverb mods, the Hot Rod models turn out to be nice, full-toned amps. Another choice for modification is a Fender clone kit (from vendors such as tedweber.com and tubedepot. com). These make great amps for a modification platform and are much less expensive than original Fenders. In fact, for the same price as a reissue blackface Deluxe Reverb, tubedepot.com sells a blackface Deluxe Reverb kit that, with its pine cabinet, American-made transformers, and original-styled fiber circuit board, is far closer to the original model than is the reissue. Also completing a kit will be the best way to learn about the circuit design of a classic tube amp, and the act of building one will help you develop proper soldering techniques.
Starting Point for This Book In looking for a Fender amp to use as the photo platform for most of the modifications presented in this book, I decided on 1970 silverface Super Reverb that I found in a local music store. The amp sounded fine in the store but had a slight background buzzing that grew worse when I turned on the bright switch or increased the treble setting. The noise didn't bother me too much, though, since I planned to replace most of the capacitors and resistors as detailed in Chapter 5. Moreover, the music store's building is old and many other amps as well as other gear were plugged into various outlets, not to mention the fair degree of fluorescent lighting. Any of these factors can induce interference in an amplifier, especially one in need of some fresh capacitors. I paid about $800 for the Super Reverb, which is more than it would have cost a few years ago, but is still a fair price in today's market. One thing to note about a decent working silverface is that its price is definitely going to go up as time goes on. Ruthless collectors are already beginning to eye these amps, poised to snatch away yet another piece of equipment from its rightful owner-the musician. Still, I think we're safe for a few more years.
Once home, I pulled the chassis from the Super Reverb to see what I might find. Not surprisingly, it looked like every guy in the neighborhood who owned a soldering iron had been inside this amp. Yet, one of the beauties of a silverface is that they are fairly easy to work on, and the damage can often be undone without great effort. In fact, I was pleasantly surprised to see that the filter capacitors had been replaced with good quality Sprague Atom types by someone who knew what he or she was doing. Judging from the manufacturer's date stamp printed on the capacitors' jackets, they looked to be about ten years old. Regarding the background buzzing I had heard in the store, after replacing capacitors, resistors, and about half of the wiring, it disappeared.
Modifications for Other Fender Amps Many of the modifications in this book can be applied to Fender amps not covered in this book. With ingenuity and by consulting the schematic and chassis layout for the amp not covered, a knowledgeable reader can apply these and other related modifications to the amp. As indicated in the appendix, schematics and chassis layouts are widely available online .
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Amps of the Stars: Stevie Ray Vaughan
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The '64 blackface Vibroverb has become legendary as the favorite amp of Stevie Ray Vaughan. In fact, Fender reissued a reengineered version of it based on collaboration with Cesar Diaz, the famous amp technician for people such Keith Richards and Stevie Ray Vaughan, from 2003 to 2008. The amp wasn't actually a straight-up version of the amps that Diaz modded for Vaughan, but was made for a little broader audience. Plus, the Diaz mods very much restricted the Vibroverb from most musicians'
uses, being tailored for Vaughan's unique playing style and large venue shows. Because blackface Vibroverbs typically cost more than $3,000, they're not really practical candidates forthe destructive (i.e., nonreversable) mods that Diaz performed on Vaughan's amps. For example, Diaz typically replaced the stock 8-ohm output transformer with a 2-ohm unit from a Super Reverb, a swap, by the way, that cuts shortthe life of the output tubes as well as severely stresses the output transformer.
Stevie RayVaughan "plays" his #1 Stratocaster at the Keystone Berkeley on August 19, 1983. Behind him is his famous hot-rodded '64 blackface Vibroverb amp, getting an equal sonic workout. Clayton Call1RedfernslGetty Images
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While I do not recommend these sets of modifications, you might find them interesting . To replicate the SRV mod, albeit notto the exact specs, a Super Reverb is a much better candidate since they are less expensive and have similar circuitry. Note that SRV also used Super Reverbs on occasion . If you don't feel comfortable stressing the tubes and transformer, replace the four speakers with lO-inch EVM types and skip the rest of this paragraph. The most pronounced SRV mod is replacing the speaker. On a Super Reverb you will not only need to pull the four speakers but
also to replace the baffle. For this you'll need a sheet of 1/2-inch plywood cutto the same width and height as the original baffle . SRV usually used 3/4-inch baffles; you can use that size, but it will take extra fashioning to get itto fit properly. Next cut a hole forthe 15-inch replacement speaker. This will not be a 15-inch hole but more like a 14inch . As forthe speaker, use an EVM-15L, 8-ohm speaker. Using a 4-ohm version will be easier on the amp, butthat isn't what Diaz used. Next, convert the rectifier from tube to solid state. The easiest way to do this is to use an SSR (solid-state rectifier) plug-in, available from mosttube dealers for about$lO. This will raise the high voltage in the amp, so make sure to rebias your output tubes. They will already be taking a beating if you go with transformer-speaker mismatch . Diaz also replaced the two 70-uF, 350-volt filter capacitors with 220-uF, 350-volt values . These are located under the pan on the backside of the chassis. Tweaks done to the circuitry include disconnecting the vibrato effect (unhook the wire from the right lug of the intensity potentiometer). disconnecting the normal channel (unhook the wire that runs from the coupling cap to the mixing resistor, identified by the "X" -markings on the schematic). replacing the bass and midrange capacitors on the vibrato channel with O.033-uF Orange Drops, and replacing the 1-M-ohm resistors on the grid of the phase inverter tube with 33-K-ohm values. As for tubes, SRV often used a 5751 dual-triode for the preamp tube (second from right when looking at the back of the chassis). This tube has about two-thirds less gain than the usual 12AX7 but is still plenty loud with a great deal of headroom . These tubes are NOS and cost about$25. More expensive and nearly unavailable are the Philips 6L6-STR output tubes. You might consider current production Tung-Sol or TAD 6L6-STR versions.
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Amps of the Stars: Neil Young
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The foundation for much of Neil Young's electric guitar amplification comes from a hot-rodded tweed Deluxe . While the tweed Deluxe usually only puts out about 15 watts max, it has a warm, richly harmonic distortion when played at full throttle. Obviously, Young uses microphones on the amp. He also uses a devices he calls
"the Whizzer," which essentially is a box with servo motors that turn the tone and volume controls remotely by a pedal signal. The only real mod Young uses on the Deluxe is using a pair of 6L6 tubes rather than the stock 6V6. When doing this, the amp needs to be rebiased. Since it incorporates
Neil Young performs with his legendary 1953 Gibson Les Paul "Old Black" on stage at Trent FM Arena on June 23, 2009 in Nottingham, England. Steve Thorne/ Redferns/ Getty Images
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Neil Young's Old Black and his Martin acoustic rest in front of his lineup of vintage Fender tweed Twin amplifiers and his tweed Deluxe (left) on stage at Munich's Olympiahalle on June 17,2009. Sitting on top of the Deluxe is the device known as the "Whizzer;' a servo-actuated mechanism designed for Young. The Whizzer allows him to automatically control the settings of the amp's tone and volume controls while playing . Stefan M. Prager/Redferns/Getty Images
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cathode biasing, the cathode resistor needs to be replaced. If you are doing this mod, replace the cathode resistor with a 250-ohm, lO-watt PRODUCED BY LIUE NAllON $ 11 .86 power resistor. These 6L6 output tubes won't make the amp any louder PLUS SPECIAL GUESTS since the output transformer is rather llolS 98X small, butthe distortion and tonal NO CA"ERAS OR RECORDERS coloration will change. One important 7518953 final step in this mod involves cooling the power transformer since the extra draw of filament current exceeds the transformers rating. Mounting or simply placing a fan in the back ofthe amp cabinet will provide the minimal protection. While you might not want to put 6L6 tubes and different cathode resistor into a vintage Deluxe, keep in mind that Neil Young does just that. This mod can be easily reversed, as well. The number of clone tweed IN THE Deluxe kits are legion these days, and cost anywhere FREE from $500 on up. This is far more reasonable price than WORLD an original tweed or a reissue (and easier to work on than a reissue) . Furthermore, the power transformers usually have higher rating than those ofthe original tweeds.
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chapter
2
TOOlS ANO flfCTRONIC COMPONfNTS While this book is aimed at guitar players who have a basic knowledge of electronics and such practices as soldering and using a multimeter, to really understand the principles of modifying amps the reader needs to have a working knowledge of the fundamentals of amplifier operation and technical terminology. (If you're already familiar with these things, you might want to skip ahead to the next chapter.)
Electrical Currents In the most basic term, current refers to the flow of electrons. Voltage is the force that causes the electrons to flow, and resistance is opposition to the flow of electrons. A common analogy for the relationship between these concepts involves a water faucet. When the valve is turned off, no water flows, but instead is stored in the pipe, much like voltage stored in a battery. Turning the valve slightly causes water to flow in a trickle; turn the valve farther and water gushes. By analogy, then, the flow of water represents the flow of electrons or current. The pressure behind the valve causing the water to flow represents voltage. The valve itself presents opposition to the water flow in the paradigmatic manner that a variable resistor opposes the electron flow. This analogy brings up another definition of voltage: As a force, voltage represents a difference in potential that causes current to flow. In our analogy, the water pressure in the pipe is greater than the air pressure outside the faucet. This difference in potential causes the water to flow from a high-pressure area to a low-pressure area. The difference in potential regarding voltage is between a negative charge or potential and a positive charge or potential. Electrons flow from negative to positive. As for measurement, current, signified by the letter I, is measured in amperes or amps, abbreviated
Oppposite:The 5-watt Fender Champ was meant as a practi ce amp, butthe rich ly harm onic distorti on t he amp del ivers at full volume made it a studi o favorite. Shown here is the '57 Champ Reissu e. Fender Musical
as A, and more commonly in milliamps (rnA) and microamps (pA). Voltage, signified as V, is measured in volts, and often given in kilovolts (kV), millivolts (mV), and microvolts (pV). Finally, resistance, signified as R, is measured in ohms, abbreviated as the Greek symbol D or just written out as ohms, and often given in megohms (M-ohms) and kilohms (K-ohms). These measurements will be described in more detail later in this chapter. The flow of current through, resistance in, and amount of voltage across the various components and pathways of a circuit can be measured with a tool known as a multimeter or volt-ohm meter (VOM), sometimes simply called a meter. Traditionally, multimeters come in two major types: the analog multi meter, which uses a needle and calibrated scale, and the digital multi meter (DMM), which uses a digital display. Today, the DMM is far more common that the older analog multimeter. Both work fine on a guitar amp, but the DMM is easier and more accurate to read. Good quality multi meters are widely available from stores such as Radio Shack and Sears and many online venders. You can buy one for less than $50 that will work fine for basic guitar amp repairs and modifications. Make certain that you get a meter that can range from 0 millivolts to 1,000 volts
Taking Caution with Tube Amplifiers Tube amp lifiers contain high voltage that can cause injury or death . These vo ltages can be stored in an amplifier's capacitors after turn ing offthe power. Do not open up an amplifier, do not attemptto repair an amplifier, and do not attemptto modify an amplifier unless you are absolute ly certain of what you are doing . Many of the procedures detailed in this book are meant for individua ls with knowledge of electronics and proper safety procedures. The author and the publ isher accept no responsibil ity for any destruction of property or for any personal injury, accident, or death .
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DC, and definitely read the manual accompanymg the meter. Essentially, a multimeter has settings for reading AC voltage, DC voltage, resistance, and current, although you probably won't use the current setting since reading current involves opening up a path in the circuit, and current can more easily be determined by measuring voltage and resistance. Each of these settings includes additional settings for range, usually in variables of 2. For example, for reading resistance, a meter might have ranges of 0 to 200 ohms, 0 to 2 K-ohms, 0 to 20 K-ohms, etc., up to 0 to 20 M-ohms. Likewise, for DC voltages, the meter might have ranges of 0 to 200 millivolts, 0 to 2 volts, 0 to 20 volts, 0 to 200 volts, and 0 to 1,000 volts. Again, it's important to have a meter that has a high-voltage range (0 to 1,000 volts being common) as guitar amps can carry more than 500 volts DC. Some multimeters include an auto-range feature that, as the name implies, automatically sets the meter on a range according to the voltage or resistance being measured. When measuring resistance, make certain you turn off all power to the amplifier. Also be aware that to get an accurate measurement of a resistor might mean
Two excellent multimeters to consider for your work. On the left is a Simpson analog multimeter. A Fluke digital multimeter is on the right.
Measuring Unknown Voltages If you do not have an auto-ranging meter and you 're measuring an unknown voltage, place the meter on the highest range, measure the voltage, and if the reading is too low, lower the range and measure again. That way, you won't burn out your meter.
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removing the resistor from the circuit because electricity always takes the path of least resistance. Therefore, when placing the meter leads on each side of an incircuit resistor, the meter will only read the value of the resistor if it is lower than the resistance of the associated circuit, that is, the path from the meter lead away from the resistor, through the circuit, and back in to the other lead. Yet, measuring resistance doesn't always mean measuring resistors. For instance, the resistance setting is used to test for open circuits (such as testing if a fuse is blown) or testing for a complete circuit (such as testing the center conductor of a guitar cable by placing a meter lead at the jack center points at each end of the cable). To test for closed or open circuits, or overall resistance of a portion of a circuit, place a meter lead at one end of the component or to whatever you'll be reading through and the other meter lead to the other side of the component or circuit. Move the range setting to a higher or lower scale to get an accurate reading, unless you are measuring for a completed circuit or an open circuit, in which case use a low range. Don't let your fingers touch the metal tips of the meter leads because the resistance of your body can affect readings of high resistance. Measuring resistance also means measuring continuity, which means connectivity as in a closed or open circuit. When measuring DC voltage, place the negative lead (black in color) on a ground, usually the chassis. You might use an alligator clip to attach it to the ground terminal of the power cord (usually the green wire from the power cord attached to the chassis). Set the voltage to a higher level than the voltage you expect to measure, turn on the power to the amplifier and let it reach operating conditions, and carefully place the positive (red) lead to the point you want to measure. When measuring an unknown voltage without an auto-ranging meter, place the meter on the highest range, measure the voltage, and if the reading is too low, lower the range and measure again. For example, suppose you are measuring an unknown DC voltage, and after placing the meter range on the 1,000-volt, you read 20 volts. You can now lower the range to the 200-volt range to get a more accurate reading of the 20 volts. If you are unsure of whether you are measuring AC or DC voltage, place the meter on AC at the highest range; if nothing shows up (or a low voltage), set the meter to DC. If you read nothing here, lower the range. If you still read nothing, then go back to the AC setting and measure the voltage as low AC. Be aware that the voltage inside a guitar amplifier can be lethal. Practice extreme care when measuring voltage. If you have any misgivings, follow your instincts and don't measure live circuits. Furthermore, be aware that when you turn off the power to an
amplifier, the large electrolytic capacitors continue to hold a charge. Bleeder resistors and tube plate circuits will eventually drain the voltage as the amplifier sits, but not right away. Always measure for voltage on the plate of a power tube to see if the capacitors are holding a charge. If there is voltage, drain the power capacitors as described later in this chapter.
Soldering and Desoldering Another tool essential for amplifier modification and repair is a soldering iron. A 30-watt "pencil-style" soldering iron works well for tube amps. Avoid using trigger-operated soldering guns because they run hotter than the typical soldering iron and therefore can damage a circuit board. While a high-wattage (40 watts or greater) iron can damage a circuit board for obvious reasons, a low-wattage iron (anything below 25 watts) can be equally as damaging since the user ends ups holding it against a component for a longer period of time to get the solder to melt. Solder is an alloy made of tin and lead with a small amount of silver added to higher quality types (most solder for electronics has the designation 60/40, meaning 60 percent tin and 40 percent lead). Currently, lead-free varieties are available and work just as well, although they tend to have a higher melting point. In solder used for electronics, the core is filled with rosin flux, which, being slightly corrosive, helps to bond the solder to metal component leads and wire. Just as with playing guitar, proper soldering technique takes practice. If you're not comfortable practicing your soldering skills on your thousand-dollar amp (and you shouldn't be if you're not experienced), a
Rather than using the typical, inexpensive 30-watt soldering iron, a soldering station usually has an adjustable heat range and better thermostatic capability, making it more versatile and safer on circuit boards. This Weller hobbyist model sells for around $60.
myriad of inexpensive electronic project kits are available online at such places as electronkits.com, canakit. com, and qkits.com. Another source for practice circuits could be broken radios, tape decks, televisions, or any number of consumer electronics goods that we humans throw in the trash hourly. With a broken piece of electronics you can also practice desoldering, which is as important to learn as soldering. One major difference between learning soldering skills and learning guitar techniques is that learning to solder generally takes less time. For a proper solder joint, the surfaces that will be soldered (be they wire or component lead) need to be free of oxidation. A few swipes with a piece of sandpaper will clean a tarnished surface. Make sure the soldering iron is hot. Usually about five minutes proves sufficient. Once the iron is hot, wipe the tip clean with a damp sponge or rag. This cleaning process should be performed periodically while soldering. Each time you clean the tip make sure to "tin" it by melting a small amount of solder on the tip and coating it. Next, set the joint to be soldered by mechanically connecting the wires or leads. With the iron in one hand and solder in the other, hold the iron to the joint and melt the solder onto the joint. Essentially, you want to avoid melting the solder on the tip of the iron and having it drop onto the joint. Rather, the trick involves heating the joint and allowing the solder to flow over it by melting the solder on the area of the joint right next to the iron. Do this somewhat quickly but not so quickly that the solder doesn't flow evenly. Such poor technique can result in a cold solder joint that won't conduct electricity properly. The knack of soldering involves balancing the contact time on the joint with adequate solder coverage. By the way, a cold solder joint can occur for a variety of reasons, the most common being moving the component lead or wire before the solder hardens (which normally takes only a second or two) and not allowing the solder to completely flow over the joint. In either case the solution is easy: Simply reheat the joint. A cold solder joint might not be easy to immediately identify. Typically, it appears grainy and often slightly pitted, while a decent solder joint has a smooth surface. The counterpart to soldering is de soldering. As the name implies, de soldering means removing solder from a joint so that a component or wire can be removed. In most cases removing a wire or lead from silverface or blackface amps, which use tag boards rather than circuit boards, simply involves heating the joint while pulling out the wire or lead, usually with a pair of needle-nose pliers. Because tag board uses eyelets rather than traces and pads found on contemporary
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Skillful soldering results from balancing contacttime with solder flow. Enough solder must melt and flow over the joint without overheating the components and wiring.
To use a trigger-operated desoldering tool, push down the plunger until it locks and hold the tip of the tool next to the joint to be desoldered. Next, heat the joint with a solder iron. As soon as the solder melts, press the trigger or button. The tool's plunger connects to a spring-loaded piston that draws the molten solder in through the tip when released.
circuit boards, it can take more heat without damage, whereas the traces and pads can easily lift and become torn from momentary heat of a soldering iron, not to mention from prolonged or extended soldering or desoldering. Someone with decent soldering skills can usually remove a component form a circuit board with minimal heat and a slight pull of needle-nose pliers. Often, though, when working with circuit boards, or with tag boards with excessive solder in the eyelets, for
•• 20
that matter, solder needs to be removed from the joint before a component lead can be pulled free. Circuit board de soldering takes skill; there's no two ways about it. The easiest, least expensive, and most practical method involves using suction, either in the form of a desoldering bulb or a trigger-operated desoldering tool. Bear in mind that these tools require practice to be used adequately. The suction from these tools can also damage circuit board pads, especially in the case
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Basic desoldering tools: The desoldering bulb produces vacuum when the bulb is squeezed and released, pulling molten solder into the bulb. The trigger-operated desoldering tool produces vacuum with a spring and piston. After the plunger has been pushed into the body, the trigger releases a lock, causing a spring to drive the piston home and pull in molten solder on the way. Both tools need to be cleaned of old solder periodically.
of the trigger-operated tool. These tools also need to be periodically cleaned.
Miscellaneous Tools Other useful tools for guitar amplifier modification and repair include several sizes of needle-nose pliers, a small wire cutter, a wire stripper, terminal crimper, and an assortment of screw drivers, both Phillips and standard. A 1I4-inch drive socket set or a set of nut drivers also will prove essential, as will long-nosed tweezers, a metal and nonmetal probe, as well as a nonmetal adjustment tool (usually a long plastic stick-like instrument with a standard screwdriver tip for adjusting trimmer pots). You can make excellent jumper leads out of good quality wire by soldering alligator clips to each end. Besides hand tools, you'll need some basic materials, such as wire and heat-shrink tubing. Use good quality wire rated at 600 volts. Insulation type can vary but should be Teflon, PVC, or cloth-covered PVC. Wire gauge can range from 18 to 22. Solid-core 20-gauge wire works well in that it is easy to bend and will hold its shape. Also, it's not too thick. Stranded wire works fine as well but doesn't have the easy manipulation of sold-core. While heat shrink tubing isn't a necessity, it works far better than electrical tape. It must be slid over the wire or lead before it is soldered into place. Next, use a lighter or match to shrink the tubing to the wire or lead.
Resistors As the name implies, resistors "resist" the flow of current. As a result, resistors lower or drop voltage and in
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The tubing shrinks with the application of heat from a few brief passes of flame from a butane lighter.
the process convert electricity to heat. The amount of heat dissipated by a resistor depends on the amount of current flowing through it along with the amount of voltage it drops. Because of this, a resistor can get quite hot, and if its power rating is exceeded, it will burn out. The power rating, measured in watts, is therefore determined by the amount of current, measured in amps, or more likely milliamps, passing through a resistor multiplied by the voltage, measured in volts, that it drops. Most resistors in guitar amps are rated at 112 watt, with others carrying larger currents rated at 1 watt or greater. The larger the power rating, the physically larger the resistor. The resistance or value of resistors is measured in ohms, kilohms, and megohms, and is abbreviated by the Greek letter omega, 0., for ohms (or just spelled out); KD, K-ohms, or K for kilohms; and MD, M-ohm, or M for megohms. While some resistors, especially the larger ones, have their values printed on them, most use colored bands to signify the value. You can easily measure the voltage dropped by the resistor, called the voltage drop, by placing each of the meter leads on each side of the resistor, making sure you've selected a meter range that exceeds the voltage to be read (if you are not using an auto-ranging meter). The current is more difficult to measure but can be easily determined if you know the voltage drop across the resistor, which you then divide by the resistor's value in ohms, kilohms, or megohms. For example, suppose you read a voltage drop of 100 volts across a 100 K-ohm resistor. Dividing 100 volts by 100,000 ohms equals 0.001 amps, or 1 milliamp. To find out the power dissipated, or the wattage used, multiply 100 volts (the voltage drop) by 1 milliamp, or 0.001 amp (the current
21 ••-
-
\ I
( Four-Band Resistor Color Codes Color
First Band
Second Band
Third Band
Fourth Band
(First Digit)
(Second Digit)
(Multiplier)
(Tolerance)
Black
0
0
1
Brown
1
1
10
1%
-
Red
2
2
100
2%
Orange
3
3
1000
3%
-=
Yellow
4
4
10000
4%
Green
5
5
100000
0.5%
Blue
6
6
1000000
0.25%
Violet
7
7
10000000
0.1%
Gray
8
8
100000000
0.01%
White
9
9
1000000000
c--.J
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c::::L
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~
Gold
0.1
5%
Silver
0.01
10%
-.'-.: L..
fI
,
(
I
Five-Band Resistor Color Codes Color
First Band
Second Band
Third Band
Fourth Band
Fifth Band
(First Digit)
(Second Digit)
(Third Digit)
(Multiplier)
(Tolerance)
Black
0
0
0
1
Brown
1
1
1
10
1%
Red
2
2
2
100
2%
Orange
3
3
3
1000
3%
Yellow
4
4
4
10000
4%
Green
5
5
5
100000
0.5%
Blue
6
6
6
1000000
0.25%
Violet
7
7
7
10000000
0.1%
Gray
8
8
8
100000000
0.01%
White
9
9
9
1000000000
Gold
0.1
5%
Silver
0.01
10%
The upper table shows standard four-band resistor color codes; the bottom table shows the five-band resistor color codes for precision resistors, such as those you'll find with many metal film types. The sample resistor represented for both is 1 megohm with a 1 percent tolerance, which means that the actual value of the resistor may vary by 1 percent of the indicated value.
•• 22
passing through the resistor). The result is 0.1 watt. In this case, a l/4-watt resistor would be sufficient. The types of resistors used in guitar amplifiers include carbon composition, carbon film, metal film, metal oxide, wire-wound, and variable (i.e., potentiometers). While you're not likely to find all of these types as stock in a single amp (for example, pre-1970s Fenders came with carbon composition resistors), these resistors have differing characteristics that make each type useful for particular purposes when replacing stock resistors. Carbon composition resistors: Also called carbon comp resistors or simply carbon comps, carbon composition resistors are made by pressing carbon particles along with a binder into a rod and attaching the ends to metal caps, which in turn connect to leads. The rod is either coated in varnish or encased in plastic or ceramic. The proportion of carbon to binder determines the value or resistance, while the physical
size determines the power rating or wattage. Carbon comps are available in resistance ranges from less than 1 ohm to more than 20 megohms. Available power ratings include Vs , 1;4, Y2 , 1, and 2 watts. Carbon comps tend to be unstable at high temperatures, meaning their resistance increases with a corresponding increase in temperature. In other words, they have what is called a large temperature coefficient. Moreover, their values tend to drift with age. Not only that, but they can be noisy at higher voltages. Yet even with these disadvantages, carbon comps are often considered the paragon of resistors for guitar amplifiers. Some designers and technicians believe this reputation is unfounded since resistors don't really contribute much to an amp's tone. However, other designers and
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Resistor Color Codes Typically, resistors do not indicate their value on the resistor as many capacitors do. Instead, a universal color code has been developed to identify resistor values by a number of colored bands on the resistor body. To allow the reader to easily identify resistor values according to the color code, the charts on the opposite page are included.
The types of resistors you're liable to find in tube amplifiers include, from left to right, 5-watt and 10-watt wire-wound, l-watt and 2-watt metal oxide, 1/2-watt and l-watt carbon composition, 1/2-watt and l-watt carbon film, and 1/2-watt metal film.
Basic Resistor Circuit Relationships
R1 50 R1 50
"/5V "/5V
1Amp
0.5
0.5
Amp
Amp
./'
10V~
"/10V
1Amp R1 200
R2 200
Total R = 50 + 50 E = I/R 10V = 1A/100 For R1 or R2, 5V = 1A/50
Total R = (R1 x R2) + (R1 + R2) = 100 E = I/R 10V = 1A/100 For R1 or R2, 10V = 0.5A/200
Two Resistors in Series
Two Resistors in Parallel
Ohm's law states that voltage equals current divided by resistance, or E=I/R.
23 ••-
-
technicians claim there is a degree of warmth and slight distortion associated with carbon comps and that their large temperature coefficient contributes to the desirable tonal qualities of a tube amp. Another factor that plays into the reputation is their association with the vintage ethos. Carbon comps were used in the manufacture of Fender amps from their conception until well into the 1970s. I would hazard to say that a 1955 Deluxe sounds different today than it did when it arrived fresh from the factory, and that vintage sound might well owe some of its distinction to worn and drifted resistors. Add to this the diversity of tastes among musicians and the bottom line becomes this: Vintage sound is a complex phenomenon. I will have more to say regarding resistor choice in later chapters. Carbon film resistors: As just mentioned, Fender amplifiers from their inception up until the 1970s came equipped with carbon composition resistors, primarily because of their mass availability and low cost at that point in history. Today, the carbon film resistor has eclipsed the carbon comp as the lowest priced, most widely available resistor. Carbon film resistors consist of a cylindrical ceramic substrate fitted with end caps and leads. A thin layer of carbon, the amount determining the resistance, coats the substrate and is, in turn, covered with paint or ceramic encasement. Their values typically range from less than 1 ohm up to many megohms, with usual power ratings of 118, 114, 112, and 1 watt. They are stable, have excellent temperature ratings, and are less prone to noise than carbon comps. Moreover, while carbon comps have usual tolerances of 5 to 10 percent (that is, the percentage that the value can vary), carbon films can be as precise as 2 percent. Metal film resistors: With the same ratings, values, and stability of carbon film resistors, metal film resistors have a more precise tolerance (1 percent or better) and are quieter than either carbon comps or carbon films. While their quiet operation has led some people to label them as purveyors of sterile sound, that claim is unfounded unless one considers noisy resistors to be desirable. As with the carbon film variety, metal film resistors have a cylindrical ceramic substrate with end caps formed to leads, although glass is sometimes used as a substrate. Rather than a thin layer of carbon, a thin layer of metal is wrapped spiral-like around the substrate, the amount of metal determining the resistance. Due to their precision, metal film resistors are more expensive to produce (although they are still relatively low-priced). As a result, they are usually only found stock in certain custom-made and highend amplifiers. Metal oxide resistors: Metal oxide resistors are constructed much the same as the film types in that they typically have a ceramic core coated with tin oxide that, in turn, is covered with a flame-retardant
•• 24
coating. Because they offer excellent stability at high temperatures and have a high power rating for their size, they are especially suited for use in power supply circuits and as screen resistors for power tubes. Metal oxide resistors typically come in values of less than 1 ohm to about 1 megohm with 5 percent tolerance and power ratings of 1/2 to 5 watts. Wire-wound resistors: As the name implies, wirewound resistors consist of a coil of resistance wire wrapped around a ceramic, porcelain, or cement core. Typically this arrangement is then encased in an insulating material, usually cement. These flame-retardant, extremely stable, high-current-capability resistors are commonly used in power supplies and, like metal oxide types, also as screen resistors for power tubes. While wire-wound resistors come in lower values (less than 1 ohm to several kilohms), they have high power ratings, from the common 5 and 10 watt sizes up to more than 100 watts. Potentiometers: While there are various types of potentiometers, or pots, the type most commonly used in guitar amps is the carbon-track variety. The two outside leads connect to opposite ends of a circular carbon track with a fixed resistance (common values in amplifiers include 10 K-ohm, 250 K-ohm, 500 K-ohm, and 1 M-ohm). The center lead connects to a wiper arm, which sweeps along the carbon track, creating a variable resistance between the wiper arm and each of the outer leads. On a typical Fender volume control, one outer lead connects to ground while the other outer lead is fed by the output of the tone controls (called the tone stack). The wiper arm is connected to the input of the next preamp stage. In this arrangement the input of the preamp stage is grounded (no volume) when the wiper is turned all the way down, grounding the wiper through the grounded outer lead. Alternately, when the wiper is turned all the way up to the outer lead that is connected to the tone stack, the input of the preamp stage receives the full signal from the tone stack (full volume). The change in resistance read at the wiper determines what is called the taper of the potentiometer, the two primary types being linear and audio, also called logarithmic or log. The function of linear taper is straightforward in that resistance changes proportionately to the wiper's movement. For example, if the wiper of a 1 megohm linear pot is in the center position, halfway between each lead of the carbon track, then the resistance between the wiper and each of the leads is 112 megohm or 500 kilohms. At one-quarter position, the wiper to one outer lead is 1/4 megohm (250 kilohms) and 3/4 megohm (750 kilohms) to the other lead. In other words, the movement on the wiper in relation to resistance is linear. Audio or log taper is more complicated and can be thought of as working in stages, meaning rather than a
gradual increase or decrease in resistance between the wiper arm and the outer leads, the audio taper has a large jump in resistance in the first quarter movement, a smaller jump at the halfway position, and from then on the resistance doesn't change much. As an example, with a 1 megohm audio, placing the wiper at one-quarter of the full run will result in a measurement of about 400 kilohms between the wiper and the lead closest to it. Going halfway gives a measurement of about 800 kilohms between wiper and the same lead while at
I ... .-~ 4--" . .j. ..~ ~
•
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'" The basic potentiometer used in guitar amplifiers consists of a circular carbon track upon which a metal wiper slides.
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Volume Potentiometer Function
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~
2nd
Tone Stack ) Full Signal to Ground
1M-ohm
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Half Volume Tone Stack
200K-ohm ==
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==800K-ohm
>== S p I'It" . 800K-ohm
between Ground and Preamp Input
Volume Full Up ~
Tone Stack
~
>1M-ohm
2nd Stage Preamp
1M-ohm
>
t Full Signal to Preamp Input
The volume potentiometer function of a typical Fender amplifier.
25 ••-
-
three-quarters of a full turn the measurement is about 900 kilohms. The reason audio taper pots function this way is that the human ear doesn't hear sound linearly. Therefore, a linear pot doesn't respond in a way that complements our hearing, whereas an audio taper pot does. However, this does not mean that our ears "hear" an audio pot as if it were a linear pot. You've probably noticed that on your amp most of the volume seems to occur by the time you turn the knob halfway. That's precisely because it has. Fender amps use audio pots for volume and tone controls (bass, treble, and midrange) and linear pots for the reverb control. Since reverb is an effect, the reverb potentiometer functions more like a mixer and in that capacity the linear taper works best. Also some models with a master volume, such as the Hot Rod amps, use a linear taper pot for the master volume, which doesn't function as a typical volume control but as a low-resistance (100-K-ohm) output level control for the full preamp signal before it enters the phase inverter to the output stage of the amplifier. The Hot Rod amps also use linear taper pots for the midrange control that, while technically a midrange tone control, is interactive with the treble and bass controls in such a way that adjusting the midrange control actually shifts the midrange in relation to the bass and treble settings. In other words, the midrange adjustment isn't heard by our ears the same way that the volume adjustment is. We will have more to say about tone control functions in Chapter 7. Specific resistor functions will be discussed throughout the book, including the vacuum tube section in this chapter.
Capacitors Broadly speaking, capacitance IS the ability of an electric circuit to store electricity. The higher the capacitance, the higher the amount of electricity, in volts, stored. To that end, a capacitor is an electronic component used to store electricity. Capacitance is measured in units called farads and given the symbol F. One farad is large, and the values of individual capacitors are much smaller. In guitar amplifiers, capacitors typically range in values of 100 microfarads down to 10 picofarads. The microfarad is abbreviated as uF, with 1 uF being one-millionth of a farad or .0000001 farad (10-6 in scientific notation). A picofarad (pF), by extension, is one-millionth of a microfarad, thus onetrillionth of a farad or .0000000000001 farad (10-12). The nanofarad (nF), which is a billionth of a farad and a thousandth of a microfarad, is also used to measure capacitors, yet with Fender amps, you'll usually only encounter the uF and pF measurements. The capacitor's ability to store voltage makes it well-suited for power supplies, not because capacitors can store voltage like a battery, but because the voltage
•• 26
they store can be released. Thus, the discharge rate is an important factor in designing a rectifier circuit to convert alternating current (AC) to direct current (DC), with capacitors storing and releasing the pulses of voltage at such a rate that it smoothes the pulses, or ripple, of voltage . I will explain the capacitor's role in rectification when discussing electrolytic capacitors later in this chapter. The capacitor's ability to store voltage also makes it useful for purposes that might not immediately bring to mind the storage voltage. For this to make sense, we have to first understand just what a capacitor is. Essentially, it is two metal plates-also known as electrodes-that have leads on the ends opposite to one another and an insulator, or dielectric, between the plates. The dielectric, which is usually used to describe the types of capacitors, can be paper, waxed paper, oil, ceramic, polyester, mica, polypropylene, polystyrene, and even air. All of these materials offer different qualities and degrees of capacitance. Now, an important feature of all dielectrics is that they block direct current (DC) while passing alternating current (AC). How they do this is a function of their ability to store electricity. In the case of DC voltage, because the electron flows in one direction only, they collect on only one of the plates of the capacitor, unable to pass through the dielectric, giving that plate a negative charge. Simultaneously, the other plate develops an equivalent opposite, or positive charge. When the DC voltage stops flowing, the capacitor holds the charge (which is often weak but with a large capacitor strong enough to cause injury). If the plates are now connected across their leads (on the other sides of the plates from the dielectric), the stored voltage discharges. Thus for any portion of a circuit where DC voltage needs to be blocked, a capacitor is employed. To say that a capacitor passes AC is a bit misleading. Actually, because AC voltage rises positively and falls negatively, the capacitor continually charges and discharges in relation to the alternating current. The ability to block DC while passing AC makes the capacitor a primary component in an amplifier, specifically as a coupling capacitor. In this application, a coupling capacitor is placed between tube stages of an amplifier. In tube circuits the voltage at the output of a tube functions at hundreds of volts DC. Yet, the input of the next tube stage often needs to be at 0 volts DC for the tube to operate properly. The AC signal at the tube's output, however, needs to remain the same at the next tube's input so that it can be amplified by the tube . A coupling capacitor makes that possible. Coupling capacitors, in their application, also shape frequencies. Being AC voltage, the frequency of audio signal is measured in cycles per second, or hertz, and given the symbol Hz. One hertz is the time it takes for an AC signal beginning at the 0 midpoint to swing
Coupling Capacitor Function 200VDC
OV
100 mVAC Audio Signal
100mVac Audio Signal Capacitor C1
100 mVAC Audio Signal
200V dc
o Vdc >- ______
-'-+-+--\--+--\--+-~+-~-A 100 m VAC Audio Signal
Graphic representation of the coupling capacitor Cl blocking the DC operating voltage while passing the AC audio signal.
Measuring the Frequency of an Audio Signal 2
- - + - - - - - - - - - - - - - - - - -1 - I I I
1
o
-- ~ --I I I I
I I I
----------- ~ -I I I
I I I
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-2 - - j. - - - - - - - - - - - - - -
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1 second
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One hertz is equivalent to one complete cycle of AC per second.
27 ••-
Capacitor in a DC Circuit Flow of Electrons
• -
Battery
_ Build-up of electrons (negative charge)
Capacitor
+
Subsequent lack of electrons on opposite side of capacitor causes a positive differential (positive charge) No Return of Electrons
The capacitor charges and blocks flow of electrons.
Capacitor Function in an AC Circuit
~
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29 ••-
-
Silver mica capacitors (such as the two on the left) are far superior for high-frequency tone and coupling application than the standard ceramic disc variety (such as the two on the right).
The two paper capacitors in the middle are wax coated, while the one on the top is plastic coated. The capacitor on the bottom consists of oil-treated paper covered with a cardboard tube and wax.
In the manufacture of film capacitors, a thin layer of synthetic material, such as Mylar, polyester, polystyrene, or polypropylene, is wrapped tightly with metal foil, as with paper capacitors, but because synthetic dielectric is much thinner than paper, the capacitor has a smaller physical size. In the case of the Mallory 150M series, a thin layer of polyester is sprayed with a fine layer of metal rather than wrapped between metal film. While further reducing the physical size of the package, this process also accounts for a more precise, stable, and warmly toned capacitor. Equal in quality is the Illinois metalized capacitor, made with the same process but using polypropylene rather than polyester. Both the Mallory and the Illinois are tubular in shape and encased in a yellow plastic wrap, although some runs of Mallory are white. Sozo capacitors use polyester for the dielectric, but instead of being sprayed with metal, a thin layer of metal foil is rolled tightly with the dielectric. This process gives the capacitors a slightly higher tonal quality at a slightly higher cost. A .1 uF Mallory 150M, for instance, costs about $1.50; whereas, a .1 uF Sozo Mustard cap costs about $2.50. Furthermore, a .1 uF Sozo blue-molded vintage cap, made as a direct replacement for the Fender blue-molded Mallory, costs about $7. Another common film capacitor is the Orange Drop. While several brands of capacitors are called Orange Drop, some of the better quality ones are made
•• 30
by SB (Sprague-Barre) Electronics and include the SBE 716P, which has excellent precision and a shimmering tonal quality. These are constructed with polypropylene film and have a hard, orange plastic shell. The Mallory and Illinois capacitors are tubular with axial leads, and the Orange Drops are more rectangular with radial leads. While some people find the Orange Drops to be a little brittle in the high-frequency range, others describe the highs as pleasantly chimey. Similar in appearance to the Orange Drop, but smaller and not as wide, the Xicon polypropylene capacitor is used in many newer Fenders. While they are a fine capacitor in their own right, they don't have the same tonal detail as the Orange Drop and are considered somewhat inferior. However, their smaller size and lower price make them an excellent choice in amps using printed circuit board, where space is often at a premium. A word about determining a capacitor's value: While Mallory and Illinois capacitors have their value printed on the package, Orange Drop and Xicons have a code stamped on their bodies. The three-number code is followed by a letter, usually a J or a K, which in turn is followed by the voltage rating, typically 400V, 600V, or 630V. Regarding capacitor voltage ratings, when replacing a capacitors always use one with the same or greater rating. If a capacitor faces more voltage than it is rated at, it will be destroyed in short order. The letter refers to the tolerance, or the precision of the
value. The letter K means that the value of the capacitor is within 10 percent of its stated value; the letter] means the capacitor value is within 5 percent. Deciphering the code is easy. For example, 103K600V means that the first number of the value is 1, the second number is 0, and the multiplier is 3, meaning three Os follow the second number (the value is in picofarads). Thus 103 equals 10,000 picofarads or .01 microfarad. This value is within 10 percent accuracy, and the rating is 600 volts. Similarly, 104J400V means 100,000 picofarads or .1 microfarad, 5 percent tolerance, and 400-volt rating. 223K600V means 22,000 picofarads or .022 microfarad, 10 percent tolerance, and 600-volt rating. Remember, to convert picofarads to microfarads, you move the decimal six places to the left. Electrolytic capacitors: Electrolytic capacitors have two primary uses in Fender amps: as filter capacitors in the power supply and as cathode bypass capacitors. Because amplifiers need a high DC voltage to function, filter capacitors are needed to smooth out the ripple present in the voltage after it has been rectified from AC to DC. In this application, larger electrolytic capacitors charge up as the unfiltered DC rises and discharge after it falls. By holding the charge long enough, the capacitor smoothes out the ripple of the falling voltage as indicated in the below illustration. In the role of cathode bypass, an electrolytic capacitor has the effect of passing the AC signal around the cathode. This keeps the cathode at a DC voltage potential and also provides a boost in gain of the amplified signal. It should be noted that the capacitor doesn't actually pass an AC voltage, but, again, through its charging and discharging properties it creates the effect of passing it.
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= =......., These film capacitors, from left to right, are Orange Drop SBE 716P, Mallory 150M, Sozo Mustard, and Xicon.
-
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To construct electrolytic capacitors, an electrolytic paste is placed between foil plates and wrapped into a cylinder. The assembly is then encased in an aluminum "can" (older types used a cardboard tube). One side of the plate is specially formed with a thin dielectric and is designated positive, the result being that this side must be kept positive in respect to the other side or else the dielectric will be destroyed. In other words, electrolytic capacitors are polarized, meaning they can only be used with DC voltage, with one lead being kept at a more positive voltage or potential than the other, negative lead. Labeling on the capacitor indicates either the positive (+) or negative (-) lead, sometimes both. In their cathode bypass function, electrolytic capacitors pass a degree of signal to ground, but the signal, even though it is AC, doesn't go below ground potential, and thus the polarity of the capacitor is maintained. Electrolytic capacitors are physically larger than other capacitors and have much higher values, typically
-.::::::> c:::>
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Electrolytic Capacitor Function Full-wave Rectified Voltage - DC with Ripple
Electrolytic Capacitor + Charges and Discharges
Transformer Produces ACVoltage
v
DCVoltage
---= ...._-- .. The dashed lines indicate the capacitor discharging voltage. Because the discharge rate is slower than the drop-off rate of the DC ripple, the discharged voltage fills in the "valleys" of the ripple .
Diode Diodes pass only the positive portion of the AC signal in this circuit arrangement.
The function of an electrolytic capacitor in a full-wave rectifying power supply.
31 ••-
-
A variety of electrolytic capacitors, blue Spragues and an Illinois on the right, a small Sprague in the middle foreground, and two "can" types on the left. The silver Sprague at the upper left has twist locks for the chassis mount, and the multisection capacitor with screw terminals uses a clamp-to-chassis mount. The Mallory on the middle left is a 1950s-era multi-section, cardboard, and wax-coated electrolytic capacitor.
To drain power capacitors after shutting off an amplifier, attach a jumper lead from the plate of a tube to ground. Make sure to remove the test lead when you are finished; if you turn the amplifier on with the test lead attached, you could damage the amplifier.
•• 32
from one to several thousand microfarads. In addition to observing proper polarity, the voltage rating must never be exceeded; doing either will destroy the capacitor. Another important point has to do with the electrolytic paste. Because it is semi-liquid, it eventually dries up, rendering the capacitor useless. Ultimately, as electrolytic capacitors age, they dry out whether they are used in circuits or kept unused. Because of this, it's a good idea to replace the electrolytic capacitors in any amp more than 30 years old, as detailed later in the book. If you turn an amplifier on without anything plugged into the inputs, turn up the volume control and hear a hum that fluctuates, you might have one or more bad filter capacitors. Yet, other things, such as the rectifier tube, could also be bad. A visual inspection will also help you determine if the capacitors are original or have been replaced. Precautions must be taken when dealing with electrolytic capacitors. Because of their large values, a dangerous amount of voltage can be stored and held. When working on an amplifier that has just been turned off, the capacitors should be discharged. Even though the capacitors will usually drain down on their own, it's a good idea to manually do it. A number of procedures can be used, but one of the easiest methods is to clip one end of a test lead to ground and the other end to either pin 1 or 6 of one of the preamp tubes (the small tubes beginning on the far right when looking into the back of an amplifier). It only takes a few seconds for the capacitors to drain, so you can remove the lead after about a minute or so. A word of warning: Make certain that you remove the test lead soon after using it so you don't forget about it. If you turn the amplifier on with the test lead attached, you could damage the amplifier.
---I
c:::> c:::> ~
~
= =,.......,
-
a:> ~
- -.
c:::>
=
~
~
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:3
This 90-ma, 4-henry choke used in most 40-watt and greater Fender amps, with Fender part number 022699, is manufactured by Triode Electronics in Chicago.
-.::::::> c:::>
= =
-
a:> ~
Pi Section Filter L1 IN
OUT
C1
C2
The pi (TT) section filter for power supply circuits looks like the Greek letter TT in its circuit configuration.
Transformers and Inductors If we pass DC current through a coil of wire, then decrease the flow of current or shut it off completely, the electromagnetic field created by the current through the coil will continue to flow a certain amount of time and resist the drop or fluctuation in current. In effect, this electromagnetic force induces voltage with a property called induction. If the rate of current flowing in our coil of wire changes by 1 ampere per second and induces 1 volt in our coil, then we have induced 1 henry (H) of induction. Since the henry is a rather large amount, inductance is often measured in millihenries (mH), which equals 1,000th of a henry. In some ways we might think of inductance as the opposite of capacitance, in that while capacitors block the flow of DC but pass AC, inductors pass DC with little or no resistance while opposing AC, especially at high frequencies. Yet, it is important to note, inductance does not actually block AC; it passes it with some
degree of opposition. This opposition is called "inductive reactance" and for our purposes can be thought of as being similar to resistance. In guitar amps this property of inductive reactance becomes especially useful in power supply circuits, specifically in filtering the choppy DC (ripple) that has been rectified from AC. Here we want the DC to flow but without the ripple of the AC remnant. Thus, a component that opposes the rise and fall of the AC ripple in the DC by tending to keep the flow of current moving in a forward direct movement proves useful. Such a component is called an inductor or, more commonly, a choke coil, a coil, or a choke. As the term coil implies, an inductor consists of a coil of wire tightly wrapped around a core, usually made of iron but it can also be a powdered-iron slug, a cardboard tube, or simply air. In guitar amps, coils have an iron core and the whole works is covered in a
33 ••-
-
metal case. The width of the coil in combination with the thickness of the wire, the number of turns of wire around the core, and the tightness of those turns determines the inductance of the coil. Most coils in Fenders have a 4-henry value with a current-capacity rating of 50 to 100 milliamps, and, in conjunction with two electrolytic capacitors, form what is called a pi-section filter, so-named because in its circuit design it resembles the Greek letter 1T (pronounced "pi"). An important characteristic of inductance in relation to a coil of wire is this: If we pass a coil of wire through a magnetic field, a current will be induced in the coil. This phenomenon relates to another important characteristic of inductance: If we pass electricity through any wire, a magnetic field will be produced. Moreover, if we wind that wire into a coil, the magnetic field becomes stronger. Now, if we were to take the coil of wire with electricity flowing through it and place it next to another coil of wire, one without electricity flowing through it, and then turn the electricity off and on and off in the first coil, current will be induced in the second coil. Depending on the number of windings we've made in the second coil, the induced voltage will vary. The reason we need to turn the electricity off and on in the first coil is that the second coil has to cut magnetic lines of flux from the first coil in order to produce electricity. Alternately, we could physically move the second coil continually through the magnetic field of the first coil, but that wouldn't be practical. Instead, we could pass AC voltage (which in effect is like turning DC on and off) through the first coil and have the second coil produce an AC voltage that is either higher or lower than the voltage we send through the first coil. This principle is called mutual inductance, and the device we have crudely created is called a transformer. A typical transformer has two separate coils of wire, called windings. The primary winding is where we apply the input voltage; this voltage, by the way, has to be AC in order for the transformer to function. In a power transformer, this voltage is the 120-volt AC wall current coming in through the power cord and switch. The secondary winding is the winding in which current is induced and that connects to the power supply circuit where its AC gets rectified into DC. Depending on the ratio of the turns of wire between these two windings, the output voltage is either higher (step-up transformer) or lower (stepdown transformer) in comparison to the input voltage. For example, a transformer with a 4:1 ratio would be a step-down type in which the secondary voltage would be one-quarter that of the primary. Conversely, a transformer with a 1:4 ratio would be a step-up type in which the secondary voltage would be four times higher than the primary voltage.
•• 34
Power transformers may be configured for vertical mounting or horizontal mounting. Most Fenders use horizontal mounting, as shown here.
The power transformer in a typical tube amplifier has a primary winding and two or three secondary windings and is essentially a combination step-up, step-down transformer. A high-voltage secondary winding delivers anywhere from 250 VAC to more than 400 VAC to supply the plates and screens of the tubes, while a low-voltage secondary winding delivers the 6.3 VAC needed to heat the tubes' filaments. Older, pre-1970s amps as well as Fender reissues that use tube rectifiers instead of solid-state diodes also have a lowvoltage secondary winding of 5 VAC used to heat the filament of the rectifier tube. Besides a power transformer, guitar tube amplifiers have an output transformer, or OT. Rather than supply voltage, the output transformer converts the high-voltage, low-current output signal of the power tubes to a low-voltage, high-current signal that drives the speaker. Because this high-voltage AC signal rides on an even higher DC voltage, the output transformer also has to isolate the speaker from this high DC. In addition to this step-down isolating function, the output transformer also converts the high impedance of the tubes' output (2,000 to 10,000 ohms) to the low impedance needed to match the speaker or speakers (2, 4, or 8 ohms). The input or tube side of the OT contains the primary winding, and its associated impedance is called the OT's primary impedance. The output or speaker side of the OT contains the secondary winding and its associated impedance is called the OT's secondary impedance. To define impedance (Z) which, like resistance, is measured in ohms, we first have to define reactance. The overall effect of capacitance and inductance in a
---I
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On the left is a replacement output transformer for a Deluxe Reverb, while the one on the right is for a Champ. The physically larger an output transformer is, the higher the wattage it will deliver.
=
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circUIt IS an oppositIOn to AC. Capacitive reactance is represented by XC and inductive reactance by XL. Now, the overall effect of reactance (X) and resistance (R) in a circuit is an opposition to AC. This opposition is called impedance (Z). For those of you who like to think mathematically, resistance and reactance in a series circuit is represented as
and resistance and reactance in a parallel circuit as
where X
= XC + XL.
As the above photo indicates, output transformers come in a range of sizes, depending on the wattage of the amp. The higher the wattage output of an amp, the larger the transformer needed to handle that wattage. Besides delivering high wattage capabilities, large output transformers also offer greater bass response and volume. In fact, replacing the stock output transformer on some of the smaller Fenders, such as the Princeton Reverb, with a larger OT having the same primary and secondary impedance will both boost the amps' output by up to 5 watts and improve the fidelity, particularly as it defines bass response. Of course, new mounting holes would need to be drilled in the chassis to accommodate the larger transformer. If you've ever taken a look at the OT in a Pro Reverb and compared it to that of a Super Reverb, you will notice a much larger, heavier unit in the former. While both amps use the same output tube arrangement-dual 6L6 tubes with fixed bias-the Pro Reverb claims nearly 10 more watts of output power, and the amp is definitely louder. Yet, the Pro Reverb doesn't necessarily put out an inferior tone or fidelity
in the comparison, one reason being the slightly lower operating voltage, another being the speaker configuration, both of which are well-suited to its smaller OT. The quality of an output transformer has a profound effect on the tone quality of an amplifier. Moreover, while a larger OT can provide more volume and bass response than a smaller one, a poorly constructed larger OT will be much more detrimental to the sound quality and not worth the boost in volume and wattage. Along with the speaker, the OT has perhaps the most pronounced effect on frequency response and range as well as the overall tonal quality of an amplifier. To that end, there are several highquality after-market output transformers offered as replacements for most Fender models. One excellent manufacturer is Mercury Magnetics, a U.S.-based company that offers replacement transformers for most brands of guitar amplifiers as well as custommade units for boutique and other independent amp builders. While I don't necessarily recommend replacing an OT in a blackface or silverface just for the sake of it, I would definitely recommend using only a highquality replacement for any damaged transformer or any inferior previous replacement. Many factors go into manufacturing a high-quality output transformer, such as the construction of the core, the uniformity of the winding, the accuracy of the impedance, and the insulation between the windings and the core. The basic construction of the output transformer is similar to that of the power transformer. The steel laminated core consists of stacked and interleaved thin E- and I-shaped pieces of steel. A bobbin surrounds the inner body of the EI-core, and copper wire coated with an insulating varnish is wrapped onto the bobbin to form the windings. Most bobbins today are made out of nylon or plastic, but prior to the late 1960s they were usually made of paper. Once wound on the bobbin, the windings are usually covered in paper, cloth, or
~
35 ••-
-
plastic and the transformer is then dipped in varnish for sealing. Two metal face plates called endbells are often installed over the bobbin and windings. Due to the usual myths surrounding anything vintage, some designers, technicians, and musicians consider paper bobbins to be superior to nylon or plastic types, so much so that certain manufacturers now use paper bobbins in their current production transformers. The original decision to use paper bobbins, however, was based more on availability and budget than on tone or quality, with paper bobbins being both inexpensive and widely available. Indeed, some of the highest quality transformers manufactured today use nylon or plastic bobbins. As with most debates involving the mythos of vintage amplifiers, evidence is subjective and consensus unlikely. Finally, a Fender amp with reverb also has a small transformer to drive the reverb tank. This is essentially an output transformer that matches the high impedance of the plates of the reverb driver tube with the low-impedance input of the reverb tank. This impedance matching transformer doesn't directly affect sound and definitely not to the extent of the output transformer. Physically, the reverb driver transformer is about the same size as a choke coil.
Diodes As previously mentioned, most Fender amplifiers made before the 1970s, as well as most Fender reissues, have tube rectifiers while those made after the early 1970s tend to have solid-state diodes for rectifiers. Tube rectifiers are actually two diodes in one envelope that together form a full-wave rectifier. Referring to the diagram in the capacitor section, full-wave rectifier means that each half of the AC signal gets sent through a diode, resulting in a fuller DC voltage.
A Fender reverb driver transformer.
•• 36
In general terms, a diode, whether tube or solid state, passes voltage only in one direction. With a tube diode this works as follows: A metallic element, called a cathode, emits electrons when heated. A metal enclosure, called the anode or plate, surrounds the cathode and is given a positive charge by being connected to the high-voltage windings of the transformer. In the rectifiers used in Fender amps, the cathode is directly heated, meaning that the heater and cathode are integrated. From the power transformer, 5 volts AC applied to the cathode causes electrons to flow from the cathode to the plate. When high-voltage AC swings positive, the plate is positive and electrons flow; in this condition the diode is said to be forward biased. When the AC voltage swings negative, the plate is negative and no electrons flow; now the diode is said to be reverse biased. Because the full-wave rectifier has two diodes, one is forward biased while the other is reverse biased, and vice versa. In effect, the rectifier conducts for the full cycle of AC, but only in one direction, providing the DC voltage that will be smoothed to full DC by capacitors and the choke network. With solid-state diodes the result is the same: AC is converted to DC, but the means are different. Most diodes used for rectifiers are made of silicon. In short, silicon, an element of pure crystal, is "doped" with an impurity to make P-type material (positive) and N-type material (negative). Doping a crystal with both N-type and P-type materials creates a junction between the negative and positive portions of the crystal. If positive voltage is applied to the P-side of the junction, electrons will flow through the silicon, forward biasing it. However, if the positive voltage is applied to the N -side of the junction, no current flows through the crystal, reverse biasing it. Putting this together, we can see that when an AC voltage is applied to the N-side, or anode, the voltage flows through the silicon diode when it swings positive, exiting out the P-side, or cathode. Alternately, when the voltage swings negative, it doesn't flow through the diode. As with the tube diode, voltage flows only in one direction.
The lN4007 silicon diode (on top) has a l-amp, 1,OOO-PIV rating, while the lN5408 has a 3-amp, 1,OOO-PIV rating.
Typical Fender Power Supply Circuits To output tu bes' screens and reverb driver transformer
To output tubes' plates
To phase inverter tube's plates
To preamp tubes' plates
Current Flow )
T1
L1
fYY"'\ ----
R3
R4
51
5VAC
R1 330+VAC
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N
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+
c co v
R2
C2T
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330+VAC
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>
V1 6.3 VAC for tube filaments (heaters)
To output tubes' screens and reverb driver transformer
To output tubes ' plates
To phase inverter tube's plates
To preamp tubes' plates
Current Flow )
T1
01 02 03 0.. __
L1
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----
c ell
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......
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R3
R4
51 R1
0405 06 __ ..0
0
N
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R2
6.3 VAC for tube filaments (heaters)
T1 = Power transformer V1 = Full-wave rectifier tube 01 through 06 = Silicon diodes in full-wave rectifier application; more diodes in series provide higher current rating . C1 through C6 = Electrolytic capacitors for filtering DC voltage. L 1 = Choke coil for filtering DC voltage. R1 and R2 = Bleeder resistors to discharge residual voltage for C1 and C2 when power is turned off; also provides a degrees of voltage regulation. R3 and R4 = Voltage dropping resistors; in conjunction with C3 - C6 and L 1, comprise the "power rail" from which varying voltages are drawn. 51 = Standby switch
Schematic diagrams oftypical Fender power supply circuits, vacuum tube (above) and silicon diode (below). Component descriptions are from both circuits.
-
it swings positive, exiting out the P-side, or cathode. Alternately, when the voltage swings negative, it doesn't flow through the diode. As with the tube diode, voltage flows only in one direction. When replacing silicon diodes, polarity must be observed for them to work properly. Most diodes, like those used in Fenders, which are almost always IN4007 types, are black with a white band indicating the cathode end. This side of the diode must be placed opposite the transformer winding (diode input) and toward the capacitor and coil network (diode output). Diode specifications must also be considered when replacing diodes. Along with current rating (the amount of current a diode can pass), diodes are specified by their peak reverse voltage (PRV), also called peak inverse voltage (PIV). This is the maximum voltage to which a diode can be reversed biased. Exceeding the PIV will destroy the diode. The usual diode used in Fender amps, the IN4007, has a I-amp current rating and a 1,000 PlY. Finally, here's some terminology regarding an amplifier's operating voltage. You'll often hear the term B+ to describe the high DC voltage applied to the plates of tubes. This term, synonymous with operating voltage, is an old term carried over from the early days of electronic technology when batteries were used to supply voltage
Tube Rectifier or Silicon Diode Circuit? A common question is this: Which is better, a tube rectifier or a silicon diode circuit? The answer varies. On one hand, silicon diodes are inexpensive, about 25 cents, compared to around 20 dollars for a tube rectifier. On the other hand, many people believe a tube rectifier gives a warmer sound to the amp. The truth is, a tube rectifier doesn't directly affect the sound, but it does have an indirect effect in that when driven hard a rectifier tube will "sag," meaning that the tube is slower to recover its voltage than a solid-state diode, which is a fast-recovery device. The sag, in turn, creates a compressed sound, which can sound warm and musical. The 5Y3 rectifier used in the tweed Deluxe, Champ, and Princeton sags easily at high volumes, while the 5AR4 used in most blackface amps takes more pushing to sag. At lowerto moderate volumes, a guitarist might not even notice an audible tone difference between a rectifier tube and a silicon diode rectifier circuit. Yet even at moderate volumes subtle differences can be heard or felt in response to various pick attacks.
•• 38
to vacuum tubes. The A-Battery, as it was called, provided low voltage to the vacuum tubes' filaments. The C-Battery, likewise, provided low voltage, below 10 volts, to be used to bias the control grids of the tubes. The B-battery, where the term B+ originates, ranged from 22.5 to 135 volts, and B-batteries were often connected in series to provide the high voltage necessary to power the plates and screens of the tubes. The + indicates positive DC voltage.
Vacuum Tubes A vacuum tube is a glass envelope of electronic elements that, in guitar amplifiers, modifies or amplifies the small electronic signal of the guitar. Like a light bulb, it contains a filament, which in guitar amps is typically run at 6.3 volts AC and produces the orange glow inside a tube. The filament heats the cathode, which in turn releases electrons that are drawn to the anode or plate because it has been made much more positive than the cathode. Generally, the plate is fed with hundreds of volts DC while the cathode is kept at 0 to a few volts DC. The tube is also equipped with a control grid on which the guitar signal is carried. Acting like a valve (which tubes are known as in the United Kingdom), the signal on the control grid influences the flow of electrons from cathode to plate. A positive or negative swinging signal on the grid increases or decreases the flow of electrons, which respond to the small grid swings with much larger corresponding swings (i.e., amplification) due to the much higher potential between cathode and plate in relation to the potential between cathode and grid (which is in the range of a few millivolts). Tubes with these three elements-control grid, cathode, and anode or plate-are called triodes. In guitar amps triodes are used for preamp and phase inverters (the small tubes). Power tubes, on the other hand, are either tetrodes or pentodes. As the name suggests, a tetrode has four elements and includes a screen grid in addition to the control grid, cathode, and plate, while the pentode has five with the addition of a suppressor grid. These extra grids are used to focus the flow of electrons since many of them bounce off the plate. By focusing the electron flow, the efficiency of the tube increases and thus allows for higher gain and amplification. The power tubes used in most Fenders, the 6V6 and the 6L6 or 5881, are designated either as a pentode or beam tetrode, depending on the manufacturer. The reason for this imprecise terminology involves patent distinctions dating back to before World War II. Specifically, the tube design designated "pentode,"
Care and Handling of Vacuum Tubes So that the internal elements of your vacuum tubes don't loosen and vib rate when you play through the amp, use common sense in handling them : Don't hit your tubes with tools; exercise care when loading your amp; leave the back cover on your amp for protection; and don't over-handle tubes by randomly removing them.
which included a suppressor grid, was originally held in patent by Philips. Therefore, the 6L6 tube, with its fifth element-a pair of beam plates-became designated as a beam tetrode. While the suppressor grid and the beam plate differ in construction, they perform
essentially the same function. In this way, the terms beam tetrode and pentode can be considered somewhat synonymous. Typically the beam plates or suppressor grids are kept negative by being internally connected to the cathodes. However, the EL34 pentode used in Marshall amplifiers has a separate pin connection for the suppressor grid and is tied to the cathode at the tube socket. The screen grid, in contrast to the beam plates and suppressor grid, is made positive, but slightly less positive than the plate. This is accomplished by using an external connection that is separated from the plate voltage supply by a choke and resistors. The slight decrease in positive voltage ensures that the electrons pass through the screen grid to the plate rather than become absorbed by the screen, which would quickly fry the tube. Vacuum tubes will be discussed more fully in the following chapters.
cot
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= c:a...
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= c=;0 ~ C)
3
-.::::::>
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= CI:>
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Vacuum Tube Noise and Microphonics There are two common maladies of vacuum tubes , stemming in part from the beating they take in a guitar amplifier: noise and microphonics. First, the two are notthe same. Noise from a tube includes rattles, crackles, pops, or hums that can range from slightly audible to quite loud. Noise usually results from age or physical abuse. All tubes age and stop working eventually. Due to the vibrations and rough handling often associated with guitar amps, their lives can be reduced. One typical problem with tubes is that the internal elements can loosen and physically vibrate when you play through the amp. Common sense rules here: Don't bang on your tubes with tools, exercise care when loading your amp (also leave the back cover on your amp). and don't over-handle tubes by randomly removing them. Pulling them out won't damage them, but each time you remove them they become susceptible to being dropped, knocked, and banged. The condition of microphonics is somewhat different. As the name implies, a microphonic tube means a tube that acts sort of like a microphone in that it picks up sounds and amplifies them. Essentially, all tubes, especially preamp tubes, are
microphonic. With your amp running, if you lightly tap a preamp tube with a chopstick or pencil eraser, you'll hearthe sound come through your speaker, amplified and possibly with a slight ring to it. Under normal playing conditions, you shouldn't hear a microphonic tube. If you do, it will probably be heard as a ringing or howling effect and possibly as feedback. This happens when the internal elements in the tube vibrate. This can happen due to age or damage. Occasionally, a new tube will be microphonic either due to rough shipping or a factory defect. Generally these are weeded out by reputable tube venders, and if you ever receive a damaged tube most venders will replace it. In the case of age or damage, it might not seem obvious which tube has become microphonic. To locate it, carefully tap on each tube with a chopstick or pencil while the amp is running on low volume. If upon tapping a tube, you hear ringing that intensifies, sustains, and possibly goes into feedback, then you've found the microphonic tube. This same process can also be used to find a noisy tube. Bear in mind that this test needs to be performed with care. Tapping on a tube can cause damage if done too hard.
39 ••-
SPfAKfRS, SWITCHfS, ANO SCHfMATICS Fender has used various brands of speakers throughout its histor y, including j ensen, j BL, Utah , Oxford, CTS, and Eminence. W hile these speakers have tonal and performance distinctions of their own, all have similar construction. Speakers identified as vintage-era, meani ng roughly those manufac t ured before the 1970s, as well as those currently m anufactured by j ensen , to name just one company, are usually low-wattage (less than 50 watts) and use alnico magnets. H igher wattage spea kers usually use cera mic magnets; however, this is not a hard, fas t rule as some current produc tion , low-wattage vintage series speakers use ceram ic speakers. Alnico, which st ands for aluminum-nickel-coba lt, is an alloy held together primarily with iron that makes a powerful permanent magnet. As a result, alnico magnets a re physically smaller tha n cera mic magnets. Aside fro m th is physi cal difference, alnico and ceramic speakers have the same basic constr uction. W hen a signal is applied to a speaker, it flows th rough the voice coil, which con sists of a coil of wire suspended inside the magnet. T he coil moves within the magnetic field according to the strength of the signal, and as it does it also pulls the speaker cone, to which it is connected back and forth in relation to the
Opposite: Fender's Vintage Modified Bandmaster head and speaker cabinet comes stocked with Celestion G12P -80 speakers designed to deliver a fat, woody bass response and clear highs . Fender Musical Instruments Corporation
The Speaker's Effect on an Amp's Sound Changing an amplifier's speaker or speakers has perhaps the most significant effect on the amplifier's sound. Yet the same speaker will sound different in various amp models.
Speaker Impedance Perhaps the most important specification of a speaker is its impedance. Most speakers used in Fenders have 8-ohm impedance; even the multispeaker amps, such as the Bassman, Super Reverb, and Twin, usually have 8-ohm speakers. Often the speakers are wired in parallel (positive + to positive + and negative - to negative -). producing an impedance of 4 ohms in the dual-speaker amps and 2 ohms in the quad-speaker amps . Sometimes, though, a quad-speaker amp will have an 8-ohm impedance that is matched by speakers wired in series-parallel, two pairs in series with the pairs in parallel. It is important to match the output impedance of an amplifier with the impedance of the speaker. Gene rally, besides possible degradation of tone, a mismatch in impedance causes stress on the output tubes and transformer. Good practice requires any replacement speaker to be of the same impedance as the one replaced. Furthermore, keep in mind that some amps use 12-inch diameter speakers while others use lO-inch diameter and others still, such as older Champs, use 8-inch . The truth is, a tube rectifier doesn't directly affect the sound, but it does have an indirect effect in that when driven hard a rectifier tube will "sag," meaning that the tube is slower to recover its voltage than a solid-state diode, which is a fast-recovery device . The sag, in turn , creates a compressed sound, which can sound warm and musical. The 5Y3 rectifier used in the tweed Deluxe, Champ, and Princeton sags easily at high volumes, while the 5AR4 used in most blackface amps takes more pushing to sag. At lower to moderate volumes a guitarist might not even notice an audible tone difference between a rectifier tube and a silicon diode rectifier circuit. Yet even at moderate volumes subtle differences can be heard or felt in response to various pick attacks .
41-.-
Speaker Hookups and Impedance Fenders with multiple speakers, such as the Twin and the Super Reverb, have their speakers connected in a specific order to match impedance between speakers and output transformers . When two speakers are connected in parallel, for example, their combined impedance is halfthat of each speaker (two 16-ohm speakers in parallel yields 8-ohm impedance). By contrast, two speakers connected in series results in a total impedance equivalentto the sum of the speakers' individual impedance (two 4-ohm speakers in series yields 8-ohm impedance). The following diagrams indicate common speaker connections in parallel and series to match an amplifier's output impedance.
+ Two 8-ohm speakers in parallel yield 4-ohm output. Ensure that the positive (+) speaker lugs connect to the center pin of the jack.
Four 8-ohm speakers in parallel yield 2-ohm output. Ensure that the positive (+) speaker lugs connect to the center pin of the jack.
louder Speakers
•
To find a louder speaker for your amp, you need to match the proper wattage and use a speaker with a higher sensitivity.
Breaking In New Speakers When installing a new speaker, the speaker should be bro ken in. Even though a new speaker won 't be f ully broken in until after hours of play, a brief initial break in helps primarily because a speaker is tight when new. Plus, a brand new speaker is susceptible to damage if hit with a full volume blast. To prevent damage, firstturn on the amp with a guitar connected and the volume at no more than one-quarter. Let it run a few minutes without playing the guitar, then strum full chords for about 10 minutes or so. Gradually turn up the volume and play the guitar a little harder for about 5 minutes. After that, playas you normally do. Eventually the speaker will loosen, perhaps after a few days or a few weeks, depending on how frequently you play.
strength of the signal. This movement changes the air pressure around the cone, which in turn creates the sound waves our ears hear. Speakers have various specifications and ratings, one being the power rating or wattage. This rating enables us to match the proper speaker to our amp. One thing to be aware of: A speaker with a higher power rating will not sound louder than one with a lower power rating. In fact, if we replace the 25-watt speaker in a Fender Princeton with a 120-watt speaker, the Princeton will actually sound quieter. Basically, a high -wattage speaker has a much st iffer cone to handle the output of a high-wattage amplifier. The Princeton will have a tough time pushing that stiff a cone enough to produce a loud sound. To find a speaker t hat will produce a louder sound, we need to match the proper wattage and use a speaker with a higher sensitivity. Sensitivity is measured in db, with a 100-db speaker generally being louder t han a 97-db speaker; however, it's important to keep in mind that a louder speaker may not sound better. For sound quality we need to also consider a speaker's frequency range and frequency response . While frequency range is generally an accurate measurement, frequency response specifications are not.
Essentially, frequency response becomes influenced by the size and material construction of the speaker cabinet as well as the dimensions of the room in which the speaker is used. The stated frequency response, therefore, isn't the best specification to go by and should be considered primarily as reference. Moreover, it needs to be taken in conjunction with two other specifications: speaker efficiency, which represents a speaker's accuracy in converting an electric signal into sound, and transient respon se, which specifies the quickness of a speaker's reaction to a signal without distorting it. It seems that interpreting a speaker's specifications is almost a science in itself. Yet, if you match the power rating of a good qua lity speaker, such as a Jensen, Eminence, or Weber to your amp 's power rating, you'll be in good shape. To become a connoisseur of speakers requires trying out a variety of speakers in a variety of amps and allowing your ears to be the judge.
Switches Fender amps use two varieties of switches: toggle switches for power, standby, and ground, and slide switches for bright control. These switches are usually of the single-pole, single-throw (SPST) type. Other common types of switches include double -pole, single-throw (DPST) and double-pole, double-throw (DPDT). A SPST-type switch operates in a typical on-off fashion . Specifically, when the toggle or slide is moved from the off position to the on position, a small slider pushes one set of contacts together. The single set of contacts make contact in only one position. In a DPST switch, two separate sets of contacts make contact in only one position. Physically, the SPST switch has two outer terminals on which to attach wires, whereas the DPST switch has four outer terminals for four wires and is essentially two individual SPST switches with actuators that are linked or "ganged" together so t hat one toggle or slide moves both switch poles. The DPDT switch functions like a DPST except that it has six terminals so that no matter which position the switch is in, a set of contacts is actuated. An SPDT (single -pole, double-throw), which has three terminals, by the same token, functions like half of DPDT in that a DPDT essentially consists of two ganged SPDT switches. The DPDT toggle switch comes in handy for such modifications as adding a gain stage. Roughly put, t he output of a preamp stage can be connected to the middle termin al of the switch so that in one position it feeds the output while in the other position it feeds the gain stage, which in turn feeds the output through the other independent set of contacts. I wil l elaborate and further clarify this switching operation in Chapter 10.
43 ••-
-
Schematics and layouts Schematics for the major models of Fender amps are widely available on the web (consult the appendix for useful websites). These are a must for amp modifications, and so it's a good idea to familiarize yourself with these documents. Fender schematics are readerfriendly and can easily be followed from the input to the output, especially in conjunction with the Fender layout, which is basically a drawing of the inside of the chassis, showing the components on the circuit board as well as the major internal wiring. See the facing page for examples of both a schematic and a layout. To describe the amp's operation as represented by the schematic, let's look at it in four pieces: the power supply (which we discussed in Chapter 2), the preamp, the phase inverter, and the output stage. The reverb circuit will be discussed in Chapter 7 and the vibrato circuit in Chapter 4. While there are two preamplifier circuits in this dual channel Fender, we will focus on only the normal channel as it is the same as the vibrato channel preamp minus the vibrato oscillator. With a guitar plugged into input 1, the first thing the signal encounters is the input jack. The sleeve of the jack, being grounded, connects the grounded side of the guitar's pickups to the amp's ground via the shield of the guitar cable. This sets the reference for the small signal coming from the pickup, passing through the center of the guitar cord, and entering the tip of the input jack. Obviously a key component in this chain is the guitar cord, which can provide a great deal of unwanted noise. Constant flexing and yanking on it can result in damage to its shield, center conductor, and the insulation between the shield and conductor. When troubleshooting any unwanted noise, be sure to check the cord as well as the guitar before turning to the amp. Often noises that originate in a guitar or cord get blamed on the amp. It should go without saying: Use high-quality cords and pickups. Referring to the schematic on page 48, the 1 M-ohm resistor located on the input jack is the grid-load resistor for the first triode and it provides impedance matching between the guitar and the triode. In addition, the gird-load resistor, which, even though it's attached to the jack, connects between the triode's grid and ground, setting the reference for the triode's cathode in its relationship with the grid. The two 68-K-ohm resistors that are also attached to the input jacks help to snub radio frequency (rf) interference (as rf constantly fills the air around us) and parasitic oscillation, which is extremely high frequencies that can be picked up by a tube's grid, causing the tube to oscillate. As a result, these resistors are often called grid-stopper or swamp resistors. Also, these resistors work in conjunction with the 1 M-ohm resistor to act as an impedance switching network making input 1 the high-level input for an electric guitar and input 2 the low-level input for
•• 44
acoustic pickups used on guitars, banjos, violins, and other stringed instruments as well as for such instruments as accordions and organs. When plugged into input 1, the 1 M-ohm grid-load resistor shunts little of the signal to ground. When plugged into input 2, on the other hand, the 1 M-ohm resistor is effectively replaced by a 68-K-ohm resistor, which shunts more of the signal to ground, thus making the overall signal lower in level. The two triodes share the same tube envelope. The Fender standard 7025 tube (which was later replaced by a nearly equivalent tube, the now standard 12AX7) is therefore known as a dual triode. The 100-K-ohm plate resistors determine the gain of each triode while the 1.5-K-ohm and 820-ohm (the reason for the differing values will be explained soon) cathode resistors set the operating bias for each tube (as explained later, "bias" signifies the idle current of the tube, the operating condition needed for a tube to amplify a signal). The 25-uF cathode bypass capacitors keep the AC signal out of the DC bias, which, in turn, provides a gain boost for each triode and, as indicated by the +, are electrolytic. Once the first triode amplifies the signal, it passes from the plate to the tone controls (also called the tone stack) where the signal is modified via filtering by the 250 pF, .047 uF, and .1 uF capacitors, the 100-K-ohm slope resistor, the 6.8-K-ohm midrange resistor, and the two potentiometers. After being shaped by these filters, the signal passes to the volume control, which adjusts its level. Because the signal passes through these capacitors, resistors, and potentiometers, it loses much of its strength through an effect known as insertion loss. The second triode functions as a second preamp stage to amplify the signal as it exits the tone stack. Besides setting the input level of the signal for the second triode, the volume control also acts as a grid-load resistor for the triode. The cathode resistor for the second triode is 820 ohms rather than 1.5 kilohms because it, along with the bypass capacitor, is shared with the cathode of the second triode of the vibrato channel preamp. When two cathodes share a resistor, the value of the resistor has to be approximately halved to give the same bias as that of a solitary cathode. Coupling these cathodes amounts to a component-saving-and therefore price-saving-method and doesn't really affect the performance of the triodes; however some people believe the tubes sound better with cathodes separated and given their own 1.5-K-ohm resistor and 25-uF capacitor. Yet, if you want to run a 12AT7 tube in one channel and a 12AX7 in the other while retaining the shared cathode resistor/capacitor arrangement, the tubes won't function properly because the 12AT7 draws more current than the 12AX7. However, using a 12AY7 with a 12AX7 works fine with coupled
Fender Model Deluxe-Amp AB 763 Schematic Notice
o
7025
~
820
a:
6V6GT
12AT7
a:l
>
~
-
~ +410V 1 50~ { : : ; : : . 4
.1
1Rnv
EXT. SPKR.
.001
+230V
~III
+-......'..'K'-+-.....4'..0_-+ +90V
, 1M
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\
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r
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~~
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~ +340V
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10K
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TR2
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450
~
450
o
450
~
Z
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TRI - 125C3A
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EXPORT MODEL - 125P2 3B
-r •
SWITCH
16· 450
--
TO All 6.3 VOLT HEATERS AND PILOT UGHT
Voltages read to ground with electronic voltmeter. Values shown + or - 20% 2. All resistors 1/2 watt, 10% tolerance. if not specified. 3. All capacitors at least 400 volt rating. if not specified . 1.
A schematic is a visual representation of an amplifier's operation while the layout shows the location of components and wiring. Shown here are the schematic (top) and layout (bottom) for a 1963 Fender Deluxe.
Fender Model Deluxe-Amp AB 763 Layout INTENSITY
speED
BASS
TREBLE
VOLUME
VIBRATO
BASS
TREBLE
VOLUME
NORMAL
~;;:t.::;:-tc ALL 6.3VOLT HEATERS
12AT7
7025
7025
NOTE - All resistors 1/2 watt, 1-% tolerance if not specified. NOTE - All capacitors at least 400 volt rating if not specified.
45 ••-
-
Schematic Symbols
-•
Ground
Ground
v 1'1.-__: •
V Input Jacks
Switch
Rotary Switch
~-I
Relay
Inductor (Coil or Choke)
OutputTransformer
4
Resistor
Capacitor
Electrolytic Capacitor
Power Transformer
Fuse
•
Potentiometer
Potentiometer with only two terminals connected
Speaker
-------
DualTriode
Tube
TetrodeTube
Pentode Tube
Shielded Cable
~I Full-wave Rectifier Tube
•• 46
Full-wave Rectifier Tube
DiodeTube
Diode (Solid-State)
Zener Diode
cathodes. In Chapter 4 we'll look at various tubes that can be used as preamps, while Chapter 9 will cover separating cathodes. Finally, the .047 uF coupling capacitor blocks DC voltage from the next stage while also shaping the signal after it's been amplified by the second triode. The 220-K-ohm resistor after the coupling capacitor acts as a mixer by being matched with another 220-K-ohm resistor at the end of the vibrato channel preamp. Effectively, the signal passes through the resistor and onto the phase inverter, while the second 220-K-ohm resistor, the one at the end of the other channel, prevents the signal from backing into that channel. As we will learn in Chapter 5, Fender has used a variety of phase inverters, but the one most commonly used is a common-cathode type called a long-tailed pair due to the "tail" created by the resistor connected to the grid and cathode bias resistors. Phase inverters are employed with "push-pull" output stages, which are used in all but the smallest Fender amps (i.e., Champ and tweed Princeton). While push-pull output will be discussed more thoroughly in the schematic description of the output stage, essentially it consists of a pair or quad of output tubes that operate outof-phase with each other. A phase inverter is used to provide two 180-degree-out-of-phase signals for the pair or quad. The long-tailed pair has two inputs, the first from the preamp and through the .001 coupling capacitor, the second at near ground potential through the .1 uF coupling capacitor. The negative feedback loop also feeds this input. The negative feedback loop (NFB) consists of a signal taken from the speaker and fed back into the circuit, here through the phase inverter. Being out of phase, NFB subtracts from the audio signal just enough to cancel high-frequency noises carried along with the audio signal. The amount of NFB is determined by the resistive divider formed by the 820ohm and 47-ohm resistors, as shown in the schematic. Much of the NFB gets shunted to ground through the 47 ohm resistor since too much NFB will adversely affect the signal. As you can see, the phase inverter uses both triodes of a dual-triode tube, usually a 12AT7 or 12AX7. The cathodes are tied together and operating bias is set by the 470-ohm resistor. The tail resistor, on the other hand, holds the cathode well above ground, allowing around +90 VDC on the cathode. The 1-M-ohm resistors tie the grids to the above-ground potential and help keep the triodes balanced. Simplifying a somewhat complex operation, when the audio signal swings positive at the upper triode grid, the cathode drives the other triode to produce an output signal 180-degrees out of phase with the output of the upper triode. Recall that electrons are drawn from the cathode to the plate, providing current flow through the tube. With current
flowing in the upper triode and its cathode held well above ground, a signal swinging positive at the grid will produce a rise in voltage at the plate and a subsequent drop in voltage at the cathode. These two signals, while not equal in voltage, are 180-degrees out of phase. The lower voltage signal at the cathode, in turn, drives the bottom triode to amplify the signal, therefore producing two amplified, out-of-phase signals at the plates of both tubes. You will note that the plate resistors (100-K-ohm and 82-K-ohm) are not of the same value. This is to compensate for the slightly unequal gain of the triodes since one is directly fed with the signal. Finally, the .1-uF coupling capacitors keep the high-voltage DC from the output tube grids and also shape the tonal quality of the signal. Most guitar amplifiers use push-pull output, meaning that, at its basic level, a pair of output tubes operate in opposite phase (180 degrees) of one another. This is accomplished by using an output transformer with a center tap in the primary winding, as shown in the schematic. B+ voltage is applied to the center tap with each end of the transformer connected to the plate of each of the output tubes. Higher power amps, typically 100 watts, such as the Fender Twin, use four output tubes (called a "quad") rather than two. In the quad arrangement, two tubes are connected in the same fashion as the pair of tubes shown in the schematic. Another tube is connected in parallel to each of these two (making the quad). A parallel connection means that the plates and cathodes of the tubes are connected together. In most Fenders, the grid and screen have their own resistors (1.5-K-ohm and 470-ohm, respectively, as shown in the schematic), which, in turn are connected at the ends opposite the tube. With this setup the signal drives both tubes, and the output power of both tubes is combined at the transformer. In a push-pull output, each tube (or set of parallel tubes) conducts on alternate cycles of the input signal. In most cases, there is an overlap, or crossover, when the two tubes conduct together near the O-volt line of the AC signal (on the schematic, the center line over which the AC signal, or sine wave, is drawn). The advantage of the push-pull output includes more efficient power delivery and quiet operation. The primary "disadvantage," although subjective, is the tendency to cancel even order harmonics, which contrasts singleended output amplifiers, such as the Champ. These amps emphasize even order harmonics, which many guitarists consider advantageous to full tonal quality. Harmonics are tones whose frequencies are multiples (either even, 2, 4, etc., or odd, 3, 5, etc.) of a fundamental tone. Even order harmonics tend to be rich, making the tone thicker, while odd levels often sound harsh. While even order harmonics sound musical, singleended output amplifiers tend to be lower wattage and require more energy and are less efficient than push-pull
47 ••-
-
Fender Deluxe Preamp Schematic and Layout
+230VDC +230VDC 100K
1f2 of 7025 (12Ax7) 68K
100K
250pf 1
Treble 250K-A
lOOK 2
Volume 1M-A
Input 2
1f2 of
7
68K 3
Input 1 1M
.047uf
6
25uf
1.5K 6.8K 25uf
+
820n
2
6.8K
--
To 220K - Mixer Resistor
+230 VAC
To Pin 8 of Vibrato Preamp Tube
F= 6.3VAC Filament Voltage
•• 48
To Phase Inverter 220K
7025 ( 12Ax7)
Bass 250K-A
.047uf
220K
To Cathode of Vibrato Channel Triode 2
1
From Vibrato Channel Preamp
Fender Deluxe Phase Inverter Schematic and Layout
Negative Feedback from Speaker
820n 1
.001 uf
v
2
From Preamp
C=~r---,-1""1---)~ ToOutputTube 3
22K
.1uf
v C\
82K
470n +340VDC
100K .1uf 7
~
c:=~~---l----i t-I---)~ To OutputTube
47
12AT7
- - To Normal Channel Preamp - - To Vibrato Channel Preamp
F= 6.3VAC
Filament Voltage
F
49 ••-
-
Fender Deluxe Output Stage Schematic and Layout
6V6 3
V
/\
1.5K
From Phase Inverter
4
5 ~
NFB To Phase Inverter
470 1W
8
0 N N
-35VDC
To SPKR ~
0 N N
From Phase Inverter
/'\.
V
10K-L Potentiometer ~+
+410 VDC
4 From Power Transformer
25uf 50v
+420 VDC
4701W
AC
Diode
10K
Output Transformer
1.5K
6V6
10K-L Potentiometer 470 n
From Power -~ Transformer
, ,
IN4007
I
+ 25uf
\
I I I
I I I I I I I
I
I I
+410 VDC
To Output Transformer
F
5
F= 6.3VAC Filament Voltage
•• 50
F
- - To Output Transformer 6V6
F
6V6
amplifiers. While push-pull output tubes conduct only half the time, a single-ended output, which typically uses only one tube, conducts all the time, throughout the full signal. By using a parallel arrangement, more than one tube can be used in a single-ended output; however, the output power and volume are still low in comparison to the amount of energy dissipated by the tubes. Obviously what we have here is a trade off. Getting back to the schematic, the 220-K-ohm grid resistors evenly distribute the negative bias voltage to the output tubes. For tubes to properly amplify, they need to have their current set to an operating point. On fixedbias amps, such as most Fenders, this is usually done by applying a negative voltage to the grid to make it more negative than the cathode. Too much negative voltage and the tube will cut off, yet with too little negative voltage, the tube will saturate. Either way the tube will not conduct. To set the bias, adjust the bias potentiometer while measuring the negative voltage at the 220-K-ohm junction. Bias is basically set when you adjust the potentiometer to read the negative voltage as stated on the schematic. However, this is a general setting. There are better methods of achieving a more efficient bias setting. Bias will be discussed in detail in Chapter 4. In Fenders, the negative bias voltage is usually obtained by using a single-wave rectifier circuit (the IN4007 diode and associated capacitor) . Note, referring to the schematic, that the diode and capacitor are installed " backwards" to rectify the negative side or cycle of the AC voltage rather than the positive cycle. The resistors are essentially voltage dropping resistors used to get the voltage in range, with a potentiometer as a fine adjustment for the voltage drop. With the bias current set to the proper operating point, the out-of-phase signal passes through the 1.5-K-ohm resistors on the grids of each tube . Like the 68-K-ohm resistors on the grid of the preamp tube, these are swamp or grid-stopper resistors, used to prevent parasitic oscillations. The 470-ohm resistors on the output tubes are the screen-grid resistors. These are normally 1 watt or greater and set the voltage at the screens of the tubes . Screen-grid resistors (also simply called screen resistors) do at least two things: One, they keep a tube's screen grid less positive than the plate so the electron flow focuses on the plate. In amplifiers that use a voltage-dropping resistor rather than a choke in the B+ power rail (the row of choke, capacitors, and resistors in the power supply from where voltage is
Push-Pull Output Most common guitar amps use push-pull output, with a pair of output tubes operating in opposite phase (180 degrees) of one another. Higher-powered amps of about 100 watts or more, typically use four output tubes .
drawn-see the discussion of rectifiers in the previous chapter) and where the plate voltage is drawn from one side of the voltage-dropping resistor and the screen voltage from the other side, the screen grids will already be adequately less positive than the plates. However, most Fenders use a choke in the power rail and while at idle a 2- to 10-volt difference exists between plates and screens; when the amp is driven that voltage difference is negligible. Because the screens and plates can be at the same positive voltage potential, screen-grid resistors become necessary, which brings us to point two: Screen-grid resistors also protect the screens from exceeding their power dissipation ratings. Basically, when the input signal on a power tube's control grid swings hard, the plate can temporarily become more negative than the screen grid. A screen resistor will limit the amount of current going to the screen grid and, by extension, limits the amount of power dissipation . Fender amps typically come equipped with screen resistors valued at 470 ohms with a I-watt or greater power rating. While the 470-ohm value isn't necessarily arbitrary, the value isn't critical. Yet, if the value is too high, the resistor will starve the screen grid, resulting in a poor sounding amp. On the other hand, a value that is too low should be avoided for the above reasons. Fender manufactured a few tweed-era amplifiers that used a choke in the power supply but no screen resistors on the power tubes. If you have such an amp, it would be a good idea to install screen resistors (see screen resistor replacement discussed in the following chapters for instruction). Installing screen resistors will not decrease the value of the vintage amp since the installation is nondestructive, meaning it can be reversed. This is especially important if you are running an expensive set of vintage output tubes.
51 ••-
chapter
4
BIAS While biasing an amp isn't technically considered a modification, it is an easy and free method to improve the sound of any tube amplifier. An amp that isn't properly biased will not sound at its peak. Furthermore, by experimenting with bias settings, you can find the tones that you prefer and not have to rely on whatever bias point the factory set or the last tech who repaired you amp believed to the optimal (or, in some cases, good enough). The term "bias" refers to setting a tube's operating condition, or current, while it is idle or determining its no-signal (quiescent) condition. Essentially for the tube to operate properly, the control grid must be at a negative potential (DC) in relation to the cathode. This relationship allows the guitar signal, which is an alternating current (AC), to rise positively and fall negatively (swing) along a "zero" line (determined by a negative bias voltage). If the control grid is not at a more negative voltage (potential) than the cathode, when the guitar signal swings it will be cut off as it reaches the cathode voltage. Basically, if the voltage on a tube's grid is set too low (more negative), the tube will cut off and not conduct (pass or amplify) the signal. On the other hand, if it is set too high (less negative), the tube will saturate and most likely burn out. While bias voltage is necessary for all tubes to operate, when speaking of bias, it is usually meant in terms of
the output tubes only. Furthermore, the term "bias" is not equivalent to "bias voltage"; "bias" is an operating condition that involves a tube's ability to conduct current, while "bias voltage" refers to the both the actual voltage applied to a tube's grid and the difference in potential between a tube's grid and cathode, measured as voltage. One of the most common questions asked regarding tube amplifiers is, "Do I need to bias my amp whenever I replace the output tubes?" The common answers are yes, no, or maybe. Believe it or not, all these answers are relatively correct. First off, with cathodebiased amps, discussed below, there is no adjustable bias, and, therefore, the answer is no. For fixed-bias amps, the answer is technically yes, but in practice maybe. Why the ambiguity? A bias voltage supply of say -60 volts maximum usually would have an adjustable range of 10 to 15 volts (for instance, -45 VDC to -60VDC) and as such replacement tubes of the same variety will probably be within a tolerable range based on the setting of the tubes being replaced. Yet, why take chances? More precisely, why not set your amp to optimum? Biasing an amp is really not difficult and requires only a minimum of tools, most importantly a multimeter. There are two methods used in most amplifiers to set or determine bias voltage for output tubes. Cathode
Biasing Your Amp " Bias" refers to setting a tube's operat ing cu rre nt wh ile it is idle . An amp t hat isn 't properly biased will not sound at its peak. And by experimenting with bias setting s, you can fin d overal l amp tones that yo u prefer.
Opposite: The Twin reissue replicates the vintage circuit design and two 12-inch nwin") speakers of the tweed-era original. Fender Musical Instruments Corporation
On this Fender Deluxe clone, the cathode resistor is the white 5-watt, 250-ohm resistor in the center. The cathode resistor in cathode-biased Fenders could also look like one ofthose being held in the foreground.
53 ••-
-
bias (found in smaller and older Fenders, such as the tweed Deluxes, Champs, and Princetons) sets the tubes bias by raising the cathode above ground potential (typically around +20 to +30 VDC), which in turn makes the control grid more negative than the cathode. This type of bias is not adjustable. Instead the cathode resister (250-ohm, 5-watt in a tweed Deluxe) must be replaced to change the bias. Typically this is not done; however, if you want to experiment, raising the value of the resistor by about 50 ohms won't harm the amp. If you decide to lower the resistor, though, you should measure the bias to ensure that the tube's plate dissipation is within limits. This procedure will be covered later in this chapter. Cathode bias is how bias is set for the preamp, reverb, and phase inverter tubes. The second method for biasing output tubes is called fixed bias, even though this type of bias is often adjustable. With fixed bias the control grid is made more negative than the cathode by supplying a negative voltage (typically -30 to -60 VDC) to the control grid of the output tube. This requires a separate power supply that is adjusted via a potentiometer whenever the output tubes are replaced. There is no "correct" voltage setting for fixed bias but, rather, a voltage range. Lower or colder bias (more negative voltage on the grids) tends to make the tubes sound clean but with a thinner tone and a tendency toward unpleasant crossover distortion
if set too low. Higher or hotter bias (less negative voltage on the grids) makes the amp louder with a thicker tone but also with the risk of saturation, which will cause the plates to glow red and eventually melt if set too high.
Removing the Chassis The first step in any modification, including biasing the power tubes, is gaining access to the tubes and circuitry. While you can often reach into the back of the cabinet or head and grab the tubes, the safest and easiest approach requires removing the rear access panel. On blackface and silverface heads and combos, this is a thin piece of wood with two screws on each side. On Fender tweed-era amps and reissues, such as the 59 Bassman Reissue, as well as the Hot Rod series, all of which have the controls on the top of the cabinet, removing the rear panel also exposes the circuitry. Easy access, indeed. In fact, most of the mods on these models can be done without removing the chassis. With blackface, silverface, and blackface reissues, the chassis must be removed to gain access to the circuitry. Before removing the chassis, it's a good idea to remove the output tubes first. The output tubes are held in by spring clamps and must be worked out gradually while holding open the clamps. Be aware also that
-==
C"C
..= c:....:>
Our silverface Super Reverb project amp with the back panel removed.
•• 54
On the Hot Rod models, remove six screws and lift the panel away.
= =
cr.>
To remove output tubes, push the clamp open with one hand and gently rock the tube out.
55 ••-
-
In time the spring clamps begin to lose their tension and need to be retensioned by squeezing the jaws together so they will secure the tubes tightly.
-==
C"C
-= c:....:>
Remove preamp tube shields by slightly twisting them counterclockwise. Preamp shields not only help to retain the tube in the socket, they also keep stray RF away from preamp tubes.
tubes get extremely hot, and therefore the tubes should be allowed to cool down before removing them. The preamp tubes, on the other hand, are secured simply from tension exerted on their pins by the socket and by a tube shield that twists slightly onto the socket. The shield has an internal spring that presses against the top of the tube. Often, though, the tube shields are missing from older amps. Since they are inexpensive, it's a good idea to replace them.
•• 56
Fender speaker connections consist of a 1/4-inch jack that should be removed by pulling the jack and notthe cable.
In addition to removing the output tubes, unplug the speaker connection, the foot switches if they're plugged in, and the reverb tank if so equipped. When removing cables, always pull by the connector and not the cable or wire itself. Note, also, that the reverb connections are color coded, red for input and white for output. Sometimes the cables themselves are colormatched and other times it's not so easy to determine. You can always mark each cable with a piece of tape. Next, remove the four chassis-to-cabinet screws. Be aware that on older amps there are nuts under the chassis that must be held while unfastening the screws. Getting a nut-driver or a wrench on them is difficult due to the tight spaces. Luckily, you can often hold them tight enough by hand. On reissues, the chassis has a lip on the back edge through which several small Phillips-head screws secure the chassis to cabinet. These are easily visible from the back of the amp. Remove the screws. The chassis is held in place across the front by the speaker baffle (the board on which the speaker or speakers are mounted) and the rear corners by the rear panel mounting strips. However, the chassis is unstable without the mounting screws as the transformers are heavy and can cause the chassis to drop with much force. Remove the chassis by pulling it back and out, using the transformers as handles. If you are going to be turning on the amp, for biasing, for example, install the tubes. If the circuitry needs to be accessible, which it will have to be for biasing, the chassis will have to be supported so the tubes have enough clearance. Wood blocks under the transformers work fine as long as they are stable. Also make sure the speaker connector will hook up. Never run your amp without the speaker attached
The color coding on the reverb connections of this amp is faint. White indicates reverb output, while red indicates reverb input. Some amps have more clearly colored jacks.
as this can destroy the output tubes and possibly the output transformer in a matter of minutes. Reverb doesn't need to be hooked up unless you are checking the reverb circuit or if you are going to use reverb when you play the amp for the ever-important ear test. If more slack is needed in the reverb cable, unscrew the cable clamp from the side of the cabinet. To work on the chassis on a bench, you'll need to either make an extension cable to connect the chassis to the cabinet speaker(s) or consider using a loose speaker or speakers-quality doesn't matter-as a sort of dummy load. An extension cable can be made from a length of 18-guage speaker wire with a male jack connected to one end and a female jack to the other. Make sure to observe polarity; that is, connect the ground lugs of the jacks to the same wire. Jacks and speaker wire are widely available at places like Radio Shack. If you decide to connect loose speakers to the chassis speaker output rather than the cabinet speakers, make certain you know the output impedance of your amplifier. A Fender with two speakers, such as a Twin, is usually 4-ohm while one with four speakers, such as the Super Reverb or Bassman, is 2-ohm. In those cases, use a 4-ohm speaker (or two 8-ohm speakers in parallel) for the former and two 4-ohms in parallel (or four 8-ohm speakers in parallel) for the latter. You could also purchase a dummy load with the proper impedance, but for the average do-it-yourselfer, these are often not worth the cost. As an alternative you can build your own dummy load with large resistors as long as the impedance matches the amp's impedance. Also make sure that if the amp is 50-watt, the combined resistor rating of the load needs to be at least 50-watt. This might mean some large resistors. Instructions for making various loads are available online.
Because the power cord comes out with the chassis, make sure to remove the screw holding the cable clamp to the cabinet.
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Each chassis strap has two screws. Use yourfree hand to hold the fastener nuts underneath the chassis. A small wrench or nut-driver may be necessary.
Warning Never, ever run your amp without the speaker or speakers attached. This can destroy the output tubes-and even the output transformer-in a matter of moments .
57 ••-
-
Using the power and output transformers as handles. pull the chassis straight back and out.
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Once the chassis is removed. place it on your bench or on top of the cabinet.
•• 58
With the chass is out of the cabinet, make sure that the tubes have adequate clearance. Here, the tubes are situated over the edge ofthe cabinet. Cautious attention must be used when the chassis sits atop the cabinet.
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Removing the speaker junction board on multi -speaker combos should allow enough slack in the speaker cable to hook the connector to the chassis. Neve r operate a tube amp without the speake r connected.
59 ••-
-
How to Set Bias To accurately adjust t he bias vo ltage, t he plate curren t and voltage of each output tu be is used to determine the tube's plate dissipation. Datasheets fo r tubes
Output Impedance of Common Fender Amps Amplifer
~
-=-
Output Impedance
Bandmaster
40
Bassman Combo
20
Bassman Head
80
Concert
20
Champ
40
Deluxe
80
Princeton
80
Pro
80
Pro Reverb
40
Showman Combo
40
Showman Head
80
Super Reverb
20
Twin
40
Vibrolux One Speaker
80
Virbolux Two Speakers
40
Vibroverb One Speaker
80
Vibroverb Two Speakers
40
~
=
C"C
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are ava ilable online and can be consulted to find a tube's maximum p late dissipat ion. Typica lly, a tu be at idle is adjusted to no more than 75 percent of maximum combined plate and screen dissipation a t idle. Any thing bet ween 60 and 75 percent is fine. I've even seen amps biased at 60 percent that st ill sou nded fine. W hatever set t ing within that ra nge sounds best is the setting you should u se. Keep in m ind, though, that biasing above 75 percent will drastically red uce the life of yo ur tubes . Mu ltiplying the plate current in milliamps by the plate voltage in volts DC equa ls the plate dissipation in watts (plus a few watts of screengri d d issipat ion) . Refer to the accompa nying t able for a quick overview of the specification s for most popular guitar amp tubes. An easy way to adjust bias is to measure plate voltage at t he tube with a mu lti meter and to determine plate current by inst alling I -ohm resistors (I-watt or greater) between the cathode of each power tube and ground and measuring volt age drop across them. Here's how it works. Accord ing to O hm's Law, current equals voltage divided by resistance (I = E/R). Therefore, if resistance is I ohm, current will be equal to voltage. For example, a reading of 0.030 volts (which is 30 millivolts) divided by I ohm equals 0030 amperes (which is 30 milliamps). For convenience, I've included bias charts in this sect ion . After measuring plate voltage, consult the chart for your particular t ube and find the recommended cathode voltage (across the I -ohm resistor) for the percent of plate dissipation you want (from 60 percent to 75 percent). After adjust ing to the cat hode voltage, measure plate voltage again since it will prob ably have ch anged. Adjust cathode volt age aga in to
Care in Working with High Voltages It always bears repeating : The voltages inside an amplifier chassis are lethal. If you are not careful, you can be seriously injured or even killed. If you are not comfortable working around high voltage , don't do it. Remember, never place anything, especially your hand, inside a running circuit except a meter lead . If you are making adjustments to the bias top, touch only the bias pot with a small screwdriver. Unless you are a skilled technician, I recommend a nonmetallic screwdriver or adjustment tool made from plastic. Even if you are skilled, a nonmetallic adjustmenttool is a good idea .
•• 60
Dummy load constructed from two 15-ohm, 25-watt res istors in parallel. The result is a 7.5-ohm, 50-watt dummy load that will work with most Fende r amps of 8-ohm impedance.
match new plate voltage reading. Continue until plate and cathode voltages match. T he following procedure will work fo r all blackface, silverface (see silverface biasing sect ion to determine if modification of bias circuit is required), and all Fender reiss ue amps . Biasing of the Hot Rod Deluxe a nd Devi lle is det ailed later as is the modifica tion and biasing of the mid-to -Iate silverface. Even though the bias adj ustment potentiometer is accessible through the bottom of the ch assis, adjusting it without measuring voltages is ill-advised. To accurately adjust the bias potentiometer, the chassis should firs t be removed from the cabinet . (Except for the later 1959 Bassman Reissue, simply remove the back cover to expose the circuit; note that ea rly Bassman reissues have nonadjustable fixe d bias like the origina l tweed version.) To avoid break ing the tubes, it 's best to remove them firs t from t he chassis. Place the chassis on top of the cabinet or on a bench close to the cabinet. It may be necessary to place blocks under the transformers to ensure tube clearance between chassis and cabinet. Unsolder the ground wire from pin 8 of each of t he two 6L6 or 6V6 output tubes. Solder a I -ohm, I -watt,
or greater resistor between the pin and the unsoldered end of t he wire. Because current product ion output tubes are matched, you cou ld get by with adjusting the bias by using only one output tube as reference. It's good practice to monitor both tubes, however. Install t he tubes. Also, make sure to reconnect the speaker to the speaker jack on the chassis (or use a spare speaker or adequately rated dummy load) . Never run an amplifier without the speaker attached because
Specifications of Popular Guitar Amp Tubes Tube
Max. Plate Dissipation
75% of Max
KT88 and 6550
42 watts
31.5 watts
6L6GC and 7027
30 watts
22.5 watts
5881, EL34, and KT77
25 watts
19.0 watts
6V6
14 watts
10.5 watts
EL84 and 68Q5
12 watts
9.0 watts
Plate dissipation is the voltage in milli volts acros s a l-ohm cathode resistor multiplied by the voltage in hundreds of volts at th e plate.
Bias Chart for 3U-Watt Maximum Plate Dissipation for 6L6GC and Sovtek 5881/6L6WXT Plate Dissipation
60%
Plate Voltage DC Measured at Pin 3 400
410
415
420
425
430
435
440
445
450
455
460
465
470
475
480
45.0
44.0
43.3
42 .8
42.3
41.8
41.3
41 .0
40 .5
40.0
39 .5
39.0
38.7
38 .2
37.8
40 .0
65%
48.7
47.5
47.0
46.4
45.8
45.3
45.0
44.0
43.8
43.3
42.8
42.3
42.0
41.4
41.0
40.6
70%
52.5
51.2
50.6
50.0
49.4
48.8
48 .2
47.7
47.0
46.6
46.0
45.6
45 .0
44.6
44.2
43.7
75%
56.2
54.8
54.2
53.5
53.0
52 .3
51.7
51.0
50.5
50.0
49.4
49.0
48.3
47.8
47.3
46.8
=
Milli volts DC measur ed across l-ohm cathode resistor from pin 8to ground 60 % = 18.0 watts
65 % = 19.5 watts
70 % = 21.0 watts
75 % = 22.5 watts
Bias Chart for 25-Watt Maximum Plate Dissipation for 5881/6L6 non-GC, KT66, and EL34/6CA7 Plate Dissipation
Plate Voltage DC Measured at Pin 3 400
410
415
420
425
430
435
440
445
450
455
460
465
470
475
480
60%
37.5
36.5
36.0
35.7
35.0
34.8
34.4
34.0
33.7
33.3
32.9
32.6
32.2
31.9
31.5
31.2
65%
40.6
39.6
39 .0
38.6
38.0
37.7
37.3
36.9
36.5
36.0
35.7
35.3
34.9
34.5
34.2
33.8
70%
43.7
42 .6
42 .0
41.6
41.0
40.6
40.0
39.7
39 .3
38.8
38.4
38.0
37.6
37.2
36 .8
36.4
75%
46.8
45.7
45.0
44.6
44.0
43.6
43.0
42.6
42 .0
41.6
41.2
40.7
40.3
39.8
39.4
39.0
Milli volts DC mea sured acros s l-ohm cathode res istor from pin 8 to ground 60 % = 15 .00 watts
65 % = 16 .25 watts
70 % = 17.50 watts
75 % = 18.75 watts
61 ••-
-
Bias Chart for 14-Watt Maximum Plate Dissipation for 6V6 Plate Dissipation
Plate Voltage DC Measured at Pin 3 365
370
375
380
385
390
395
400
405
410
415
420
425
430
440
450
60%
23 .0
22.7
22.4
22.0
21 .8
21.5
21.2
21.0
20.7
20.4
20.2
20.0
19.7
19.5
19.0
18.6
65%
24.6
24.3
24.0
23.6
23.3
23.0
22.7
22.5
22.2
21.9
21.6
21.4
21.2
20.9
20.4
20.0
70%
26.8
26.4
26.0
25.7
25 .4
25.0
24.8
24.5
24.0
23.9
23.6
23 .3
23 .0
22.7
22.2
24.7
75%
28.7
28.3
28.0
27.6
27.2
26.9
26.5
26.2
26.6
25.6
25.3
25.0
24.7
24.4
23.8
23.3
Millivolts 0 C measured across l-ohm cathode resistor from pin 8 to ground 60% = 8.4 watts
65% = 9.0 watts
70% = 9.8 watts
75% = 10.5 watts
Making aBias Board Because the voltages inside an amplifier are lethal, extreme care must be taken when measuring them . The use of a bias board makes adjusting bias safer (essentially a small board with a terminal strip attached).
=
th is ca n destroy the output tubes a nd the output t ra nsformer. Attach jumper wires fro m the tu be side of each resistor and pin 3 of one of the power tu bes to the bias board . Ground the multimeter negative lead to the amp chassis . Set your mu lt i meter to read DC voltage . You w ill be using bot h t he highest (at least 1,000 -volt) and lowest (200 -millivolt) DC voltage ranges. Set volume and reverb to 0 and turn on the amp. Let it ru n for about 60 seconds in standby. After taking the amp off of standby, wait about 30 seconds or so, and then with the positive meter lead measure the plate voltage (at bias board jumper from pin 3 of one power tube) on the 1,000-volt scale. Write down this figure. Next, measure the voltage at each jumper end of the I-ohm resistors. Voltage will be in millivolts so set the meter range accordi ngly. Record these numbers
=-
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-= c:.....:>
A bias board can be made from a piece of wood and terminal strips. Before applying power to the chassis , connect jumpers from the components to the terminal strip. Measuring voltages ofthe components can then be done remotely to reduce risk of shock or meter probe slippage.
Whi le the bias adj ustment pot is accessible f rom the outside of the cha ssis and can be turned with the chass is mounted inside the amp, voltages cannot be me asured and monitored unless the chassis is removed.
•• 62
as well. If the readings vary by less than 10 millivolts, the tubes are matched within adequate range. Multiply each of the millivolt readings with the plate voltage to determine plate dissipation of each tube. If the reading is low or high, adjust the bias potentiometer while measuring the voltage at one of the resistors. Measure the plate voltage again (it will have changed). Continue until proper dissipation is found. For example, if plate voltage is 460 VDC and one resistor measures 43 mVDC (or 0.043 VDC) and the other reads 40mvDC (or 0.040 VDC), tube dissipation is approximately 20 watts and 18 watts, respectively. This is approximately 65 percent of maximum plate dissipation for 6L6GC (30-watt rated) tubes and is thus fine, just slightly cold. Play through the amplifier for several minutes, and recheck the bias as it might have changed. Also take a look at the power tubes to make sure the plates are not turning red. This would most likely appear as a dull, dark red in a fold or center of the plate. It is normal to see the orange glow of the filament and sometimes a faint bluish, cloud-like color around the plate. If one or more of the power tubes are red-plating, make the bias colder. While this process of adjusting bias might seem complicated, practicing it will make it second nature.
When soldering the l-ohm resistor between pin 8 and ground, wires should be pushed away from the socket to prevent them from being burned.
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Remove excess resistor leads ("pigtails") once the resistors are soldered in place. After completion ensure proper wire dressing by keeping the filament wires up and away from the other wires.
63 ••-
-
Using an alligator clip to attach the negative meter lead to chassis ground will keep your hand safely clear of high voltage. Any of the lugs attached to the power supply mounting bolts provide excellent grounding points.
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I should mention that when taking your amplifier to a shop to have tubes installed and bias adjusted, there are occasions when the technician won't be as precise in checking bias as has just been demonstrated. Sometimes bias is set in the ballpark range by simply adjusting the potentiometer for the negative DC bias supply voltage as measured at the junction of the 220-K-ohm output tubes' grid resistors. Schematics for Fenders usually show this to be somewhere around -SOVDC. The main problem with this adjustment is that the current flow through the output tubes cannot be determined accurately, only at best estimated; the result being optimal tone as well as tube performance is neglected. Obtaining a schematic for your amplifier from one of the free online sources listed in the appendix will go a long way to help you understand the practical working of the bias supply and circuitry as well as the overall operation of the amplifier. Most of the schematics show voltage readings at various locations within the circuitry; bear in mind that these voltages will vary slightly among amplifiers.
Using a bias board allows for safe, hands-free-fromchassis measuring of voltages. Attach one lead from the board to the cathode resistor (the l-ohm resistor just installed) at pin 8 and attach another lead from the board to pin 3 for plate voltage.
•• 64
Measure plate voltage, which will be at high DC voltage.
Biasing the Hot Rod Deluxe and Deville Biasing Hot Rod Deluxe and Deville and other newer Fender tube amps is a relatively straight-forward process, easier than for the older Fenders. First of all, each of these amps has a I-ohm resistor installed between ground and the output tube cathodes, easily accessible at a designated test point. One important detail to bear in mind is that this resistor connects to both tubes' cathodes and therefore passes the combined current of the tubes . What this means is that the voltage you measure will be twice as high as in amps where you install one resistor per cathode. Plate dissipation of each tube is approximated by halving this voltage reading before multiplying it by the plate voltage. Bias instructions on the Hot Rod schematic are simple: Adjust pot R82 to read 60 mV at test point TP30. All of these points and parts are clearly marked on the printed circuit board. As indicated in the photo on the next page, TP30 is a solder point located on the front left corner of the tube socket board and labeled bias test point. Plus, the chassis doesn't have to be removed from the cabinet. Simply unfasten six screws to remove the rear panel and expose the circuitry. While the above procedure will set the bias to factory specifications, these specifications are rather on the cold side. A common complaint regarding these amps is that they tend to feel somewhat sterile and colorless, symptoms of too cold bias. To bring these
Adjust bias potentiometer while reading cathode voltage. Voltage will be low in the millivolt range.
amps to life and really get coloration from a decent set of output tubes, try adjusting the voltage on TP30 to around 70 mY. Even the less than desirable overdrive feature on these amps will have a notably improved sound, the sterile harshness often characterizing the overdrive becoming warmer and more complex. Above all, when you want to set your amp for optimum bias operation, or when using other tubes, such as 6V6 or KT66, plate voltage should also be measured and multiplied by the cathode voltage (found at TP30) to determine plate dissipation. The process is similar to that detailed previously for blackface and early silver face amps. Plate voltage can be read at TP27 or TP28. Note that the voltage reading printed on the schematic for these points is for signal voltage (which is in AC) and not for the +DC voltage of the plates. Following the process detailed above, you will find approximately +430 volts on the plates of the Deluxe and +480 volts on the plates of the Deville, which will change when the bias pot (R82) is adjusted. I've gotten quite a different range of breakup on the Hot Rod Deluxe by varying the bias setting. As discussed in the next chapter, there are a variety of output tubes that can be used in these amps to provide a range of sonic quality. It is vital to provide proper bias for each of these variations. The factory setting of 60 mV isn't accurate for these tubes.
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65 ••-
-
Outputtube bias can be roughly setto a ballpark range by adjusting the bias potto obtain the predetermined bias supply voltage atthe junction ofthe cathode resistors of the outputtubes (as shown). For a blackface Super Reverb, the voltage should be -52 VDC, while for a blackface Deluxe Reverb it is -35 VDC. Refer to the appropriate schematics.
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After grounding the negative meter lead to the chassis, adjust the pot while measuring voltage, in millivolts, attest point 30. The factory setting is 60 millivolts.
•• 66
Biasing the 6l6-Tubed Silverface The most common and affordable pre-197S Fenders you're likely to find are the 6L6-tubed silver face models, such as the Bandmaster, Bassman, Pro Reverb, Showman, Super Reverb, Twin, and Vibrolux. One major difference between blackface and silverface circuitry lies in the output tube bias design. While blackface Fenders have adjustable fixed bias, most silverface Fenders with only a few early exceptions have what amounts to a nonadjustable fixed bias. Technically, the silverface has an adjustment pot, but rather than adjust tube bias, it balances the negative bias voltage going to the output tubes' grids. Why did Fender switch from bias adjustment to bias balance? The common belief is that converting to a balance control allowed unmatched output tubes to be used. This would not only save money at the factory but would also allow the user to replace only one tube at a time (commonly recommended practice is to replace both output tubes). Whatever the reason, a balance adjustment for output tubes is no longer needed as current production tubes are normally sold in matched pairs (or quartets, for Twins and other four-tube, lOO-watt amps). Replacing the balance circuit with a bias control is perhaps the most common modification of Fender silverfaces. Moreover, many guitar shops selling used amps often perform this and other blackface modifications routinely. In fact, it's getting rare to find a silverface amp that hasn't been blackfaced or at least had the bias control modification. For the most part, this is a good thing, as long as the mod has been done correctly. There are various extents to which this mod can be made, as I indicate later. A quick way to see if your silverface has already had the bias modification done is to locate the wire on the number 2 lug (middle lug) of the adjustment pot. Follow this wire to where it connects on the circuit board. If it connects to the junction of two 220-K-ohm resistors (red-red-yellow banded), then you don't need to modify your bias. It is already adjustable. If, however, lug number 2 of the pot has a resistor soldered to it, which, in turn, has the other end soldered to the pot, the bias modification hasn't been done. If there is a wire coming from lug number 2 that connects to a single lOO-K-ohm resistor (brown-black-yellow banded), which in turn is attached to the circuit board, the bias modification hasn't been done. Silverface balance pots are easily recognizable because most have four connection lugs rather than three. Some, however, have three lugs but are distinguishable by a pair of lO-K-ohm (brown-blackorange-sliver banded) resistors with one end connected to each other and a lOO-K-ohm resistor (brown-blackyellow-silver banded) going to the circuit board and the other end connected to the outermost lugs of the pot.
Plate voltage of a Hot Rod can be measured at pin 3 of either of the outputtubes. Here the measurement is taken at test point 27, which is a short distance from the pin and allows a safer measurement.
This 1970 Super Reverb has a balance adjustmentthatwill be converted into a bias adjustment. If your amp has a fourlug pot but nothing attached to the fourth lug (on the left by itself), your amp most likely has the bias modification done already.
In addition to the balance pot, some silverfaces also have ISO-ohm, 7-watt cathode resistors going from pin 8 of each output tube to ground. A S-uF capacitor bridges the cathode end of the resistors.
67 ••-
1971 Silverface Bias Circuit
(3)
~ (2)
--
--1
+ 50uf
lOOK
or 68K lOOK
or 68K
•
(5 )
(4)
•
The wires and resistors in red should be removed or replaced to match the 1963 blackface circuit. Numbers correspond to the steps for converting to bias control.
1969 Silverface Bias Circuit
The wires and resistors in red should be removed or replaced to match the 1963 blackface circuit. Numbers correspond to the steps for converting to bias control.
A 1968 Silverface Bias Circuit
--1
I •
+ 50uf
15K
( 5)
The wires and resistors in red should be removed or replaced to match the 1963 blackface circuit. Numbers correspond to the steps for converting to bias control.
A 1963 Blackface Bias Circuit
4701W
- --II
,,,
27K
(1 )
(1 )
220K
220K
+ 25uflI-- -•• '
(3a)
The wires and resistors shown in red indicate bias circuit. Numbers correspond to the steps for converting to bias control.
-
First, refer to the diagrams and photographs to identify your silverface, then do the following: 1. Locate and remove lOOK resistors (and 10K resistors on pot, if so equipped). 2. Next, locate the resistor that is soldered to the balance potentiometer and unsolder the lead that is soldered to the middle lug of the pot. 3. Unsolder the wire from the upper lug. 4. Remove the 3.3K resistor and electrolytic capacitor, if so equipped, attached to the left lug of the pot. The other end of the capacitor is soldered to ground while that of the resistor is soldered to the small bias power supply circuit board located in the left upper corner of the chassis; if your amp has no resistor and capacitor at this location, remove, instead, the wire from the left lug of the pot. 5. It's also a good idea to replace the electrolytic capacitor located on the bias power supply board. A 47-uF capacitor rated at 100 volt works well here.
=
(1) Remove the 100-K-ohm resistors from the board and the potentiometer.
(2) Unsolder the resistor on the pentiometer from the
middle lug and (3) the wire from the top lug.
(4) Completely remove the 3.3-K-ohm and
the electrolytic capacitor that are attached to the potentiometer.
(5) Replace the electrolytic capacitor
on the bias supply board. Use a 22-uf or 47-uf cap rated at 100 volts. Make sure to observe polarity. The positive end of the cap goes to ground (left side of board).
•• 70
Referring to the diagram for the 1963 blackface bias circuit and the following photographs: 1. Install two 220K resistors (red-red-yellow-gold banded or red-red-black-orange-brown banded for metal film resistors) in the positions indicated. 2. Solder wire that was originally on the upper lug to the middle lug of the potentiometer. 3. Solder the wire from the bias supply board to the lower lug of the pot; if you removed a 3.3K resistor from between the bias supply board and the pot, (3a) install a wire (20-guage is fine) as just indicated. 4. Finally, solder the loose lead from the resistor that is attached to the pot, to the upper lug of the pot.
(1) Two 220-K-ohm resistors should be installed here.
(2) Solder the wire that comes from the circuit board to
middle lug ofthe potentiometer.
(3) If not already equipped, solder a wire from where the
negative lead of the electrolytic capacitor on the bias supply board connects to the lower lug of the pot. (4) Solder the resistor connected to the pot to the upper lug.
The Silverface Super Reverb with completed blackfaced bias control.
71 ••-
-
If your silverface has 150ohm resistors on pins 8 of the outputtube sockets, replace them with l-ohm, 2-watt resistors to modify the amp for adjustable bias.
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Afterfinish ing the bias modification, turn the amp on but leave it in standby. With your meter set to read DC voltage, ground the black lead and place the red lead atthe junction of the newly installed 220-K-ohm resistors . While turning the bias pot from one end to the other, the voltage should read between approximately -47VDC and -54VDC.
•• 72
If your amp has the 150-ohm cathode resistors with adjoining 5-uF capacitor, remove them and replace the resistors with l-ohm, 1- to 5-watt resistors. Once these modifications are completed, set your multimeter to read DC voltage with range set to 100 volts or greater. Attach the negative lead to ground and, once the amp is turned on, place the positive lead to the junction of the two 220-K-ohm resistors just installed. You'll be reading negative voltage. Turn the amp on, but leave it in standby. While holding the positive lead to the 220-K resistor junction, turn the bias pot adjustment from one extreme to the other. The voltages you read while adjusting the pot should be about -47 volts to -54 volts, give or take a volt or two. Adjust the pot to read the highest negative voltage and turn off the amp. When you adjust the bias, you will be starting from the extremely cold bias. If the voltage range you measure cannot rise above or below -50 volts, the 15-K-ohm resistor on the pot will have to be changed. If the voltage range cannot rise above -50 volts, try a 27-K-ohm resistor (a higher value resistor will raise the voltage threshold). If the voltage range will not drop below -50 volts, try a 10-K-ohm resistor (a lower value resistor will lower the voltage threshold). The 15-K-ohm resistor will most likely be fine, but it is good to know how the value of this resistor will affect bias. By the same token, when you adjust the output tube bias, following the procedure described earlier in the chapter, if the voltage across the l-ohm resistor for one of the output tubes can't be adjusted high enough to reach 75 percent plate dissipation (usually around 35 to 40 millivolts), the 15-K-ohm resistor on the pot should be lowered in value (less negative DC voltage at 220-K resistor junction). On the other hand, if the voltage is too high across one of the l-ohm resistors (can't go below about 35 millivolts), the 15-K-ohm will have to be raised in value (more negative DC voltage at the 220-K resistor junction). What this means is that the plate current of the tube (what you're measuring as DC millivolts across the cathode resistor) is indirectly proportional to the negative voltage being applied to the control grid of the tube. The more negative the control grid voltage, the less current drawn by the plate; the less negative the grid voltage, the more current drawn by the plate. With the Super Reverb photographed in this chapter, which is equipped with a pair of Sovtek 5881WXT, we ended up adjusting the bias for 45 mV (0.045 VDC) across the l-ohm resistor of one output tube and 42 mV (0.042 VDC) on the resistor of the other. With a plate voltage of 430 VDC, the plate dissipation of the higher drawing tube is 19.3 watts, a little cold for the 30-watt tube, but the amp sounds great. As a loud, clean amp, the Super Reverb actually seems a little mellower at this lower bias. It just goes to show that the best judge for setting bias is your ears.
Biasing the Deluxe Reverb, Princetons, and Tweeds The Deluxe, Deluxe Reverb, Princeton, and Princeton Reverb all use a pair of 6V6 for output tubes. The Princeton and the Princeton Reverb, while being fixed bias, have nonadjustable bias. With these amps the bias circuit is more integral to the tremolo or vibrato circuit than the Deluxe Reverb, which is adjustable. In fact, the silverface Deluxe Reverb didn't employ the change toward balance control as did its 6L6 big brothers and, typically, is adjusted in the same manner as blackface amps discussed earlier. It should be noted that while tweed-era and brownface Fenders also use nonadjustable fixed bias, the reason that the bias is nonadjustable in the Princeton, Princeton Reverb, and brown face Deluxe is due to the vibrato feature. (On a side note, the vibrato circuit in Fender might more accurately be called a tremolo circuit, the difference being that tremolo involves amplitude modulation rather than the frequency modulation associated with vibrato. But that is probably talking apples and oranges here.) Princetons, like the brownface Deluxe models, use what's called bias modulating tremolo, which manipulates the bias on the output tubes to give the familiar oscillating sound of tremolo. Bias modulating tremolo works fine on smaller amps, but on the larger Fenders this doesn't sound good. The Deluxe Reverb, like the 6L6 blackface and silverface, use an opto-isolator for the oscillator circuit and therefore does not manipulate bias.
On Princeton Reverb amps, lowering the value of this resistor will give the output tubes a hotter bias; raising the value, on the other hand, will give them a colder bias.
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To give tweed and brownface amps adjustable bias, replace the 56-K-ohm resistor with a 27-K-ohm resistor in series with a 50-K-ohm trimmer pot. The trimmer pot can be adjusted with a small screwdriver.
For the most part, the Princetons, and any other Fender with bias modulating tremolo, are probably best left as is unless installing anything other than recommended or stock tubes. The integral relationship between the bias and tremolo circuits requires changing a resistor in the bias power supply to determine the range of bias voltage that is effectively varied by the intensity control. If you choose to experiment with the bias, locate the 22-K-ohm or 27-K-ohm resistor that is connected across the leads of the electrolytic capacitor on the bias power supply circuit board mounted in the upper left corner of the chassis. Lowering the value of this resistor (for example, to 18 K-ohms) will give the output tubes a hotter bias, while raising it (for example, to 28 or 33 K-ohms) will give the tubes a colder bias. Follow the procedures above for measuring voltages to determine plate dissipation, using the bias chart for 6V6 tubes. Make sure that the vibrato effect is turned off.
The Value of Proper Biasing Learn ing to properly adjust outputtube bias is probably the most important step in modifying any amplifier. Not only will you save yourself money by not ha vi ng to hire someo ne to adjust yo ur bias whe never yo u change output tubes, but you wi ll also be ab le to quick ly switch between various outputtubes. In fact, running differenttubes, power as wel l as preamp, can prove to be one of the most nondestructive (i.e., reversible), tone-effective mod ifications you can perform.
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By the way, the above procedure of replacing the resistor situated across the electrolytic capacitor of the bias voltage power supply, also works for adjusting the nonadjustable fixed bias on the tweed-era and brownface amps, both 6L6 and 6V6 models. Again, though, unless you are going to use KT66s or EL34s on the 6L6 models, or 6L6 on the 6V6 models, it's best to leave the nonadjustable fixed-bias circuits as is. Yet, if you're interested in achieving optimal output tube performance, being able to adjust the bias is essential. A common trick to use on tweeds and brownfaces is to remove the bias resistor and replace it with one of about half the value (specifically, the 56 -K-ohm of the type used on tweed and brownface amps with a 27-K-ohm, and the 22 -K-ohm of the Princetons and brownface Deluxe with a 10-K-ohm). Attach one end of the resistor to the first or third leg of a 50-K-ohm, small (112-watt) trim potentiometer (with the 6V6 amps, use a 25 -K-ohm trimmer). Next, tie the middle leg of the trimmer to whichever leg was not installed to the resistor (the trimmer has three legs). From the connected legs, run a short wire to the negative end of the electrolytic cap. Attach the free end of the resistor to the positive (i.e., ground) end of the electrolytic capacitor. Now you can adjust the bias using the blackface procedure discussed earlier. Note that tweed amps don't have a separate bias supply board. Instead, the resistor and capacitor is on the left end of the main circuit board between the two 8-uFbias supply electrolytic capacitors. On the early Bassman reissues (later reissues come with adjustable bias) the resistor is located in the center of the board, below and between the mid and bass potentiometers.
Things to Keep in Mind Regarding Biasing Power Tubes While we've just gone into depth regarding basic techniques for biasing power tubes of a typical Fender guitar amp, a few overall points still need to be addressed . There often exists disagreement concerning not only the proper method for biasing tubes, but also the need for doing it in the first place . This last point speaks to the fact that while most amps have a bias adjustment potentiometer, the variance among similar tube types won't be enough to warrant making a precise adjustment. Indeed, when replacing a pair of output tubes, many technicians will simply adjust the negative bias voltage at the junction of the 220-K-ohm output tube grid resistors according to the voltage indicated on the schematic for that particular model (usually between -48 and -52 VOC). Obviously, this is an inaccurate procedure, and guitarists often pay good money just to have that done. Furthermore, even certain reputable amplifier manufacturers tell us that there is no need to set the bias on fixed-bias amps since they believe it's not a critical measurement. It should be stated, though, thatthis critique of procedure does not apply to cathodebiased amps, because, as has been discussed, they use a different method of setting bias than the more prevalent fixed-bias amps .
Atthe other end ofthe spectrum reside the gurus and venerated experts who claim that proper tube biasing can only be accomplished with an oscilloscope and a signal generator. Of course there's nothing wrong with being overly precise; it's just unnecessary. Most technicians land in the middle of these two camps of extremes. So, who's right in all of this? Maybe it's not a question of right but of enjoyment. Many guitarists who learn aboutthe various techniques for biasing an amp eventually try it on their own. They might bias hot, bias cold, bias in the middle, bias, and rebias again perhaps for the challenge of getting the optimum tone from an amp, or perhaps forthe fun of it and the enjoyment of being able to work on your their amps. On a related, closing note, back in the old days when people used to replace their own spark plus in their cars, they were often warned about the need to properly gap the spark plugs. Yet, some people felt comfortable installing them as is, knowing thatthe factory setting was close enough. Other people bought gapping tools and set the gaps of the new plugs precisely, even if many ofthem seemed to be already set pretty close. All in all, most everyone's cars started, and invariably some ran more efficiently than others. Did it have anything to do with gapping their own plugs, or did it have more to do with the care given during the tune-up?
=
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5
QUICK, BASIC, ANO fSSfNTlAl MODIfiCATIONS Fewer things in the amp world receive more hype than the NOS tube. In fact, not only has the term become synonymous with the "best" tubes but also an entire industry of collectors and vendors has developed over the past 20 years. But just what is NOS? Basically, NOS stands for "new old stock," meaning tubes manufactured, primarily in the United States and Western Europe, more than 25 years ago but that are still new in that they haven't been used. Because the companies that once manufactured these tubes no longer do so and haven't done so for decades, NOS tubes are in finite supply and will eventually run out. In fact, many are becoming quite rare . Along with the diminishing supply, the collecting-or hoarding-of these tubes has driven their price beyond, in my opinion, their actual value, and soon the common guitar amp types, such as the 6L6, will no longer be available at all. Don't get me wrong. There are excellent NOS tubes out there; however, there
are also some not-so-excellent ones and worse yet, used and substandard tubes being sold as NOS. The fundamental question for most musicians is this: Should I pay $150-plus for a pair of NOS tubes that may not be closely matched, or should I pay less than $50 for a high-quality, matched pair of current production tubes? While guitar amps are quality equipment, they are not high-fidelity audio amplifiers. In other words, the precision stylus on an audiophile quality stereo system comes from a different world when compared to a high-gain guitar pickup with its tendency to act as an antenna for electromagnetic force . I can't dispute the claims of those who hear a significant difference between NOS tubes and current New Sensor or JJ tubes. As in all cases musical, let your ears be the final judge. If your amp sounds its best with NOS tubes, and you don't mind the high prices, definitely use only NOS tubes. At the same time, if
Here's a typical tube layout in Fender silverface and blackface amps. From left to right, the tube functions are rectifier (5U4 or 5AR4/GZ34), output (6L6/5881 or 6V6), output (same as previous), phase inverter (12AT7), vibrato (12AX7), reverb recovery (12AX7/7025), reverb driver (12AT7), preamp vibrato channel (12AX7/7025), and preamp normal channel (12AX7/7025). Most amps will have a tube layout chart with designated tube types attached to the inner cabinet.
Opposite: Using a current rather than reissue design, the Hot Rod Deluxe has been available in a variety of coverings and colors. Fender Musical Instruments Corporation
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your budget has you deciding between a pair of NOS output tubes for your amp or a quality hum bucker for your guitar, and your goal is improved sound, I'd go with the hum bucker.
Preamp Tubes The standard issue preamp tube for Fender amps has usually been the 7025 or the later 12AX7. While these tubes are interchangeable and not much different, the 7025 is considered a lower noise version of the 12AX7. The 7025, being no longer produced, is becoming rare and expensive. Luckily, there are many varieties of the 12AX7 in current production and many of them are excellent tubes for guitar amp application. Some manufacturers, such as Electro-Harmonix and JJ, make gold-pin variety 12AX7 tubes that cost slightly more than nongold pin versions. The idea behind this involves making better contact with the tube socket. This is vital in the first-stage preamp since this is the first tube that the tiny guitar signal reaches, and adequate tone shaping at this stage is vital. Yet, if the pins of your tube socket aren't also gold plated, these tubes won't really matter.
The Value of Good Preamp Tubes One ofthe easiest and quickest modifications you can make to your Fender amp is to install an excellent preamp tube . And keep a variety of preamp tubes available to alter or shape your sound for different uses .
The JJ EC83S and Sovtek 12AX7LPS preamp tubes have spiral filaments for reducing background hum.
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A spiral filament tube, such as the JJ ECC83SI12AX7S (ECC83 is a European designation for a 12AX7) or the Sovtek 12AX7LPS, is a better choice for standard tube sockets. In fact, the JJ ECC83S is considered by many to be the best sounding current production preamp tube. It has rich tones and excellent frequency separation, and the complex harmonics make this tube musical. By the same token, the Sovtek 12AX7LPS is extremely responsive tonally and as musical and rich as the JJ tube. It should definitely replace the Sovtek 12AX7WA, which, while being the standard issue for new Fender amps (and many others), is markedly inferior to its spiral filament, long-plate brother (LPS standing for long-plate spiral, by the way). Both the JJ and Sovtek tubes cost less than $15. But what is spiral filament? Because the preamp tube has to amplify the tiny guitar signal, it has the capacity of amplifying every other tiny signal presented to it. Often in this situation, the AC filament used to heat the tube's cathode can also generate a signal of its own in the form of a background hum. One method of eliminating this interference that you may have noticed is the twisted wires going to the pins of the tube socket feeding the filament. Tightly twisted wire forces the signal radiating from the wires to cross itself at 90-degree angles which, in effect, works to cancel the signal radiation. This same principle is applied to spiral filament tubes. Simply put, the 90-degree wrapping of the wires is continued inside the tube. The result is a much quieter tube, not quiet in the sense of gain, but quiet in the sense of canceled background hum. One of the quickest and easiest modifications for your Fender is installing an excellent preamp tube. Another quick and easy mod is keeping on hand a variety of preamp tubes to alter or shape your sound for
The Sovtek 12AX7LPS on the left has larger plates than the Philco 12AX7A (center) and Sovtek 12AX7WXT (right). The larger plates render an excellent balance across the frequency spectrum with rich tones; however, in small combos at high volumes, the larger plates can sometimes become micro phonic and pick up vibrations.
Preamp Tube Distortion The distortion you hear from your Fender amp is usually from the preamp tube . Using a lower gain, higher current preamp tube will give you more headroom before preamp breakup and a cleaner sound.
various applications. For this purpose, other preamp tubes in current production include 12AY7, 12AT7, and 12AU7. The primary difference between these tubes is their voltage gain, which is a tube's amplification factor. The chart below indicates the amplification factor of common preamp tubes used in Fender amps. One thing to keep in mind is that the gain of tubes is not in direct ratio; that is, an amplification factor of 100 is not twice as loud as that of 50. In fact, with the tubes listed, those with lower gain have a higher current capacity, meaning, they can push the following stage harder. This becomes important for phase inverters and reverb drivers where the higher current capacity and ability to drive power tubes and reverb tanks makes the lower-gain tubes more suitable for these applications. The 12AY7 tube is perhaps best known for its preamp application in the famous Fender tweed Bassman and Deluxe, where their rich tone and warm coloration contributed to the bluesy breakup distortion these amps are famous for. In fact, Fender's move to the 7025112AX7 wasn't so much aimed at improving preamp tone as it was at developing a cleaner sound. Simply put, the higher gain, louder 12AX7 stays clean at low to mid-volume; therefore, the tube was used not for its higher gain but for its ability to stay clean longer. Of course at the volumes rock musician use, the 12AX7 is definitely used for its high gain, which brings with it a rich breakup. NOS versions of 12AY7 tubes currently sell for around $25 and thus make excellent choices for trying out good NOS tubes (the 7025/12AX7, on the other hand, goes for well over $100). Electro-Harmonix, the only current producer
The 12AY7 was the standard preamp tube for tweed-era Fenders. It has lower gain than the 12AX7 and a warm, bluesy quality.
,
The 12AU7 and 12AT7 are normally used as phase inverter or driver tubes due to their high current ratings. They are also good choices for experimenting with different preamp tones.
of this tube, puts out a fine version, the 6027A/12AY7, which is also worth trying. This tube is well-balanced along the frequency spectrum and produces a smooth, warm, thick tone. The bottom line, here, is that the 12AY7 has a characteristic sound of its own, and while it is not as loud as the 12AX7, it does provide plenty of drive. Indeed, a popular and easy modification to the Bassman reissue is to replace the first preamp tube
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Gain of Common Preamp Tubes Tube Type
Amplification Factor
Max. Plate Dissipation
Max. Plate Current w/25DV on Plate
12AX7
100
1.20 watts
1.2 milliamps
12AT7
60
2.50 watts
100.0 milliamps
12AY7
40
1.50watts
3.0 milliamps
12AU7
17
2.75 watts
10.5 milliamps
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Replacing Vacuum Tubes How do you replace tubes? This is probably the easiest modification you can perform since all that needs to be done is remove the back panel, pull out the tube (in silverface and blackface, the first one on the rightfor normal channel and the second one from the right for vibrato/reverb channel), and push in the replacement. Preamp tubes are not keyed like larger power tubes. Instead, they have a gap between the first and ninth pin, which needs to line up with the gap in the tube socket. Furthermore, preamp tubes don't need to have bias adjusted as they are cathode biased.
Sockets for 9-pin tubes have a gap that matches the space between pins 1 and 9.
Sockets for 8-pin tubes have a gap on the side of the center hole that matches the key in the tube stem between pins 1 and 8.
on the right (12AX7) with a 12 AY 7. This will help to restore some of the original vintage Bassman sound. The 12AU7 and 12AT7 tubes are perhaps best known as phase inverters and output stage drivers, as discussed below. When used as preamp tubes they provide varying degrees of gain and variety in sound. Some people find their sound to be thinner than that of the 12AX7 and 12AY7, but they are worth a try, especially since even NOS versions of these tubes are relatively inexpensive. Why replace preamp tubes? First, as stated, this is a quick, cheap, easy, and reversible modification that will definitely alter the sound of your amp. Second, substituting a 12AY7 for a 12AX7 will result in a more responsive and wider range volume control. As you've most likely noticed, a common feature of high-gain amplifiers is an often overly sensitive volume control that goes from nothing to loud in about 2 notches. The lower-gain tube will allow you to dial in your volume more easily. Third, a lower-gain, higher-current tube can complement effects boxes used between guitar and amp input. Also, for acoustic instruments and harmonica, a lower-gain input tube is a must. Finally, on the Fender 6L6-pair amps, it takes a great amount of volume and drive to get the output power tubes to break up into distortion. Often the distortion you hear is that of the preamp tube, which for some musicians is more harsh than the dynamic texture of output tube distortion. Using a lower-gain, higher-current preamp tube will give you more headroom before preamp breakup and a cleaner sound. When the amp does go into distortion, more of the dynamic character of the output tubes will come through. Another popular preamp tube used primarily in some boutique amplifiers and made famous by its use in early AC-30 Vox amps is the EF86 pentode. Being a pentode means that the EF86 has well over twice the gain of a triode such as the 12AX7. While this is a beautiful sounding tube with a warm contoured distortion (unlike a triode), this tube is not without problems. One reason
While the EF86 pentode will physically fit into the standard dual-triode socket, extensive modification needs to be performed to make it work in Fender amps.
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Vox stopped using them was due to their susceptibility to vibration damage, especially considering that the AC-30 is a medium-sized combo stuffed with either a 1S-inch speaker or a pair of 12-inch speakers. Because of its more complex construction (the pentode has the added screen grid), shock and vibration can rattle the tube innards, which, when amplified, become extremely noisy. Also the tubes tend to be microphonic, meaning that without proper shielding (but sometimes even with proper shielding) the high gain of the tube basically turns it into a small microphone that picks up rattles and hums and sends them through the amp along with the guitar signal. But having said all that, I have made several 20-watt heads using the EF86 and can report that it is a magnificent tube in the proper application. The overdrive produced by this pentode is nothing less than stellar. Back to the downside, however, modifying a 12AX7 preamp circuit to work with the EF86 is not for the squeamish. While the tube will fit easily into the 9-pin socket, all the pins need to be rewired and virtually every associated component has to be replaced. Nonetheless, I will provide step-by-step instructions in Chapter 10 for those with more-thanbasic technical skills and for those who feel it's time to boldly dig into their Fender chassis. To those in the latter camp, though, no guarantees will be made.
Phase Inverter, Reverb, and Vibrato Tubes The phase inverter, reverb, and vibrato tubes are the middle tubes in the standard Fender chassis. On silverface and blackface, the third tube from the right is the reverb driver, which, as the name implies, drives the signal through the reverb tank mounted on the floor of the amp cabinet. Since the signal suffers immense insertion loss in passing through the reverb tank, a reverb recovery tube, fourth from the right, brings the signal back up to strength (and then some). The fifth tube from the right is the vibrato tube, which produces the tremolo effect. The sixth tube from the right is the phase inverter/driver tube, which essentially splits the signal into two out-of-phase signals, one for each of the two output tubes (or, in the case of the Twin and other 100watt amplifiers, the four output tubes). Through the 1960s and 1970s, Fender's standard tube for both the phase inverter and the reverb driver was the 12AT7. In older amps, the 702S/12AX7 was used, but most of these amps have different types of phase inverter circuits, most notably the cathode follower (used in the famous tweed Deluxe), which isn't as symmetrical and doesn't have as much drive capability as the long-tailed pair 12AT7 circuit. The name "long-tailed pair" refers to the long tail of resistors that tie the cathodes of the 12AT7 together. This type of phase inverter is actually a differential amp and, basically, what it does is take the difference between
the two inputs to the grids of a dual triode tube (the 12AT7) to produce two out-of-phase outputs to drive the power tubes. The 12AT7 is especially suited for this task, since it has the high current plate capability needed to drive the output tube grids, plus a smooth, well-balanced character. Recently, with the Hot Rod series, for example, the phase inverter was switched to the 12AX7 and the reverb became solid state (thus no tube). Replacing the phase inverter with a 12AT7, with its lower-gain and higher-current capability, will tighten up the sound of the Hot Rod and place the sonic focus more on the output tubes. In effect, the driver signals have more current to back them up, while the lower gain helps to reduce the phase inverter distortion. On the other hand, replacing a 12AT7 inverter with a 12AX7 boosts overall preamp gain (preamp plus inverter) as well as pushes the output tubes into breakup earlier, and even though the current capability is lower, the 12AX7 will still function well. Therefore, if you want higher gain and more preamp breakup from your silverface or blackface, definitely try a 12AX7 in the phase . . . lllverter posItIOn. As for the reverb driver tube, there are mixed opinions regarding the types of tubes that can be used. Given the fact that in a Fender, between the high plate voltage and the heavy drive requirements, this tube takes a beating, I recommend only using the 12AT7. Moreover, of all the tubes other than the two output tubes, the reverb driver is likely to be the one with the shortest lifespan. Yet, if you find your Fender's reverb to be a little overwhelming (as some guitarists do) or if you'd like to have a lower range on the reverb control, you might consider using a 12AU7 for the reverb driver. This tube will perform just as well as the 12AT7 but with much lower gain (and thus less reverb drive). I've even used a 12AY7 in this position with good results since it falls between the 12AT7 and 12AU7 in gain. Again, give it a try. As I've said all along, guitarists can really tailor their sounds by incorporating tube changes. Another means of manipulating the amount and control of reverb involves the reverb recovery tube . While the 12AX7 is standard for this duty, a 12AY7 in the recovery position not only lowers the reverb threshold, it also results in higher headroom and a cleaner preamp signal. I recall one guitarist who wanted a cleaner amp so that he could get more versatility out of his effects pedals. I swapped out the first preamp tube with a 12AY7. He indicated there was improvement but something was still not right. After a couple of inquiries I realized that he was reacting more to the distortion in the reverb section rather than in the input preamp. I reinstalled a 12AX7 in the input preamp and used the 12AY7 as the reverb recovery tube. This fixed it for him. The point is that gain and
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Long-Tailed Pair Phase Inverter Function
Signal
Out
I
II
Signal In
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I
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.... .....
____ 1
II
C .....
..........
~~
~ A ~
~~
B ..... ....
~~
A ~
I
II II
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• Signal"
Out
.
Negative Feedback In
The "long-tailed pair" phase inverter so named for the resistor "tail" tying the dual triodes' cathodes together. The meter lead in the photo indicates the junction of the two grid resistors (A, in the schematic), the cathode resistor (B). and the "tail" resistor (e), which sets the bias for the phase inverter.
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headroom obviously aren't things that are equally dispersed throughout the amp. Rather, there are differing types of distortion associated with the input preamp, reverb, and the phase inverter. By experimenting with these tubes, you'll discover the interesting sonic qualities that your Fender is able to produce. The last tube to discuss in this section is the 12AX7 vibrato tube. Essentially, this tube is not part of the signal chain and has little sonic effect on the amp. Instead, it functions in the tremolo circuit to either cut the bias on and off in the smaller Fenders or to run a phase-shifting oscillator in the larger Fenders that in effect cuts the signal on and off prior to its entering the phase inverter. Replacing this tube with other types won't change your sound . It might, however, alter you tremolo effect. One useful feature of this tube not really intended by Fender is as an emergency place holder. This means if your preamp tube becomes noisy or microphonic, you can swap it with the 12AX7 vibrato tube (providing that tube isn't noisy or microphonic). Basically, it doesn't matter if this tube is replaced by a noisy one since all it does is oscillate.
Winged "C" 6L6GC has a warm, clear breakup and amazing definition. In general, the Sovtek varieties are bold, loud, and have a distinctly heavy breakup, and the 11 is tonally complex, being both bright with highs and thick with lows. While being loud like the Sovteks and 11, the Electro-Harmonix 6L6 tends to be a little darker and slightly more subtle in saturation during breakup. Overall, I've found that the differences between brands and types are relative to the amp you're using. In other words, a pair of 11 6L6GCs will sound different in a Super Reverb than in a Pro Reverb, because even though these amps have similar circuitry and wattage rating, the output transformers, not to mention the speaker arrangements, differ considerably. The best way to determine which tubes sound optimal in your amp is to try a few for yourself. Most current production 5881/6L6 tubes are less than $50 a pair (EH, 11, and most Sovteks are less than $30 a pair). Finding out what rates as the best sounding tube is a relative and subjective process indeed.
Output Tubes The mainstay of Fender power has been the legendary 5881/6L6 output tube. While some tubes are marked with both designations (for example, the Sovtek 5881/6L6WGC), the 5881 and the various 6L6 types are slightly different. In fact, the 6L6GC has a 30-watt maximum plate dissipation rating, while most 5881 as well as 6L6WGB tubes are rated at 25 watts. However, the Sovtek 5881 WXT is a 30-watt tube. This can definitely be confusing. Yet, keep in mind that most 6L6 tubes manufactured today are of the GC or WGC, STR, and WXT variety and thus have the 30-watt rating. One way through this is to treat tubes without a GC in the suffix as if they have a 25-watt rating; they'll still sound good biased a little cold. Sonically, there are subtle textural differences, mainly in the definition (some feel a little "punchier" than others). For example, I've found the Sovtek 5881WXT, a standard in many amps, to be a little lackluster when it comes to overdriven dynamics and tonal harmonics. The Groove Tube 6L6GC has a similar crispness, but more dynamic harmonics, while the
The Sovtek 5881/6L6WGC and Sovtek 5881WXT are found in many Fender amps.
Power Output An amp's overall power outp ut is determined primarily by the output transformer as well as the amount of voltage .
The Groove Tube 6L6 is standard equipment in many Fender amps.
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The new production Tung-SoI6L6GC STR comes in a short bottle and is a rugged, bold tube.
The Electro-Harmonix and JJ 6V6tubes have a very different look. The EH bears the classic 6V6 shape, while the JJ uses a larger bottle. Both tubes have vastly improved ratings over earlier current production 6V6s.
To me, the classic Fender sound comes not as much from the 6L6 amps as it does from the lower power 6V6 amps, such as the Deluxe Reverb and Princeton Reverb, and the even lower power Princeton and Champ, the primary difference between the two groups being that the former uses a push-pull design (using a pair of output tubes) and the latter uses a single-ended design (using a single output tube). The downside for many guitar players regarding these amps is precisely their lower power capability. The Deluxe Reverb pushes an albeit loud 20 watts, while the Champ measures a mere but sonically impressive 5 watts. Yet, the relatively lower power of the 6V6 amps is largely responsible for their excellent sound. Simply put, cranking these amps to get loud volume drives the output tubes into distortion much earlier than their 6L6 counterparts. Add to this the fact that the 6V6 has a creamy, smooth distortion that comes on early. Now, even though these tubes are considered lower power, when used at the higher voltages that they are in the Deluxe Reverb, these tubes are plenty loud. The tweed Deluxe, on the other hand, and more so the Champ, can be overpowered on stage by even a moderately loud drummer; however, in the studio or when microphoned and run through a venue's sound system, the warm tones and deep, smooth overdrive of these amps are hard to beat. As fine as the 6V6 sounds, it's interesting to experiment with different tube types in the Fender 6V6 amps. The obvious choice is to try a pair of 6L6s. This is a piece of cake with a Fender Reverb, which has adjustable bias allowing various levels of plate dissipation to be dialed in. The matter is a little more complex with the tweed Deluxe and the single-ended Fenders. These use cathode bias and can't be adjusted (as discussed in the previous chapter). However, don't let that deter you. While it is a good idea to increase the
wattage of the cathode resistor and tweak to find the right value, if you're experimenting with tone, a simple swap won't hurt things. For permanent use of 6L6s, you should up the rating of the cathode resistor to 10 watts. A more pressing matter is the extra filament current drawn by the 6L6 in comparison with a 6V6. On the other hand, my experience has been that the Fender power transformers do fine with the extra filament draw. Still, you should perform the simple test regarding filament draw that I discuss later in this chapter. One thing you really don't want to do is to burn out your power transformer. One thing warrants mentioning here. While swapping lower powered 6V6 tubes with their higher powered 6L6 counterparts will give your amp a different tone, it will not make the amp much louder. An amp's overall power output is primarily determined by the output transformer as well as the amount of voltage. Yet, there will be an appreciable difference in sound and it will be somewhat louder. I've found the current production Tung-Sol 5881 to be an excellent candidate to replace the 6V6. In fact, this particular 5881 has a lower plate voltage rating, about 400 VDC, much lower than other current production 5881 or 6L6 tubes. It's an excellent choice, by the way, for the Bassman Reissue or blackfaces using a tube rectifier. These Tung-Sols have a complex compression and vintage tone quality. While an excellent pair of NOS 6V6 tubes runs around $90, a pair of Electro-Harmonix runs around $20. Indeed, EH tubes are an excellent choice not only price-wise but sonically, especially with their great sustain and smooth tone that grows ever richer when overdriven. Another excellent new production tube is the JJ 6V6. Besides having a well-defined character, crystal clear when clean and thick and detailed when overdriven, these tubes can take higher voltages
The current production Tung-Sol 5881 is a 23-watt tube with a 400-volt plate rating, making it unfit for use in amps with solid-state rectification; however, it's a perfect choice for Bassman reissues, tweeds, early blackfaces, and 6V6 amps.
The 7027, or "Ampeg tube," is virtually the same as a 6L6 except for internal pin connections. This fact and the necessary removal of wires and resistors from pins 1 and 6 on the output sockets mean that installing these tubes in Fender silver- and blackfaces is not worth the bother. They can be used, however, in the Hot Rod series without any problems.
Typical Fender Power Tube Sockets Screen Grid (+420 to 460 VCD) Filament (6.3 VAC) Screen Grid
Plate (+420 to 460 VCD) Filament (6.3 VAC)
....:::::::= Signal Pins 1 & 2 are not connected inside tube.
The typical Fender power tube sockets have two unused pins, 1 and 6, which serve as binding posts for the output tubes' screen and control grid resistors in blackface and silverface models. Screen-grid resistors are 470 ohms, 1 watt, while the control grid resistors, or "grid stoppers" as they are often called, measure 1.5 K-ohm, 1/2 watt. Note that many ofthe early Fenders didn't use grid stoppers on the outputtubes. Also, in Hot Rods and reissues with printed circuit (pc) boards, pins 1 and 6 have nothing connected to them.
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than most other 6V6 representatives. Because of their ruggedness and higher power capability, 11 6V6s can successfully replace the 6L6 tubes in a Hot Rod Deluxe, and really make that amp come alive. Currentproduction Tung-Sol6V6s also work great. If you have a Hot Rod, I recommend giving these tubes a shot; they add more growl and color to the sometimes uninspiring tone of the Hot Rod when run at lower
volumes. Also, they sound better and their detail really comes out when you play the Hot Rod with its clean channel and avoid the often brittle overdrive function. The biggest thing to remember is that you have to adequately bias the Hot Rod after installing the tubes, as described in the previous chapter. With simple bias changes, other output tubes that can be used in the Hot Rod amp series include the KT66
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The bigger bottle KT66 requires different clamps and won't work in all applications. Shown here are a Tung-Sol and a Sino.
A 616 "bear trap" clamp is on the left and KT66 spring retainer clamp is on the right. To replace tube clamps, remove the two socket mounting screws.
(discussed below) and the 7027 (used in most Ampegs). One thing you need to be aware of is that the 7027 is basically the same tube as a 6L6, except internally pin 1 is connected to pin 4 and pin 5 to pin 6, thus making them unusable in blackface or silverfaces unless you remove the wires and resistors from pins 1 and 6 of the sockets. The 6L6 does not use these pins and, as a result, Fender used them for binding posts. Since the Hot Rod series and most of the reissues don't use pins 1 and 6, you can swap in the 7027 with a basic bias adjustment. Yet, it is always a good idea to ensure that nothing is connected to pins 1 and 5. You can do this by removing an output tube and measuring for ohms between the socket pins 1 and 5, then between socket pins 1 and 6, with the meter set to about a 20-K-ohm range. There should be no reading (or infinite ohms). One tube to keep in mind, albeit somewhat cautiously, is the KT66, which was originally a British complement to the American 6L6 and was reputed to be a better quality tube. Those distinctions no longer apply since tube production ended in Britain and the United States. With the proper bias, the KT66 does offer a louder, bolder tone with dynamic lows. Overall,
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it's a better sounding tube than the 6L6. As with the 588116L6, subtle differences in tone and texture exist among the various brands of KT66, and sound distinction depends on the amp used. Note that in blackface and silverface, the KT66 Sino generally works best at a higher bias, typically 70 to 75 percent of max plate dissipation. See the bias information in Chapter 3. That being said, though, whenever changing power tubes, it's a good idea first to bias them cold-around 60 to 65 percent max plate dissipation-and see how you like the sound. Also, check to make sure the plates aren't getting red, which indicates too high a bias setting. In general, KT66s have a lower rating (25 watts) than 6L6GCs (30 watts). There are exceptions, though. I've found that Tung-Sol KT66s sound best in the Hot Rod Deluxe when biased around 80 mV (for the pair, which equals 40 mV per tube). There are several caveats regarding the KT66 that need to be considered. First, this tube is physically larger than the 6L6 and sometimes won't fit in an amp head. In combos, the standard tube clip has to be replaced with the spring-type retainer made for these larger tubes. That replacement, though, is pretty straightforward and not difficult. It would be foolhardy to install KT66s in a combo without any clamps since vibration combined with the tube's heaviness will certainly cause them to work loose from the socket. Even in the somewhat roomier combo, the tubes sit close together. Thus, adequate airflow is a must. This can be as simple as keeping plenty of open room behind the amp, or even running a small fan into the back of the amp. Perhaps the most important point to consider regarding the use of KT66s is their increased filament current. At 1.3 amps compared to the 0.9 amp of a 6L6, a pair of KT66s will draw an extra 0.8 amp. Depending on your amp's power transformer, this could overheat the transformer and eventually cause it to burn out (although, I've only had this become an issue with single-ended Champ and Princeton amps). If the extra current draw becomes a concern, one way to reduce the load on silverface and blackface models is to remove the vibrato tube, which in turn will reduce the current draw by about a third of an ampere. The overall extra current draw will now be about a half ampere, which the transformers on these amps can easily handle. I've used KT66s in early Bassman reissues, as well as various silverface and blackface 6L6 models with no problem, but the filament draw should still be checked as described at the end of this section. The same goes for the Hot Rod series. A pair of KT66s sounds especially dynamic and bold in these amps and vastly improves their sonic qualities. When using the clean channel, the Hot Rod comes alive with such a smooth, rich breakup coming early on that you will forget all about using the Hot Rod's noisy overdrive channel.
The Tung-Sol KT66s, in combination with a Sovtek 12AX7LPS as the first-stage preamp tube, gives this Hot Rod Deluxe a bold, richly dynamic sound that rivals many vintage amps.
There is one thing to be aware of regarding any KT66 that has a metal base (namely the current production Tung-Sol version). The TungSol KT66 uses the same bottle as the Tung-Sol 6550, which has the metal base attached to pin 1. In the Fender reissues and Hot Rod models this doesn't matter since pin 1 of the socket is not connected to anything, but on the silverface and blackface models, pin 1 is used as a binding post for the grid resistor and its input wire. Never use the metalbased tubes in these amps unless you unsolder the resistor leg and wire from the socket, leaving them connected and free-standing. See the photo and instructions at the end of this section. If you have any suspicion regarding pin 1 of the tube, place your multi meter on resistance or ohms setting, hold one lead on the metal ring and the other lead on pin 1. The meter should read infinity. If it reads or close to ohms, then pin 1 is connected to the shield. Finally, there is the EL34/6CA7 "Marshall tube." First the obvious point: Replacing a Fender's 6L6s with
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On any KT66 with a metal base, check for resistance between pin 1 and the base. If it reads 0 or near 0 ohms, and you still want to use this tube, read the instructions regarding the EL34 tube.
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a pair of EL34s will not make the amp sound like a Marshall since, for one thing, the output transformers have different specs. Yet, both types of output transformers will work with both types of tubes. In fact, I once built an amp for a guitarist who wanted a Fender output transformer driving a pair of Marshall tubes. For that build, I used an OT with Bandmaster specs and fed it with a pair of 6CA7 Electro-Harmonix tubes (excellent tubes, by the way). The amp didn't sound like a Fender or a Marshall, and that was the point. The guitarist wanted a somewhat unique sound without paying big bucks for a custom output transformer. This hybrid did the trick. But back to the discussion at hand. The primary difference between the 6L615881 and the EL34/6CA7 is a thicker midrange and quicker breakup on the part of the EL34, while the 6L6 tends to remain cleaner yet bold longer. Again, this is relative to the amps. A common belief is that the EL34 has more midrange than the 6L6, and while that is partially true, the primary reason why Marshall amps sound more mid-heavy than Fenders lies with the tone circuit. A case in point is the tweed Bassman (which, by the way, is the amp that "inspired" Jim Marshall's amplifier circuitry). Situated at the end of the preamp stages and driven by a cathode follower, the Bassman's tone stack delivers a rich midrange tone due to the capacitor, resistor, and potentiometer values. The sound produced through the 6L6s in that amp are mid-heavy indeed. So, what makes the EL34 unique? Aside from its use in Marshall amps, the tube does have a distinct tonal definition with harmonics that are extremely rich and lush. And, yes, the characteristic mids of the tube are there, but closely tied to the lows without a great deal of separation in the mid- to low-frequency range, making for the legendary sound of the Les Paul coupled to the Marshall stack. While this isn't a book about Marshall amps, if you want more separation in the mid-low range with slightly later breakup in a Marshall amp, swap out the EL34s for a pair (or, more likely, a quad) of KT77s or 6550s, both amazing tubes in their own right, loud yet smooth and thick. While the EL34/6CA7 tube and its cousins the 6550, KT77, and KT88 have been successfully installed in Fender, I would advise against it, primarily due to the fact that, first, these tubes draw even more filament current than the KT66 and, second, the bias circuit has to be modified. I have ran 6550s in Hot Rod Devilles with no perceivable problems, but I've never ran them for long stretches at a time, and I do not endorse or recommend it. Basically, when using 6550s, replace R61 and R62, the large 470-ohm, I-watt resistors to the right of each output tube socket, with even larger l-K-ohm, 5-watt resistors. This might be tricky, since the tube socket circuit board needs to be removed and the resistors are physically larger. After that, replace R76 (a 1.5-K-ohm, 112-watt resistor) with a
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The 6550, 6CA7, and EL34 can only be used in Fenders with modifications to the tube sockets and bias circuitry.
larger value; try 22-K-ohm, I-watt up to 39-K-ohm, 1-watt until you can read around 33 mV to 38 mVon pin 5 of one of the output tube sockets. Since the 6550, with its 35-watt plate dissipation, draws more current than should be drawn through the Deville, bias it cold, around 65 percent, or better yet use JJ EL34s or Electro-Harmonix 6CA7s (a darker, more complex tube). Just to stress the point, if you use 6550s or KT88s in your Fender, beware that you are operating the transformer beyond its ratings. Don't be surprised if everything goes dead and puffs of smoke rise in an expensive disaster from the back of your amp. If you would like to try 6CA7 or EL34 tubes in your Fender, be warned that you will not be able to use 6L6s again unless you reverse modification. In other words, you can't easily switch between EL34s and 6L6s like you can switch between KT66s and 6L6s. First, if the amp is a blackface or silverface, referring to the next photograph, remove the resistor and wire from pin 1. Skip this step if the amp is an early Bassman reissue (those with printed circuit boards) or Hot Rod model. In other words, if nothing is hooked up to pin 1, skip this step. Second, run a short piece of wire from pin 1 (which you've just freed up) to pin 8. Don't forget this step or your EL34/6CA7 won't work. If you have freed up pin 1 in order to run metal-based 6550s in your Fender, though, you don't need to use the jumper. Confusing? Well, some tube swaps aren't all that straightforward. If you want to know the reason, here it is: Beam tetrodes, such as the 6L615881 and the 6550, have a pair of beam plates (like a pentode's suppressor grid) that are internally connected to the cathode in the tube, while the EL34/6CA7 pentode has a suppressor grid connected to pin 1; therefore, you're connecting the suppressor when you add the jumper. Some hi-fi applications, by the way, make use of the separate suppressor connection for added stability and distortion elimination at higher
volumes, which is something we don't want with guitar amps. As a side note, there is somewhat of a historical controversy over whether a beam power tube such as the 6L6 is a pentode or a tetrode. Svetlana, for instance, calls its 6L6 a beam power tetrode. The confusion dates back to the 1930s when RCA first developed the 6L6 and designed it as a beam tetrode to avoid a patent dispute with Phillips over the pentode design. There are interesting differences between the designs that you can research on your own. Third, referring to the diagram on page 85, replace the 470-ohm screen resistors between pins 4 and 6 on the output tube sockets with 1-K-ohm, 5-watt resistors. For Hot Rod models, replace R61 and R62, as previously discussed. Fourth, these tubes will draw more filament current from the power transformer. If you proceed with this modification, make sure that you monitor the power transformer. An accurate method for monitoring the power transformer is to measure the 6.3 VAC filament voltage. If you are measuring the filament
When using EL34s or any tube that has pin 1 tied to the tube shield, remove the wire and resistor (1.5-K-ohm, brown-green-red banded) from pin 1 of the outputtube sockets and leave them attached only to each other. Make sure the exposed junction of the wire and resistor won't come in contact with the chassis or other socket pins.
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To install EL34tubes in a Hot Rod model, replace R61 and R62 with 1-K-ohm, 5-watt resistors and R76 with a 1-watt resistor (22-K-ohm to 39-K-ohm)thatwill bring voltage to pin 5 on one of the outputtube sockets in a range of about 33 mV to 38 mV while the bias potentiometer is turned .
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The filament wires are the greenish twisted wires that run from tube to tube. Turn the amp on, but leave it in standby. Allow the filaments to heat for about 60 seconds. Next, with your meter setto read AG voltage on a low scale, place one lead on pin 2 of an outputtube and the other lead on pin 7. You can measure the filament voltage at any tube, but the output tubes offer more room to help prevent meter lead slippage. Also, for added safety, you can run jumper leads from the pins to the meter leads.
Filament voltage can also be measured across the power lamp because it runs on the same 6.3 VAG line as the tube filaments. Here, I'm checking the filament voltage across the power lamp socket of a Hot Rod Deluxe after having installed a pair of KT66s.
voltage of a Hot Rod amp or a Bassman reissue, you simply need to remove the back cover, which you will already have done since you've changed tubes. For most Fenders, you'll need to remove the chassis to gain access to the tube sockets. The good news with this method is that you leave the amp in standby and don't run high voltage to the tubes. The photograph above demonstrates the method. If the filament voltage reads 6.2 to 6.4 VAC, you're fine (note that some amps can run as high as 6.7 volts). A reading of 6.15 VAC is marginal and may be fine, but the life of your power transformer will be reduced. Anything below 6.1 VAC, even 6.0 VAC, means that the current draw on your transformer is too great. If you still want to use the tubes, you'll need to do one of two things: Either find a replacement power transformer or install an extra filament transformer. Both require rewiring, drilling holes in the chassis, and, with a replacement transformer, possibly enlarging the mounting holes (not much fun, by the way). Finally, and very important, if you decide to use these tubes, bias them as per the directions found in
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Chapter 4. This will mean replacing the bias supply resistor. On silverface and blackfaces the value will be either 470 ohms, 1 watt, or 1 K-ohm up to 1.8 K-ohms, while on the Hot Rod models the value will be 1.5 K-ohms. Try various resistors from 22 K-ohms, 1 watt up to 39 K-ohms, 1 watt until you can read about 33 mV to 38 mV on pin 5 of an output tube socket while turning the bias adjustment potentiometer. You can also use a 50-K-ohm, 1-watt potentiometer in place of the resistor.
Rectifier Tubes While the choice of rectifier tube doesn't affect sound directly, it does determine the voltage level that in turn does affect sound. Note that not all Fender tube amps have rectifier tubes. Newer amps such as the Hot Rod models as well as some mid- to late silverface era amps use solid-state diodes instead of tube rectifiers. The easiest way to tell if your amp has a tube rectifier is to check the first tube on the left as you look in the back of the amp. If the tube is labeled 5AR4, GZ33, 5U4, or
5Y3, it is a rectifier. If, on the other hand, you find an output tube (6L6 or 6V6) as the first tube on the left, you have a solid-sstate rectifier. The three primary types of rectifier tubes Fender uses are the 5Y3 in tweed Deluxe and Princeton models; the 5AR4/GZ34 in tweed 6L6 models, most blackfaces, and a few silverfaces; and the 5U4 in early silverfaces and some late blackfaces. The basic differences between these tubes are their current capacities and the output voltage levels. Even though these tubes may be somewhat interchangeable, there are a few factors you need to keep in mind. First, the 5Y3 has a lower current rating and a lower output voltage than the other two. Most single-ended amps originally equipped with this tube, such as the Champ, should probably stick to using it since replacing it with a 5U4 or a 5AR4 will result in a higher operating voltage that might exceed the ratings of the circuitry made for the 5Y3. NOS versions of this tube are still plentiful and should be your first choice in replacement. Sovtek does make a 5Y3, but the tube is actually closer to a 5U4 and puts out more voltage than the standard 5Y3. A tweed Deluxe will work with this tube, but the voltage on the 6V6 plates can increase up to about the +400 VDC range rather than the usual +365 or so VDC, making the amp less bluesy. In other words, it will change the sound that made this amp famous. Yet, there are guitarists who prefer the increased headroom and louder sound this amp produces with the Sovtek 5Y3 or 5U4. The case of the 5AR4/GZ34 and 5U4 swap is a little more forgiving. In a Fender, the 5AR4 will probably put out around 20 to 25 more volts than the 5U4. It's common practice to replace the 5U4 with the 5AR4, especially as it concerns blackface models. Replacing the 5U4 in a silverface with a 5AR4 is also quite common, but you should consider that in most cases, silverface amps employ higher voltages than their blackface counterparts, the main reason being that higher voltages usually result in cleaner tones with higher headroom. Boosting the voltage even more means you have to push the volume even louder to get the output tubes to breakup. One other point to consider is that some silverfaces use a smaller value cathode resistor (1.5-K-ohm) on the reverb driver. This tube already operates at or beyond its rating with a 5U4. Punishing it with more voltage by using a 5AR4 might not be a good idea. As discussed later in the book, you should consider replacing the reverb driver's cathode resistor with a 2.2-K-ohm value if it is stocked with anything smaller. Fender wasn't necessarily consistent with this resistor value in mid- to late silverface models. My advice is to stick with the type of rectifier tube designated for a particular amp. As for replacement tube choices, NOS versions of the 5AR4/GZ34 are becoming scarce and so the price is high (well over $100). Fortunately, JJ makes
Converting from Solid-State to Tube Rectification Converting a Fender from solid-state to tube rectification requires not only drilling a hole in the chassis for the tube but also replacing the power transformer. The bottom line is that converting to a tube rectifier is by no means practical. The exception, of course, is the early Bassman teissue. From 1990 to around 2004, this amp came with solid-state diodes inside of a plastic plug that sits in the chassis where one usually finds a rectifier tube. Because the socket of the solid-state plug is wired the same as a rectifier tube socket, all you need to do is replace the plug with a 5AR4/GS34. Be aware that the high voltage will drop about +40 VDC, which means a few less watts of power but also a sound more akin to the vintage Bassman. In fact, if you buy a used reissue, chances are pretty good that it might already have the tube installed. If not, this is one of the easiest and most essential modifications you can give this amp.
These are 5Y3 NOS, 5U4 NOS, 5AR4/GZ34 JJ, and 5AR4 Sino rectifier tubes. Plate voltage can increase by 25 VDC when going from a 5Y3 to a 5U4 and increase by another 25 VDC when going from a 5U4 to a 5AR4/GZ34.
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an excellent version for around $20, as does Sovtek and Sino. NOS versions of the 5U4 are a little cheaper than those of the 5AR4 but are still expensive. Fortunately, again, JJ and Electro-Harmonix make great versions of this tube for less than $20. While tube purists will insist that these current production tubes have nowhere near the ruggedness of their NOS counterparts, you could buy more than a half-dozen JJ 5AR4/GZ34 tubes for the price of one NOS. I'd much rather have to replace a JJ prematurely than have to worry about my $130 NOS rectifier getting shaken to pieces every time I crank my Deluxe Reverb (which is all the time).
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OVfRHAUliNG THf SllVfRfACf, HOT RODOING THf HOT ROD, AND RfBUllOiNG THf RflSSUf The chances are good that if you purchase a Fender silverface amp, it will already be "blackfaced." For decades the much maligned silverface sound led not only technicians but almost anyone with a soldering iron to attempt reverting the silverface circuit to that of a blackface. Several books published in the 1990s as well as numerous web sites take the reader on a step-by-step course of rebuilding his or her silverface to blackface specs. At least two problems exist with this approach. First, the obvious: Circuits were mutilated. Because proper soldering is a skill, many would-be techs burned wires as well as components to get the blackface sound. Not only that, but often substandard parts were used or the wrong part altogether. The second problem is much simpler: No matter what you do, you will never turn a silverface into a blackface. While replacing the capacitors and resistors with higher quality components and rewiring the amp to blackface specifications will definitely improve the silverface's fidelity, the amp will never sound like a blackface, because the output transformer, power transformer, and speakers are not the same. These parts can be replaced, but the cost may not be worth the result. This chapter, therefore, will describe the silverto-blackface conversion not with the goal of making your silverface sound like a blackface, but rather to offer suggestions for improving silverface tone. In fact, I've met more than a few people who like the silverface sound, especially its ability to stay clean. Surf
and country music guitarists find the silverface's high headroom beneficial, but guitarists who extensively use effects pedals also benefit from these characteristics. The clean, loud sound of the silverface definitely takes pedals well. However, as discussed in Chapter 5, the silverface's lack of a true bias adjustment makes at least that blackface mod necessary.
Overhauling the Silverface After purchasing a silverface it is essential to remove the chassis and inspect the circuitry. If you're lucky, you'll find the old parts replaced with high-quality capacitors and resistors. Unfortunately, you're more likely to find a hodge-podge of parts with a wire or board burned here and there by a reckless soldering iron. However, that's fine in that these amps are relatively easy to work on and you can replace all the components with good quality parts for a reasonable price. One silver-to-blackface modification is vital: performing the bias modification demonstrated in Chapter 5. Another common, less vital, but easy modification is removing the 2,000-pf suppressor capacitors from pin 5 of each output tube. While these capacitors do shunt some of the higher frequencies to ground, don't
Silverface lore •
Opposite:The Hot Rod Deluxe boasts a bold, dynamic sound that rivals many vintage amps. This amp and extension cabinet are finished in the White Lightning scheme. Fender Musical Instruments Corporation
Many guitarists love the classic silverface sound and the amps' high headroom . And if you use effects pedals often, the silverface's clean, loud sound is ideal.
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This silverface shows the typical hodge-podge of replacement parts normally encountered.
expect to hear a great deal of a difference after clipping them from the sockets. However, removing them does eliminate an impediment to fidelity in the overall signal chain. The suppressor capacitors were installed by Fender to reduce the chance of parasitic oscillation caused by a change in lead dressing. Legend has it that when Fender changed from cloth-covered wire to plastic-covered wire around 1969, proper lead dressing (the way the wires are arranged) became difficult since the plastic wires were not as rigid as their cloth counterparts. Whether that's true or not, lead dressing did get messier with wires often jumbled together and randomly placed. Moreover, wires no longer passed under the board through holes drilled for that purpose; instead, they passed over the board in a rather haphazard manner. Why is lead dressing important? Because wires carrying high voltage emit slight electrical waves while other wires, such as grid input wires, are extremely sensitive; they must be kept at a distance or pass each other at 90-degree angles. When this doesn't happen, noise as well as oscillation can be induced. Removing the suppressor capacitors will improve the tone; however, if you do encounter oscillation, which would sound like your amp has suddenly lost half its power, simply replace the caps. You might also try to redress the wiring by referring to a blackface chassis as a model. There are many photos of
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blackface chassis on the web if a blackface amp isn't available. Toward the end of this section, I've provided a photo of a blackfaced silverface with proper lead dressing. Basically, keep the heater wires tightly twisted together and away from the other wires (running up above the sockets) and the input wires (those connected to pins 2 and 7 of the preamp, reverb, and phase inverter tubes) away from or at least not parallel to other wires. Also if the wires that run from the input jacks and volume controls to the tubes are not shielded, replacing them with shielded cable will help. In all likelihood, removing the suppressor capacitors probably won't cause any problems. After removing the suppressor caps, replacing input wiring with shielded cable, if necessary, and performing the bias modification, bias the amp and play through it. If you are happy with the tone, then you won't need to do any more "blackfacing." If, on the other hand, you want a little more gain feeding the output tubes as well as what many consider to be improved fidelity, try replacing the two 47-K-ohm phase inverter plate resistors with a 100-K-ohm resistor on the left and an 82-K-ohm resistor on the right (when viewing the circuit board with the tube connection side down), the standard blackface values. The reason these two resistors have different values is, broadly, that only one of the phase inverter's inputs receives a signal (the second input receives negative feedback, which technically is a
Shielded wire from the input jacks and volume controls should have the shielding connected to ground at only one end-that of the jacks and pots.
The shield of the cables is left ungrounded at the tube end.
signal but not a signal straight from the preamp). The side of the inverter that receives the preamp signal will have more gain than the side that doesn't; therefore, a lower value resistor is used as a means of providing balanced output signals. So why use the same value of 47 K-ohms for the plate resistors? Without getting into the technicalities of the role that biasing and plate resistance play with the phase inverter or of the overall functioning of the output stage regarding such things as impedance and loading, with the lower gain resulting from the lower valued plate resistors, the degree of unbalanced output is also lowered and, as a result, plays less of a factor. If you replace the phase inverter plate resistors, make sure that the grid-to-cathode, cathode, and tail resistors on the phase inverter (the infamous "longtailed pair") are 1M-ohm, 470 ohms, and 22 K-ohms, respectively. The tail resistor might also be a 27-K-ohm, which is fine. While you are modding the phase inverter, you might want to replace the coupling capacitor connected to the right-hand 1M-ohm resistor (as indicated in the top left photo on page 100) with a Mallory 150 or an SBE Orange Drop 716. Blackface amps generally used a O.OOl-uFcap while silverfaces tended toward a O.Ol-uFcap. Using a .0047-uF will give great bass response but not be flabby as it could be with anything larger than a O.Ol-uF, or not as attenuated as could be the cause with anything less than O.OOl-uF. I've seen a 0.022-uFused here as well. If you have several capacitors of varying values, go ahead and try each to find the sound you prefer. Finally, another blackfacing modification involves swapping the silverface SU4 rectifier tube with the SAR4 used in blackface models. Later silverface models use solid-state rectifiers, so this mod is for early
silverface amps. While switching the tube might seem like a restoration to the blackface models, this isn't entirely so. The SAR64 drops less voltage than the SU4, and, as a result, the voltages will be raised in the silverface. This isn't necessarily a bad thing, but again it isn't restoration because the silverface power transformers differ from those of the blackface in that the high-voltage windings put out more voltage than the blackface models. Thus, the silverface voltage will probably be closer to the blackface voltages by retaining the SU4. Some people claim, though, that rectifier tubes have different sonic qualities. This is doubtful. Probably more to the point is that the SAR4 has higher current capability than the SU4, and when pushed extremely hard the SU4 might sag, resulting in a compressed sound. I recommend leaving the SU4, but if you do use a SAR4 make sure that you check the cathode resistor on the reverb driver tube (the 12AT7 tube that is third from the right when looking into the back of the amp). This cathode resistor was lowered in some silverfaces from the usual 2.2-K-ohm blackface value. The reverb driver in Fender amps is already driven at or near its maximum rating so raising the voltage on the plate without raising the cathode resistor will decrease the tube's life. In fact, even if you retain the SU4, or if your amp has a solid-state rectifier, you should change the cathode resistor to 2.2 K-ohms (red, red, and red banded) if it hasn't already been changed. Bear in mind that a higher amp voltage yields a cleaner, louder sound. If your goal is high gain with warm tube distortion, you probably don't want to raise the voltage. Regardless of whether or not you blackface your silverface, you should replace the capacitors with Mallory lS0Ms or high-quality Orange Drops. Each replacement coupling capacitor should have a
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RG-174 shielded from Hoffman Amplifiers is 1/4-inch thick with 24-gauge center conductor.
I
•• 96
I
Carefully trim the plastic coating from the cable without cutting the shielding.
A wire stripper can be set to the same gauge as the wire being stripped.
minimum rating of 400 VDC, and preferably a 600 or 630 VDC rating. Furthermore, all resistors should be checked (by measuring the resistance) and those more than 10 percent out of range should be replaced. Few things cause more argument in the world of amplifier restoration than the choice of resistors. As discussed in the first chapter, types of resistors include carbon composition (used in blackface and silverface amps), carbon film, metal film, and metal oxide. I recommend metal oxide for the power supply (under the capacitor panel), metal film for plate, cathode, and slope resistors, and carbon composition for signal resistors. While silverface Fenders haven't really reached the collectible status of their blackface counterparts, who knows what the future may hold . Thus, if you are concerned about retaining the originality of a silverface, make sure to keep all the capacitors and resistors you remove. Store them in a zip-lock bag and if the day arrives when an original silverface becomes more valuable than it's worth as a working amp, you can reinstall the old parts and sell the amp to a collector who will probably never playa guitar through it. Replace all carbon composition resistors with metal film type where the resistor value is most critical (plate and cathode resistors as well as the tone stack
slope resistor). The value of the reverb mixing resistor (3.3-M-ohm), on the other hand, needs not be precise. The carbon comp can stay there, as well as on the input jacks (one 1 M-ohm and two 68-K-ohm resistors on each pair of jacks). The grid resistor used for the reverb driver and recovery tubes can also retain the carbon comps (1 M-ohm and 220 K-ohms, respectively, with the latter being mounted on the reverb input jack) as can the 470-K-ohm isolation resistor associated with the reverb control pot wiper arm. Another use where resistor value isn't critical and where the carbon comps can be retained is as mixing resistors for the normal and vibrato channels (each at 220 K-ohms). The output tube grid stoppers could stay with the carbon comps except that the screen resistors mounted on the same sockets usually run hot, exposing the grid stoppers to constant high temperatures over decades. I recommend replacing the screen resistors with 470-ohm, 2-watt, or greater, metal oxide types and the grid stoppers with 1.5-K-ohm, 112-watt metal film or new carbon composition resistors. In all cases, when retaining a carbon composition resistor you should check its value with a meter, being aware that the resistance can drift with temperature and voltage, meaning that it might check fine while
When blackfacing the phase inverter circuit, first remove the 47-K-ohm phase inverter plate resistors.
Second, replace the 47-K-ohm resistors with a 100-K-ohm on the left and an 82-K-ohm on the right (when viewing the circuit board with the tube connection side down).
cold and disconnected from the circuit but could change to a different value during operation. Yet, in the few places mentioned above, if a carbon comp resistor is in range when cold, the possible drift during use shouldn't affect the amp's operation. 1 usually change out all the resistors of the long-tailed pair with metal films; however, you could retain the I-M-ohm grid resistors if they are matched to within 5 percent of each other. As for the plate resistors, 1 recommend using new metal films there. Carbon comps are often noisy as plate resistors (hiss and crackling). There is debate involving resistor choice for the second-stage preamp (the plate resistors wired to pin 6 of the first two tubes on the right). Legend has it that "mojo" resides here. If you decide to use carbon
comps as plate resistors, 1 recommend using new ones with a 1/2-watt rating. Finally, as for the bias splitter resistors, 1 recommend any resistors that are closely matched (1 prefer within 1 percent) to aim for the same amount of negative bias voltage on the output tube grids. I've used both carbon film and metal film, again, as long as their resistance is closely matched. As for the vibrato circuit, check the values of the resistors; more than likely they will be fine. Since this is an oscillator circuit and not part of the signal chain, I usually retain the original caps and resistors unless they are out of range. The type of resistors and capacitors employed here won't affect the sound. Regarding the reverb mixing resistor mentioned earlier, you can reduce it to 1 M-ohm (carbon comp
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Blackface Circuit Board
Output Coupling Ca acitors
Reverb Control Isolation Resistor
Ne ative Feedback Resistors
Phase Inverter Bias Resistor
Bias Splitter Resistors
Vibrato Resistor Capacitor Network
Channel Mixing Resistor
o
to-Isolator
Reverb I Resistor
Reverb Recovery Grid Resistor
25
25
IJF +
IJF +
.001 ~F
Phase Inverter Plate Resistor
NFB Coupling Capacitor
Phase Inverter Cathode Resistor
Phase Inverter Coupling Capacitor
Phase Inverter Grid Resistors
Vibrato Plate Resistor
Reverb Recovery Cathode Resistor & Capacitor
Reverb RecovE Plate Resisto
Opto-Isolator Resistors Final Preamp Coupling Capacitor Vibrato Cathodes Resistors & Capacitors
Typical blackface circuit board showing component functions.
Reverb Driver Cathode Resi r & Capacitor
lixing
Vibrato Channel Treble, Mid, & Bass Capacitors
Reverb Driver Grid Resistor
Normal Channel Coupling Capacitor
Normal Channel Treble, Mid, & Bass Ca citor
~everb
Bypass apacitor
Reverb Input Coupling Capacitor
250pF
OpF
25
25
!-IF
!-IF
+
+
100K
y
s
Normal & Vibrato Channels 2nd Preamp Cathode Resistor & Capacitor
100K
Vibrato Channel Slope Resistor Vibrato Channel Preamp Plate Resistors
Vibrato Channel 1st Preamp Cathode Resistor & Capacitor
Normal Channel 1st Preamp Cathode Resistor & Capacitor
Normal Channel Slope Resistor Normal Channel Preamp Plate Resistors
Vibrato Channel Coupling Capacitor Reverb Output Coupling Capacitor
Typical blackface circuit board showing component functions.
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•• 100
For a completed blackface phase inverter circuit, the resistors should be a 22-K-ohm on top, l-M-ohm on left and right, and a 470-ohm in the center. If not, replace them. Also replacing the coupling capacitor atthe phase inverter input will improve overall tone.
For optimum reverb driver tube life, make sure its cathode resistor is 2.2 K-ohms .
It's a good idea to replace the old silverface capacitors. Here, the bass capacitor in the normal channel tone stack is being removed.
The originaI3.3-M-ohm reverb mixing resistor (on left) as well as the 220-K-ohm channel mixing resistors (on right) can be retained unless they have drifted far out of range (beyond 20 percent).
or metal film} without losing substantial reverb. Also, make sure to replace the bypass capacitor with a silver mica. The original is 10 pf, but you can raise the value up to 68 pf. The purpose of this resistor/capacitor combination is to shunt most of the signal to the reverb driver and pass a small, high-frequency signal, which will mix with the returning reverb signal. If the capacitor is too large or the resistor too small, more frequency or a wider frequency signal will pass without going through the reverb, resulting in weak reverb with a possibly muddier tone.
After replacing the coupling capacitors with Mallory lS0Ms or high-quality Orange Drops (including 0.0033 blocking cap between plate and reverb control), also make sure to replace the SOO-pf reverb coupling capacitor as well as the 2S0-pf treble capacitors with silver mica capacitors. (As will be discussed in Chapter 8, you might experiment with 330 pf and 500 pf for treble capacitors.) If you were to replace only one type of capacitor on the circuit board, make sure it would be the electrolytic cathode bypass capacitors. If these are
These plate resistors are being replaced with metal film variety. The color-band resistor code often differs with metal film resistors; refer to the appendix for an explanation.
These original carbon composition power resistors will be replaced with metal oxide resistors. The 220-K-ohm bleeder resistors on each side of the 100-uF capacitor on the right are less critical so the carbon comps could remain; however, it's best to replace them as well.
The electrolytic power capacitors in this amp have been replaced with Sprague Atom brand capacitors while the power resistors are now high-quality metal oxide type.
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Ceramic disc treble capacitors should be replaced with a higher quality silver mica variety.
A silverface board with newly installed Mallory 150M capacitors, silver mica caps, and metal film resistors.
the original white plastic type or the golden brown cardboard variety they are sure to be leaky. When a preamp cathode bypass cap opens, the gain drops and with it volume. If they short, the resulting bias shift of the tube produces nasty distortion-and not the kind of distortion you want-as well as reduced volume. Often these don't completely short or open, but leak, in which case you get fluctuating volume with awful tone. The stock value for these capacitors is 25 uF with a 25-volt rating. Sprague Atom brand capacitors are available in that value; however, you can use any good quality 22-uF electrolytic capacitor with a 35-volt rating. Make sure to observe polarity when replacing these caps. The positive end should go toward the bottom, or tube-side, of the circuit board. Finally, replace the electrolytic power capacitors and power resistors located under the metal pan fastened to the back of the chassis. There are two to four I-watt power resistors under the pan, depending on year and model, usually two at 220 K-ohms, one at 1 K-ohm, and one at 4.7 K-ohms. Again, just as with the number, the values of the resistors may vary. In any case, replace all of them with 2-watt or greater metaloxide resistors of the same value. As for the electrolytic capacitors, these too should be replaced if they haven't already been. Sprague Atom capacitors come close in physical size to the originals.
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There are other physically smaller brands, such as Illinois, that can also be used. While Sprague Atoms tend to be more expensive, sometimes by twice as much, most technicians prefer them, largely because of reputation but also due to their size proximity. Whichever type you use, make sure that the voltage rating is at least equal to the originals (normally 500and 350-volt). You can be less precise with the capacitance value. For example, it is difficult to find the 70-uF, 350-volt original capacitor values. In that case, you can use either an 80-uF or 100-uF at 350-volt. Also, a 20-uF can be replaced with a 22-uF and a 40-uF with a 47-uF. Check, double-check, then triple-check the polarity when replacing electrolytic capacitors; that is, make sure the positive end of the new cap goes to the same point as the positive end of the old cap. And, once again, make sure not to go below the voltage ratings. Few components put on as spectacular of a display as do electrolytic capacitors when they explode due to being under-rated and put in backwards.
Speakers One distinct difference between the blackface and silverface models is the type of speaker used. In fact, changing an amplifier's speaker or speakers has
When removing speakers on most amplifiers, the wires need to be unsoldered.
When briefly tapping a low-voltage battery to the speaker terminals, clicks will be heard through a functioning speaker.
perhaps the most significant effect on the amplifier's sound. While Fender used various brands of speakers, the most prevalent brands used during the tweed and blackface eras were Jensen and Oxford. Again, replacing the speakers in a silverface with Jensen Alnico types won't make the amp a blackface, but it will definitely improve the sound. One of the first things to check on an older model Fender amp should be the speaker, especially in a multi-speaker unit. If, for instance, one of the four speakers of a Super Reverb isn't working, it might not be readily apparent . With the amp set to a low volume, strum your guitar and listen to each of the speakers. If it seems like one is dead, pull it out and check it out. First, if the speaker has slide-on connectors, remove them from the speaker terminals, making sure to mark which connector goes to which terminal. More than
If the speaker won't budge after removing the screws, carefully run a putty knife or other flat blade between the baffle and speaker, being especially cautious not to damage the speaker.
Jensen Alnico (right) and ceramic (left) speakers are great choices for replacement speakers in any silverface.
likely, rather than slide-on connectors, the wires will be soldering directly to the speaker input terminals. In that case, place a small piece of cardboard or shop rag behind the terminal board to keep solder from dropping onto the speaker cone and unsolder the connectors. Next, remove the nuts or screws that attach the speaker frame to the baffle. Support the speaker by holding its magnet. Once the fasteners are removed, if the speaker isn't loose, gently rock it free, using the magnet as a handle. Sometimes on an old amp, the speaker gasket can stick like glue to baffle. In that case, carefully and gently run a flat blade, such as a putty knife, around the edge of speaker against the baffle, trying not to damage the gasket. Once the speaker has been removed, attach a jumper lead to each of the terminals. Hold the other end of one of the leads to the end of a flashlight battery. When
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tapping the other lead to the other end of the battery, you should hear clicks and pops coming from the speaker if the voice coil and thus the speaker are functioning. Don't hold the lead to the battery as even the small DC voltage can overheat the coil over time. To replace the speaker, simply reverse the removal procedures, making sure not to let solder drip onto the speaker cone. Currently there are some excellent guitar amp speakers on the market. Jensen manufactures an Alnico series that works extremely well in the Fender silverfaces. While a set of four PI0R speakers sound amazing in a Super Reverb, it will cost around $400. For half the price, a Super Reverb could be equipped with Jensen's Vintage Ceramic speakers (CI0R). These are fine speakers in their own right, having a wider tonal range and more clarity at high volumes than the Alnicos but not as warm and reactive at low volumes. For a guitarist looking for a definitive vintage sound, Alnico is the choice; however, for a guitarist who wants versatility at a lower price tag as well as plenty of bite in overdrive, ceramic is the way to go. Other excellent speakers to consider are the Eminence Legend series as well as the various lines of Weber speakers.
Hot Rodding the Hot Rod As discussed in the previous chapter, the easiest modification to make to a Hot Rod is to switch tubes. In that chapter we discussed mainly the power tubes; however, the preamp and phase inverter tubes also can be swapped. First, the I2AX7 phase inverter, third tube from the right, provides plenty of gain along with adequate signal swing for the output tubes. A I2AT7 in this position will lower phase inverter gain while also providing ample signal swing for the output tubes. Lower phase inverter gain will help to tighten up the overall tone, increase headroom, and, as a result, deliver a cleaner sound at lower volumes. Of course, many guitarists want the full tonal blast of the I2AX7 phase inverter, so you should judge for yourself, especially since this is an inexpensive, easy, and easily reversible experiment to perform. The same goes for second preamp tube, located second from the right, between the first preamp tube and the phase inverter. The second triode of this tube is used for the overdrive channel, specifically providing the distortion, while the first triode is used
The tube layout in the Hot Rod models is, starting from the left, the two power tubes, the phase inverter, the overdrive and third-stage preamp tube, and the first- and second-stage preamp tube. The foam-like material around the two preamp tubes serves as a shield against interference and should be kept on the tubes.
•• 104
for both channels. Try substituting the 12AX7 tube with a 12AY7 to reduce some of the overdrive's edge while warming it. Otherwise, for the first and second preamp tubes, try a JJ EC83S or Sovtek 12AX7LPS. There has been a misconception that the overdrive of the Hot Rod models is solid-state generated. On the contrary, the overdrive is tube-derived. Two JFETs, which are solid-state devices, merely act as switches to turn on the more drive effect, essentially by engaging bypass capacitors across the cathode resistors of both triodes of the second preamp tube. When a triode is run without a cathode bypass resistor, a form of negative feedback works to keep the triode from distorting, resulting in a cleaner, quieter output. Adding a cathode bypass capacitor reduces this negative feedback effect, thus allowing the tube to produce more gain as well as distort. The reason that the overdrive channel might sound less than stellar has more to do with the voicing of that tube, as will be discussed. If you would like more range on the volume control and reduced volume at lower settings, try a 12AY7 or an NOS 5751 in the first preamp position. Because the Hot Rods are loud amps, either of these tubes will give a lower volume with smooth tone. Otherwise, again, go with a JJ or Sovtek or experiment with the variety of 12AX7 tubes currently available. Finally, for fullbodied tone and seat-grabbing growl, install a set of KT66s. Working bias will probably range from 78 to 84 mY, both tubes combined, measured at the test point 30. Keep in mind that you might not be able to set the bias voltage any lower than 82 or 84 mV (depending on the brand of KT66). The KT66 draws current differently
than a 6L6 and thus the negative bias voltage range differs as well. KT66s should be fine biased at 84 mV as they will still be idling around 70 to 75 percent of maximum plate dissipation. In fact, I've run these tubes at 90 mY, which gives amazingly warm overdriven tone yet at the cost of somewhat shortened tube life. In order to get the bias to drop below 80 mY, the negative bias voltage supplying the KT66s would need to be increased, but there's only about -53 VDC available from the power supply. You may be able to get the bias down a few millivolts by lowering the value of R76 from its stock 1.5 K-ohms to 820 or 680 ohms but probably not below 80 mv' Yet, it's really not worth the bother, since KT66s sound best when biased slightly hot. If you're more into sweet, warm distortion that comes on early and gets richer with more volume, try a pair of JJ or Electro-Harmonix 6V6s. Working bias for these will be much lower than for either KT66s or 6L6s, between 42 to 46 mV measured at test point 30.
The Hot Rod Printed Circuit Board The biggest challenge to modifying the Hot Rod models is dealing with the printed circuit board (PCB). Removing the board can be a chore and because the potentiometers and jacks are mounted directly on it, care must be taken not to damage the board, pots, or jacks when sliding it out of the chassis. Also complete removal of the board is difficult due to the wires and cables soldered to it. I recommend leaving them in place and only pulling the board out as far as is necessary to gain an adequate work area.
Remove the knobs by using a small standard screwdriver to loosen the set screws.
105 ••-
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Remove the chassis mounting screws with a medium-sized Phillips screwdriver.
Gradually and carefully pull the board down and tiltthe top toward you. If you encounter resistance, stop until you find the source, which will usually be a wire bundle.
•• 106
Once the board is turned backwards there will be plenty of access to the solder point.
When removing components from a PCB, first remove the solder. A desoldering tool makes the job easier and helps prevent damage to the circuit trace . First push down the plunger and place the tip next to the trace to be unsoldered. As soon as the solder begins to melt, remove the soldering iron while pushing the button on the tool to release the plunger, creating vacuum that draws in the solder. Remember to only hold the iron to trace long enough to melt the solder.
107 ••-
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To improve the tone of Hot Rod models, replace the components indicated.
To remove the circuit board, first loosen the set screws recessed in the back of the eight knobs. Do not remove the set screws all the way to avoid losing them. The knobs will easily pull off. Remove the nuts from the five jacks (14 mm and 16 mm) as well as the nuts (11 mm) from the eight potentiometers. Nut drivers, sockets, or small wrenches work best. Avoid using pliers or an adjustable wrench since they can slip and strip the edges of the nuts or scratch the faceplate. When installing the nuts back onto the jacks, be careful not to over-tighten them. Newer Fenders used plastic jacks that can be easily broken. It's a good idea to place the loose knobs, nuts, and washers in a small box or bag so they won't be misplaced. Next, remove the six Phillips-head screws from across the lower-middle portion of the board. Also remove the screw attaching the terminal of the ground wire to the chassis on the right-side of the board. Pull the board down slightly until the potentiometers clear their installation holes on the chassis. You have to work the wires at the bottom left of the board, pushing them down and back against the chassis, in order to clear the potentiometers from their holes and from the top corner of the chassis. Carefully pull the top of the board toward you until the board rests upside down with the back facing you. This requires close attention, care, and patience as the jacks, potentiometers, and board can be damaged by reckless and raging practices.
•• 108
To reinstall the circuit board, carefully slide it in place, aligning the potentiometers, jacks, and switches with their respective holes. When pushing the board upward, if you feel any resistance, stop and investigate. Sometimes a wire can catch or the "more drive" lamp become misaligned. Once the board is in place, install all washers and attach all jack and potentiometer nuts. Be careful not to over-tighten the nuts. Next, attach the six screws located across the middle of the board, and secure the ground wire to the chassis with its screw. Finally, install the knobs. Turn all the potentiometers to their lowest position and align the knobs so that the indicator lines point to "1." Don't push the knobs all the way down, but maintain a slight clearance between the bottom of the knobs and the potentiometer nuts so that the knobs won't drag when turned. A second challenge to modifying a Hot Rod involves the more intricate soldering required when replacing components on the PCB. The circuit traces can be easily damaged if a soldering iron is too hot or held too long on them, which can often happen when unsoldering a component. Refer to Chapter 2 for proper soldering practices. If you damage the mounting trace for a component, all you can really do is to run a short jumper wire from the component associated with the damaged trace to the next component to which the trace runs. While this will work, it looks
sloppy and unprofessional. It's best to avoid this last resort altogether by being careful when soldering and de soldering. Practice is the best avoidance. In our culture of throw-away appliances there is no shortage of broken electronic gear on which to practice. You can quickly gain adequate skills by desoldering the components from a broken radio or tape player and then resoldering them back in. The largest cause of damaged traces comes from holding the soldering iron too long on a component. Even two seconds of an overly hot iron can destroy a trace.
Hot Rod Reverb Tanks and Tone Stacks Because the Hot Rod models use a solid-state reverb circuit, some musicians automatically condemn the reverb as having an artificial sound. While the amps do use solid state op-amps to drive and recover the reverb signal, the tank itself is a traditional spring and transducer unit; such as those found in older tube amps. To be honest, the Hot Rod reverb can be overwhelming and somewhat mushy at middle and high settings. The culprit isn't so much the solid-state driver or recovery op-amps, as is often claimed (although they do contribute), but the use of a bypass resistor and capacitor around the reverb control. The purpose of the bypass is to boost high frequencies at low reverb settings, which it does while also sending some of the full reverb signal around the control potentiometer. There is a common modification that techs perform to help alleviate the overwhelming reverb. Yet, you can easily do this yourself without shelling out the money to have it done-essentially, eliminate the treble bypass cap and resistor from the reverb control circuit. In fact, since the resistor and capacitor are in series, you only need to remove one of them. Some techs don't even bother to remove the board for this mod, but simply
clip the resistor leads from the face of the board. You can do this also, but if you plan on doing the total overhaull'm suggesting, then pull the board and unsolder the resistor. After performing this mod, don't expect the Hot Rod's reverb to suddenly sound like that of a Deluxe Reverb or any other tube-driven reverb circuit. However, you should notice a more easily controllable reverb level with a clearly defined sound. Now that the reverb circuit has been dealt with, it's time to tackle the tone stack. As will be discussed in Chapter 8, lowering the value of the slope resistor boosts midrange tones by adding more low-end and reducing the top-end; however, it also limits the effectiveness of the tone controls. Essentially, the slope resistor sets the center of the frequency range over which the tone controls operate. By slightly lowering the slope resistor in the Hot Rod while also lowering the bass capacitor, the lows become more defined and tightened.
The reverb bypass resistor is removed.
The clean space of a properly removed reverb bypass resistor.
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The tone capacitors and resistor are in close quarters with the tone controls.
Newer Fenders use polyester capacitors that are of fairly decent quality. Yet, the tone can be improved by using high-quality Orange Drops. Sozo or Mallory 150Ms are also excellent choices and, like Orange Drops, are larger than the standard polyester variety. Whatever capacitor you use, make sure it has a minimum rating of 400 VDC. To fit them into the mounting holes, you'll need to run the leads along the capacitor body and then bend them outward to match the distance of the mounting holes. Getting back to the slope resistor, replace the 130-K-ohm carbon film resistor with a resistor of the 100-K metal film variety. Actually you could go as low as 56-K-ohm if you prefer more bass and low midrange. Next replace C27, the 250-pf ceramic disc treble capacitor, with a 220- to 390-pf higher quality silver mica (I prefer a 220-pf). Replace C6, the .022-uF midrange polyester capacitor, with a same value Orange Drop. Finally, replace C5, the .1-uF bass polyester capacitor with a .047 Orange Drop. Replacing the .1 bass cap with a .047 passes a narrower spectrum of bass frequencies and has the audible effect of tightening the lows but not enough to attenuate them. I've also used a .022-uF capacitor in this position for guitarists who want a more upperlow emphasis. An occasional complaint regarding the Hot Rod models is that they can sound "boomy" when the bass is set at around the halfway level and beyond. While the cabinet and speaker baffle material and construction contributes to this, tightening up the low frequencies via the tone stack modification helps to reduce the boominess. Another complaint involves the tone stack design. In this case, there are modifications that essentially replace the Hot Rod tone stack with that of a blackface. Yet, I find the Hot Rod's tone stack to have a wide range of tonal possibilities. Setting the
•• 110
bass to 2, the midrange to 4, and the treble to 8, and then, alternately, setting the bass to 6, midrange to 8, and treble to 4, for instance, sounds more diverse than respective ranges on a blackface. This is not to say that one is better than the other; they just have different voices. The most prevalent complaint I hear regarding the Hot Rod amps involves the overdrive channel. While I don't believe the overdrive to be as horrible as reported, I do think there is plenty of room for improvement. Let's start with C23, the 1.5-nf (or .0015-uF) overdrive channel coupling capacitor. As well as blocking the DC plate voltage from the previous tube stage, a coupling capacitor also attenuates the signal being passed. Because the overdrive channel functions by adding an extra gain stage, a capacitor with a large value would pass too wide of a signal, resulting in a muddy, over-emphasized low-mid tone. While it might seem like passing a wide signal would give more sound, that's not the case. By the same token, if the coupling capacitor is made too small, the resulting tone will be thin and brittle. Most complaints regarding the Hot Rod overdrive address the latter case: that the amps are brittle, harsh, and sterile. The tone can be improved by increasing the value of the coupling cap. I recommend a .0047-uF Orange Drop or Mallory. If you don't like the sound of the overdrive after that, try going midway between stock and .0047 with a .0022-uF capacitor. Or even go a little higher, say .01 uFo Be aware, though, that the higher you raise the capacitor value, the looser the tonal definition will be. Yet, I've seen values as high as .022 uF. For some musicians, myself included, that value renders the overdrive completely useless, while for others, that value delivers the distortion they've been seeking from this amp. Perhaps you find the overdrive to be fine
To avoid damaging solder traces as well as repeatedly removing the circuit board when experimenting with various component values, solder short pigtails to the original component's mounting holes.
Components can be easily soldered and unsoldered to the pigtails. Be aware that even short runs of wire can pick up interference that could influence your sound.
The mounting screws for the tube socket PCB are easily accessible. Use a medium-sized Phillips screwdriver. Make sure to remove the tubes.
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The phase inverter input coupling capacitor has been replaced with a .0047-uF high-performance SBE Orange Drop rated at 600VDC.
This Hot Rod has had the plate resistors replaced with metal-film resistors and the coupling capacitors with SBE 716P Orange Drops.
just as it is. In that case, leave the C23 alone or replace it with a high-quality capacitor of the same value. Keep in mind, though, that it doesn't hurt to do some experimenting to try to find the elusive prime tone. One thing to be aware of when using repeated component changes is the damage to the delicate PCB traces. When searching for the right sound means repeated component change, I recommend temporarily soldering some short wires (pigtails) into the component's mounting holes and using these to solder in and out the components you are trying. It's best to keep the wires short as they will pick up interference if
•• 112
too long. The excess lead lengths that you cut from a resistor or capacitor work well as pigtails. Once you've found the capacitor or resistor you want to use, you could wrap the pigtails around the leads, close to the component and flush with the circuit board, and solder the leads in place. On the other end of the overdrive circuit sits the master volume. If you look at a Hot Rod schematic, you'll see that it is bypassed with Cll, a 390-pf ceramic capacitor. The purpose of this capacitor is to shunt some of the higher frequencies to ground since the extra gain doesn't really discriminate what it amplifies and, as a result, can
give you unwanted noise. Basically, this capacitor, in conjunction with the resistance of the master volume pot, works as a filter to help tame unwieldy high-frequency preamp distortion. The stock value of 390 pf might be fine, but if you've made any changes to the tone stack (different value capacitors and/or slope resistor) you might find that the overdrive has become a little thin. Be aware that changing this capacitor will have only a subtle effect and you might not find it worth the trouble to replace it. Changing C23, as just discussed, will have a more noticeable effect. Yet if you want to hear for yourself, or if you just want to experiment, try replacing CII with a 250-pf cap or even a 47-pf. You can use a less expensive ceramic disc as C11 doesn't pass any signal through the preamp chain, but just shunts top edge frequencies to ground. You might even go the other way slightly; that is, increase the size of C11 to, say 500 pf, for a tamer overdrive. I'd be reluctant about raising the value of the cap much more than that, because you can really knee-cap the overdrive with too much signal-to-ground filtering, making it sound truly pathetic. Overall, the best way to tame the overdrive is to use a 12AY7 or even a 12AU7 in place of the 12AX7. The capacitor change can be thought of more as fine tuning the tube's circuit. With the tone stack modification and overdrive revoicing, it's a good idea to lower the value of C24, the phase inverter input coupling capacitor, from the standard .022 uF value to .0047 uFo The lower value will further tighten the lows and clarify the highs as well as complement the revoiced overdrive channel. Keep in mind that the change is subtle yet noticeable. Also note that C24 is located on the tube socket PCB, which must be removed to access it. To do so, first remove the tubes. Next remove the nine mounting screws from the bottom of the chassis. Once the screws are removed, pull the circuit board up and flip it to access enough room to desolder the capacitor and solder in the replacement. Be careful to avoid burning the ribbon connector that runs from the socket board to the main board. Once the replacement capacitor is installed and the extra lead lengths clipped, install the circuit board by reversing the removal process. Finally, replace coupling capacitors C2 (.047), ClO (.047), and Cl8 (.022) with higher quality capacitors of the same value. Again, Orange Drops rated at 600 VCD or Mallory 150Ms rated at 630 VDC are good choices, as are Sozo capacitors. Next, replace plate resistors R4, Rll, R16, and R22 with 100-K-ohm, 1/2-watt metal film resistors and, on the tube socket board, R58 with a 100-K-ohm, 1/2-watt metal film and R57 with 82-K-ohm, 1/2-watt metal film.
Rebuilding the Reissue Externally, the Fender reissue amps look identical to their antecedents. One look under the hood, however, reveals
a completely different layout, yet with the same design. In other words, even though the schematics are the same, the original amps use the traditional tag-board layout. The reissues use printed circuit boards. This distinction has led a number of users to have their amps converted to the original blackface (or tweed, in terms of the Bassman reissue) layout. Essentially, this is a complex modification that involves replacing the printed circuit board with an epoxy-based board replication of the original tag-board. The conversion also entails replacing the jacks and potentiometers since on the reissue they are mounted to a PCB. While the conversion improves sounds and also makes further modifications easier to perform, it does not really make the amps sound like the originals. To get the original sound, you'd need to replace the speaker, or speakers, as well as the speaker baffle, the output transformer, and ultimately the cabinet-in other words, the entire amp. Seriously, though, the reissues do sound similar to their original counterparts, and, in their own right, they sound excellent. In this section I will describe how to rebuild a reissue with the aim of both making it sound closer to the original and, more importantly, how to improve an already excellent amp. To that end, some users might be content to leave their amps as is. This section is for those who want to experiment with sound improvement, beginning with basic modifications and culminating with the advanced modification of converting the PCB layout to a vintage-type layout. Let me state right up front, this is an advanced modification, perhaps the most complex one in this book, and should be performed only by someone with advanced technical skills. Therefore, rather than step-by-step instructions, which due to the complexity could easily be a book in itself, I will provide an overview. This will be enough information to get you familiar with the procedures of a total rebuild and perhaps inspire you to research and learn the essential skills required for a total rebuild. Yet between leaving the reissue alone and completely rebuilding it, there are a number of other modifications to consider. The first modification to perform on any reissue is easy and has one of the most pronounced effects. The reissues have a factory-set bias that is cold by any standards. Setting the bias at about 75 percent of plate dissipation, as detailed in Chapter 4, will go a long way toward improving the sound, even with the factory-equipped tubes, which leads me to the next equally easy modification that will provide results that are just as pronounced: tube replacement. Tube choice is covered in Chapter 5. Of the many choices for preamp tubes, J], Sovtek LPS, and, of course, NOS varieties stand out as great choices. For power tubes, try a pair of Electro-Harmonix, Tung-Sol, or J] 6V6s in the Deluxe Reverb and Princeton Reverb reissues. Winged C, J], or TAD 6L6s are good choices for Super Reverb, Vibroverb, Vibrolux Reverb, and Twin
113 ••-
-
The location ofthe coupling capacitors and plate resistors on a typical Fender reissue are indicated here with the letters C and R, respectively.
Component Changes for Popular Fender Amp reissues Capacitors
65 Princeton Reverb
65 Deluxe Reverb
65 Super Reverb
65 Twin Reverb
59 Bassman
Preamp Coupling
C4.0220
C5.0470
C5.047
C12.0220
C2,C3.0220
C12.0220
C12.022
C18 .0470
C18.1000
C18.1
C19.1000
C3500pf
C14500pf
C14500pf
C13500pf
C13.0033
C17 .0033
C17 .0033
C16.0033
Phase Inverter Input
C14.0220
C25.0010
C25 .001
C20.0010
C8.0220
Output Coupling
ClO, C16 .1000
C27, C28 .1000
C27, C28 .1
C22, C23 .1000
C12, C13 .1000
Reverb Bypass
C5 lOpf
C1310pf
C1310pf
C1710pf
NA
Phase Inverter NFB
n/a
C26.1000
C26.1
C21 .1000
C9 .1000
Treble
C19250pf*
C2, C7 250pf*
C2, C7 250pf*
C2, C7 250pf*
C5250pf*
Midrange
C23.0470*
C4,C9.0470*
C4, C9 .022*
C4, C9 .0470*
C7 .0220*
Bass
C24.1000*
C3, C8 .1000*
C3, C8 .1*
C3, C8 .1000*
C6.1000*
Slope Resistors
R46100K*
R6, R18 100K*
R6, R18 100k*
R6, R17 100K*
R17100K*
Plate and
R4, R36, R37, R45100K-1W
R4, Rll, R16, R23, R29,R34100K
R4, Rll, R16, R23, R29, R34, R55 lOOK
R4, Rll, R15, R22, R28,R33
R8, R9, R14, R29100K
Other Resistors
R17, R26 56K
100KR5482K-1W
R5482K
100KR42100K-1W
R2882K
Reverb Coupling
NA
c:ct
-
"-
R55100K-1W
R4382K-1W
CI:I
t :::L cal
..=: ~
•• 114
• Components located on the tone control board . Component designations as printed on PCB Capacitor values in microfarads unless otherwi se designated. Resistor ratings are 1/2 watt unless otherwise noted.
reissues, while Tung-Sol 5881s work well in the Bassman reissue. There are many decent new production tubes from which to choose. A set of quality power tubes, properly biased, proves to be the most practical improvement you can make to any reissue. As discussed earlier in this chapter, one of the quickest ways to change the sound of any amp is to replace the speaker. As with tube changes, this is a subjective process. To top it off, the same speaker will sound different in the various amp models. For a single-speaker amp, such as the Deluxe Reverb reissue, replacing the speaker can be done for around or just under $100, depending on the quality. The situation gets more expensive, however, with multi-speaker amps like the 2x12 Twin or, especially, the 4xlO Super Reverb. Replacing speakers in these amps can really be an expensive proposition. When shelling out $300 to $400 to replace speakers in a Super Reverb, you should think twice about following someone's recommendation regarding speaker choice. The reissue amps all come with fairly decent speakers and so the choice of
replacing them is something you might want to weigh with some research. An excellent choice for speaker replacement in the reissue models can be found among the variety of Weber brand speakers. The Weber Vintage series offers both ceramic and alnico speakers that work well with the reissues .
When in tight quarters the leads of a capacitor need to be kept long; use heat shrink tubing to help isolate them.
After removing the knobs and pot and jack nuts, pull the tone control board down and away from the chassis.
After removing the mounting screws, flip the top of the main PCB down to expose the solder points. If you can't gain enough access, refer to the next photo.
115 ••-
-
As described on the Hot Rod models, replacing coupling capacitors with high-quality Orange Drop, Mallory, or Sozo brands will improve tone and fidelity. Also, replace the ceramic capacitors used for reverb bypass and treble with silver mica. Whichever type you use, make sure they have a minimum rating of 400 VDC.
To allow more freedom of PCB movement, remove some of the power wires by carefully pulling them off by their connectors. Mark their positions for easier reconnection if not already marked on the terminal connectors .
An epoxy-based circuit board can be cut to desired length using a hacksaw. You can drill holes fairly easily with an l/8-inch general purpose drill bit and crimp into the holes either turret lugs around which to wrap the component leads or eyelets into which the component leads are inserted. With either method, the leads are then soldered.
•• 116
While you're at it, replace the plate resistors with metal film or carbon composition types. A number of the reissues use resistors rated at 1 watt. Most metal films are 112 watt but will still work as replacements; however, you might want to go with I-watt and 112-watt carbon composition resistors as these were the types used in the original blackfaces. The table on page 114 identifies the resistors by rating (wattage) as well as value (ohms). Since the reissues typically use the same value components as the originals, you can stick with those values. However, due to differences in cabinet construction and materials as well as speakers, some improvement can be found by using a higher value phase inverter input coupling capacitor. The blackface reissues usually come with a .00l-uF value. I recommend anything from .0047 uF to .01 uFo By the same token, on the Bassman reissue the phase inverter input coupling capacitor is .022 uFo Here, you could go lower, say .01 uFo This will tighten up the bass and clarify the tonal separation. Bear in mind, though, that the Bassman is highly prized for its bluesy sound, which is partly attributable to this .022 coupling cap. On the other hand, there are potentials beyond that designation. Whatever components you decide to change, I highly recommend obtaining a schematic and board layout for your particular reissue. Some can be downloaded from the Fender website while others can be found at schematicheaven.com. As with the Hot Rod models, the printed circuit boards of the reissues present challenges to component replacement, especially the PCB holding the control potentiometers. Tight quarters are found here. When replacing capacitors with larger types (which will be the case if using high-quality caps), you may have to keep the leads somewhat longer if the capacitor needs to be situated above or alongside the mounting holes and other nearby components. Where this positioning is required, make sure the leads don't touch anything, especially metal. It's a good idea to use heat-shrink tubing on them. To gain access to the solder points on the back of the PCB, you'll need to unfasten it from the chassis. Rather than disconnect the wiring, you only need to remove it partially. I recommend reviewing the previous section on the Hot Rod PCB removal and installation as it is similar to that of the reissues. Furthermore, the same care and precautions should be taken. First, if you're going to be replacing the tone stack capacitors, the control PCB needs to be removed. Loosen the set screws on the knobs without removing them. Pull the knobs from the potentiometers. Next, remove the nuts and washers from the jacks. Disconnect the ribbon cables from the main board by carefully pulling them by their connectors. Some boards have a plastic strap that needs to be cut in order to slide the board out. These can replaced with a small zip tie. Once the control board is free, remove the mounting screws
Use double-face tape on the board and line up the components, sticking them to the tape. Here, a blackface Princeton Reverb circuit is being laid out.
·. f
I r -
-I
T \
After the parts are laid out, mark their positions on the board as you remove them.
from the main circuit board and carefully pull it back and flip it over. If you encounter even slight resistance, stop and investigate. Wires can get easily snagged. The board should now be free enough to replace the components. See the Hot Rod section earlier in this chapter and Chapter 2 for soldering instructions. With the control board removed, you might consider a tone stack modification as described in Chapter 8. Regardless of whether you replace the tone capacitors with different values as discussed in Chapter 8, you should install higher quality component, silver mica types for treble capacitors and Orange Drop, Mallory, or Sozo varieties for midrange and bass capacitors. Slope resistors can be improved by using metal film or carbon composition types. As discussed in Chapter 8, changing the value of the slope resistor will shift the center frequency of the tone controls. A common tone modification for the tweed Bassman, and therefore also for the reissue, is to change the slope resistor to 56-K-ohm value. The resulting midrange enhancement will resemble that of vintage Marshall amps but still be uniquely Fender.
To boost midrange on the Deluxe Reverb reIssue, replace R9 for the normal channel and R21 for the vibrato channel, both being a 6.8-K-ohm resistor located below the bass potentiometers on the control board. Using a lOoK-ohm resistor will emulate the tone of a three-band blackface with the midrange control turned all the way up. For an even greater mid-boost, you could try a 15-K-ohm resistor, but I wouldn't go too much higher than that, as the tone will get muddy. You might consider leaving one channel stock and modifying the other. Further, you might bring reverb to both channels according to Chapter 7, and thus have two differently voiced reverb channels. On a Princeton Reverb reissue, the midrange resistor is R52, also a 6.8-K-ohm resistor, located between the volume and treble potentiometers next to the ribbon connector on the tone control board. Try the same values as with the Deluxe Reverb. Before reinstalling the tone control board, make sure that with the replacement caps the board will still fit. You might need to reposition the caps, making sure that the leads don't come in contact with anything else.
117 ••-
-
Left: You will need to drill1/8-inch holes for the eyelets and then crimp the eyelets in the holes. The pair of holes for each component should line up. To do this accurately, measure equal distances from the edges of the board for the holes. Below Left: After the holes have been drilled, crimp the eyelets into place. An eyelet crimping tool, which will make this task easier, is available from Hoffman Amplifiers, but a pair of needle-nose pliers, an awl, a punch, or a similartool with a tapered, pointed end works if used carefully. First, widen the edge of eyelet or turret hole by stretching it and pressing it outward toward the board. Below: Next, use a pair of pliers or a small C-clamp to crimp the edge of the eyelet ortu rret to the board.
As mentioned earlier, the most radical modification you can perform on a reissue is to replace the PCBs with an epoxy-based circuit board to replicate the tag-board construction of the blackface and silverface models. As well as giving the reissue more authentic circuitry, replacing the PCBs also simplifies the component layout, making it easier to perform other modifications, such as bringing reverb to both channels or using the normal channel for an extra gain stage. You need to weigh the benefits with the risks, however. You can literally destroy your amplifier if you're not familiar with the complexities of this modification. Before ripping the guts out of your reissue, try building a circuit board first. Epoxy-based board material is sold by the inch and can be purchased from Hoffman Amps as well as other vendors listed in the appendix. Make sure to order eyelets or turrets. I highly recommend studying Doug Hoffman's website regarding board building. Measure the size of the original board to determine the length to order,
•• 118
rounding up instead of down (for example, order a 14-inch piece of material for any board size between 13 and 14 inches). Before laying out the components on the board material, line up the mounting holes with the board material and mark where the mounting holes will go. Next obtain a circuit layout of the amplifier you are replicating. Schematic Heaven and other places online have free downloadable Fender schematics that also contain the circuit layouts. You can also refer to the diagram of a typical blackface circuit layer earlier in this chapter. Using the schematic and layout, make a list of all the components you'll need and order them. If the idea of building your own board seems like too much work, consider purchasing an already populated board, commonly called a Hoffman board. These boards are so-named because Doug Hoffman originally made kits to replace printed circuit boards in a variety of amplifiers. While he no longer sells these kits, other manufacturers do. It is well worth your while to read about these kits at www.el34world.
com/boardmakerloldboardkitpage.htm. You can also purchase these kits by visiting the online vendors I've included in the appendix. Note that you may have to drill new mounting holes in the chassis to properly mount some of these boards. Whether you build your own board or purchase a kit, the next step is removing all pots, jacks, and PCBs from the chassis. Potentiometers can be replaced with Alpha pots, which will fit easily into the mounting holes. Standard jacks, such as Switchcraft brand, will also fit the pre-existing mounting holes, but if you decide to isolate the jacks from the chassis to reduce the potential of hum, you will need to enlarge the holes to 112 inch. Jack isolating washers are available from Hoffman. Some of the original power wires can be used, but you will more than likely have to replace the
wiring for the tube sockets and definitely for the pots. Shielded wire should be used for all jacks. If this seems to be a complex operation, that's because it is. Techs will usually charge anywhere from $450 to $600 (and even higher), parts and labor included, to do this for you. You might find it to be worth it. Then again, you might be able to do this yourself for less than $100. What lies in between is the Hoffman board kit sold by most vendors for around $250. The nice thing about these kits is that they include everything you need and are really a great step for anyone with moderate technical experience. Check out the kits at the Airtight Garage online, for example. The bottom line, however, is that you do not need to blackface the reissue to make a great-sounding amp; it's just one option to consider.
Populate the board with the components and tack them in with solder. The large Orange Drop capacitor on the right end of this clone tweed Princeton board is used to make a variable gain control as described in Chapter 8.
This is a modified Bassman clone board. Note the extra cathode capacitor used to enable gain changes via a boost switch.
119 ••-
chapter
Originally derived from the reverb unit used in Hammond organs, the Fender reverb has proven to be a standard for reverb effects in guitar tube amplifiers. While major manufacturers, such as Ampeg and Gibson, use different reverb circuit designs than Fender, many manufacturers, especially in the booming boutique amp market, have similar-if not the same-circuit design as traditional Fender tube amps. Apart from circuit design, though, the defining sound of reverb has been that of the classic Fender blackface with its warm, wet treble-focused reverb.
Fender Reverb Operation and Basic Mods In Chapter 6 we discussed modification of the reverb circuit in Fender Hot Rod models, which use solidstate reverb units. In this section we're going to cover the reverb circuits in Fender amps that don't feature solid-state reverb units. As the schematic on the next page indicates, the signal from the preamp runs up against a large resistor (3.3 M-ohms) with a 10-pF bypass capacitor that allows high-treble frequency to pass across the resistor and directly on to the phase inverter. However, this high frequency also mixes with the out-going reverb signal. It would be a mistake to think that the 3.3-M-ohm mixing resistor blocks the signal; most of
7
it passes, but with much lower gain. While the value of this resistor isn't critical, lowering it drastically, say below 1 M-ohm, will definitely start swamping the reverb signal. The 3.3-M-ohm value was originally chosen as the optimum value for allowing the proper mix of reverb to preamp signal, giving the distinctive reverb-heavy Fender sound. However, that doesn't mean you shouldn't experiment by trying lower value resistors. One of the easiest modifications of the reverb circuit simply involves lowering this resistor, although many guitarists find the Fender reverb to be fine as is. Another important factor in determining not only how much of the preamp signal goes to the reverb circuit, but also how the reverb sounds, is the 500 pF coupling capacitor at the input of the reverb driver tube. This capacitor may not be the best focus for modification, since going much higher than .001 uF can start to muddy the reverb. The treble-rich signal reaching the driver tube through the 500-pF coupling capacitor is boosted by the tube. Note that this dual triode 12AT7 tube has a parallel connection, meaning the plates, grids, and cathodes of the triodes share the same connections. While running the triodes in parallel won't add more gain than if only one triode is used, it will increase the current capacity of the driver. With well over 400 VDC on the plates of the triodes, drawn from same power rail node as that of the power
The current production Vibro-King incorporates the reverb circuitry of the 1963 stand-alone unit. The array of Vibro-Kings here belongs to Peter Townshend of The Who. Rick Gould
Opposite:The '63 Fender Reverb is a reissue of the famous stand-alone reverb unit. Fender Musical
Instruments Corporation
121 ••-
-
Blackface and Early Silverface Reverb Circuit
-
'--
=
From Preamp
100 K-ohm .1uf
61----i I 500pf
3.3 M-ohm
)
To Phase Inverter
7 Vz 12AX7
P
10pf
To
[8J
12AT7 .002ul
1
2
E .t:: o, :2:
6
Reverb
Tra nsfo rmer'
c==r---C==~r7--~ 3 8 25uf 25V
2.2K-ohm
-
H--(>
200 K·ohm
:
1
.003 E
2
11 1: II II II II
II
"
II II II
. c:::>.. -: c--,
~
0
~
~
~.
c::::>
=
DPDT Toggle Switch
To Center Lug of Bass Potentiometer
To Center Lug of Volume Potentiometer
When installing a deep switch, it often proves easierto solderthe components to the toggle switch before it is attached to the chassis. Once it is attached, connect the components by short runs of wire, if necessary, to the points indicated.
Completed Dumble-inspired modification with added deep switch .
145 ••-
AODING GAIN When technicians and musicians speak of modifying an amplifier, they most often mean modifying an amplifier's gain. Indeed, the various techniques commonly used for modifications, ranging from the basic to the mind-numbingly complex, usually involve manipulating gain in some form or another. Almost always the manipulation involves boosting preamp gain, which not only makes the amp louder but also enables preamp tube distortion. Through modification, preamp gain can be increased to a level similar to that of a typical distortion pedal. Without a means of switching this high degree of gain in and out of the circuit, the modified amplifier's versatility would be drastically narrowed. Besides a switch, another means of controlling the added gain is to use a potentiometer to vary and control the amount of gain produced in the preamp. Yet when adding only a modest boost of gain, neither a pot nor a switch is necessary since the slight gain boost simply thickens tone and adds a warm edge of distortion. Indeed, the questions to consider when it comes to boosting gain is not only how much gain to add but how much gain is too much gain. For our purpose and at its most general, gain can be defined as the ratio of an amplifier's (or amplifier stage's) output signal, measured in AC volts, to its input signal, also measured in AC volts. In other words, the degree to which an AC signal increases is a function of gain. Each tube or, in the case of a dual triode, each triode, is limited in the amount of gain it can deliver as defined by its amplification factor. Because only a certain amount of gain can be produced by a tube, a high-gain amplifier will employ several tube stages. Even in a guitar amp, though, there could be such a thing as too much gain. If you couple together even more than several high-gain tubes stages, you could end up with a buzzing, squealing, and loud amp. Although such an amp might have its place in the diverse world of music, most of the mods presented in this chapter won't get us into the territory. Yet a couple of them add an additional tube stage to the preamp, which delivers a healthy dose of overdrive. While the
Opposite: The Twin Reverb reissue reintroduces perhaps the most popular-and loudest-of the blackface amps.
Fender Musical Instruments Corporation
•
Gaining Background Noise
•
One thing to keep in mind: With added gain co mes the pote ntia l fo r added backg rou nd noise.
gain can be controlled with the amp's existing potentiometers, some guitarists might find the gain to be excessive. However, it's worth detailing the technique if only to demonstrate the high-gain capability of a typical Fender amp.
Basic Gain Boosting Modifications As the first gain-boost modification in this chapter shows, we don't need to do much to get an incredible amount of gain out of a typical Fender. With a simple patch cord and a resistor you can give any Fender with tube-driven reverb a rich overdrive effect without removing the chassis. The reverb will be disabled; however, the effect can be easily reversed and the reverb restored in about five seconds. This was a fairly common technique a few decades ago. Essentially, you disconnect the reverb tank from the reverb circuit. By bridging the input and output reverb connections on the back of the chassis with a resistor cable, the signal passing through the reverb circuit is first boosted by the 12AT7 reverb driver tube and, instead of passing through the reverb tank, runs directly to the recovery triode of the 12AX7 via the resistor cable, where it is boosted again. Instead of reverb, this circuit now produces tube overdrive. First, if you don't already have one, buy an inexpensive audio or AV patch cord with phono plugs, like the ones that come with most DVD players, televisions, or audio equipment. Using a wire cutter, clip off two of the plugs with about 3 to 5 inches of cable attached to each. Strip back and remove about 1 112 inches of the shielding and then strip about 112 inch of insulation from the center conductor. A wire-stripping tool works best; be careful as the center conductor usually consists of a small-gauge collection of thinly stranded wire. Make a resistor cable by soldering a resistor-any
147 ••-
-
~
II" I
......._
....__~/
-"• ••
An inexpensive AV cable can be converted into a resistor cable to be used for a reverb channel jumper when converting the reverb circuit into extra gain stages for the preamp.
value between 330 K-ohms to 470 K-ohms will do-to each of the center conductors leading from the plugs. Cover the exposed leads and conductors or the entire resistor and solder joints with heat shrink tubing or electrician's tape. Remember to place the heat-shrink tubing over the cable before connecting the resistor. That might sound obvious, but it is an easy thing to forget. To complete the modification, unplug the reverb cables from the back of the chassis, and insert the resistor cable in their place. The vibrato channel will now have an overdrive effect that can be turned on and off by the reverb pedal. Moreover, the reverb level control will now be the overdrive level control. Be aware that you will no longer have reverb, but you can simply unplug the resistor cable and reconnect the reverb cables. Another common modification, even easier than the previous one, for any dual channel silverface, blackface, or blackface reissue is to remove the normal channel preamp tube, the first tube on the right when looking into the back of the amp. The normal channel will be inoperable, but the vibrato channel will have a
A silverface with the resistor cable replacing the reverb tank. To reverse the mod, simply unplug the resistor cable and plug the reverb cables back in.
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modest boost of gain that will enable preamp crunch at moderate volume levels. While removing a tube might seem counterintuitive when it comes to boosting the gain of amp, the reason it works involves preamp tube bias. As discussed in Chapter 2, the useable gain of a tube is determined by the value of both the plate and cathode resistors. Typical Fender values of 100 K-ohms and 1.5 K-ohms, respectively, deliver a healthy amount of gain. As an aside, the combination of a 220-K-ohm plate resistor with a 2.2-K-ohm cathode resistor also delivers another optimal combination for gain and overall performance. Regarding the dual channel Fenders under discussion here, the preamp tube for each channel has an independent cathode resistor (1.5 K-ohms) for its triode, while the other triode of each tube serves as the second-stage preamp for its respective channel. These secondstage triodes are tied together and share an 820-ohm cathode resistor (believed to be a cost-saving measure, according to Fender lore). The 820-ohm resistor is roughly half the value of the typical1.5-K-ohm because preamp tube biasing works in such a way that when cathodes of two triodes share a cathode resistor, to get the equivalent bias of an independent cathode resistor, the shared cathode resistor needs to be half the value. Pulling the normal channel preamp tube, therefore, has the same effect as replacing a 1.5-K-ohm cathode resistor with an 820-ohm resistor, which in turn readjusts the triode's bias voltage, increasing the gain of the triode. Although raising the value of a triode's plate resistor is a more effective and better way of boosting gain, manipulating the tube's bias by lowering the value of the cathode resistor will also yield an increase of gain. This is an extremely nondestructive modification; it can be undone by simply reinstalling the tube. As discussed in Chapter 8, an amplifier's tone stack introduces insertion loss, meaning that when the signal passes through the potentiometers, capacitors, and resistors of a tone stack, the signal becomes weakened. As a result, a second-stage preamp is employed to restore the signal's gain. Deductive reasoning tells us that if we eliminate the tone stack, we eliminate the insertion loss. Furthermore, because a second stage of preamp is in play, the full strength of the signal, freed from its insertion loss, will get a large boost from this second stage. Therefore, our next gain boosting mod involves removing the tone stack from the circuit. Of course, we're not going to physically remove it but simply disconnect it, which has the same effect. This process is much easier than it sounds. For the normal channel, simply disconnect the 6.8-K-ohm midrange resistor from the left lug of the bass pot. For the vibrato channel, unsolder the ground connection from the left lug of the midrange pot. The obvious disadvantage of disconnecting the tone stack involves the lack of tone control. The
coupling capacitors provide the only attenuation of this pure, high-gain signal, and, by the way, that isn't much attenuation. If you don't want to disable the tone controls, a similar boost of gain can be garnered by disconnecting the vibrato effect. Viewing the schematic for a dual channel Fender with vibrato shows a connection between the vibrato intensity 50-K-ohm potentiometer and the 220K channel mixing resistor of the vibrato channel. This connection has the effect of putting a 50-K-ohm resistor (the value of the potentiometer) between the vibrato channel's preamp output and ground, and, as such, pulls some of the signal to ground. Removing this wire from the right lug of the intensity potentiometer will free the signal from the 50-K-ohm drain and, as a result, add some gain to the signal. Of course, vibrato will be lost. Ultimately, using either one of these gain boosts means sacrificing something; however, one solution would be to sacrifice instead one of the bright switches to use as a boost switch that lifts either the vibrato pot from the signal path or the mid pot from ground. If you want to lift both vibrato and tone stack, and you never use either bright switch, you could use one bright switch to lift the tone stack and one to lift the vibrato pot. Another option would be to replace the bright switch for the vibrato channel with a miniature DPST (double-pole, single-throw) or DPDT (doublepole, double-throw) toggle switch and wire it to lift both the vibrato channel's tone stack and intensity pot simultaneously. To do this, simply combine the general instructions for tone stack and vibrato intensity pot lifting, as detailed below. Finally, if you use the bright feature most of the time, consider "hard-wiring" it to the circuit by disconnecting the lead of the bright cap from the switch and soldering it to the middle lug of the volume pot. By the way, this permanent bright feature is the stock configuration of the Deluxe Reverb's vibrato channel. Locate the intensity potentiometer and the midrange potentiometer for the vibrato channel. To remove the intensity pot from the circuit (and by extension the vibrato effect), disconnect the wire from the right lug of the intensity pot. To make the modification switchable, remove the wire and the capacitor from the bright switch. In their place, connect the disconnected wire from the intensity pot to one of the terminals, and to the other terminal solder one end of an 8- to lO-inch wire; solder the other end of the wire to the right lug of the intensity pot. Now, when the switch is closed, the signal modulation effect from the vibrato circuit functions as the intensity pot remains connected, through the switch, to the 220-K-ohm resistor on the circuit board. Conversely, opening the switch prevents signal modulation since the intensity pot's connection to the resistor opens, effectively disabling the effect but keeping the intensity pot from pulling any of the preamp signal to ground.
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To liftthe vibrato effect, remove wire A from the intensity potentiometer. To liftthe tone stack, remove ground wire B from the midrange potentiometer.
To lift the tone stack of the vibrato channel, disconnect the ground lead from the left lug of the midrange potentiometer. To make this gain boost switchable, remove the wire and capacitor from the bright switch terminals and run a 4- to 5-inch wire from one terminal of the switch to the disconnected left lug of the midrange pot. Next, run a short jumper wire from the other terminal of the bright switch to the left (grounded) lug of the volume pot. To add both of the mods-vibrato disabling and tone stack lifting-I suggest replacing the bright switch with a DPST or DPDT miniature toggle switch, as mentioned earlier. The hookup is the same as with the bright switches, but make sure to use the proper terminals. Specifically, if the toggle has six terminals (as a DPDT does), when viewing it from the back, connect the wire you removed from the intensity pot to either one of the center terminals and connect the new wire running to the intensity pot to the terminal above that one. Likewise, for the tone stack lift, connect the wire you ran from the left lug of the mid pot to the other center terminal of the switch and the jumper wire from the left lug of the volume pot to the terminal above that center terminal. Now, if you are using a DPST switch, which has only four terminals, the hookup configuration should be the same; that is, the pair of wires for the intensity pot should be attached with one above the other and, similarly, the pair from the mid and volume pots should be attached one above the other. If adding either or both of these modifications to a Deluxe Reverb or similar Fender without a bright switch and you want the mod to be switchable, you can remove the number 2 input jack on the front of the amp or the external speaker jack on the back. To use a miniature toggle switch, add a washer on either side of the empty jack hole since the switch mount is too small for the existing hole. Otherwise, a standard-sized toggle
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switch will work. If using the external speaker jack, make sure the negative feedback wire connecting to it from the circuit board gets connected to the speaker jack at the same connection point. This negative feedback configuration will be discussed shortly. If you want to use the typical two-band normal channel for the tone stack lift, the process is similar except, of course, there is no midrange pot from which to remove the stack from ground. Instead, you'll need to locate the resistor connected between the left lug of the bass potentiometer and ground. Next, unsolder the resistor's connection to ground and run a wire from the disconnected resistor lead to one of terminals of the bright switch (after disconnecting the existing wire and capacitor from the bright switch). The other terminal of the bright switch should then be connected via a jumper wire to the left (grounded) lug of the volume potentiometer. So far, we've increased gain by locating areas of the amp where gain becomes most apparently reduced, specifically, the tone stack and the effects (reverb and vibrato). Another area of gain reduction involves the negative feedback loop. All Fender amps except for some of the small, early tweed models, such as the Champ, incorporate negative feedback as a means of reducing interference and noise while increasing headroom and overall stability. Feedback is perhaps most familiar when describing the squeal that occurs when placing a guitar against a live amp's speaker. While this effect is indeed called feedback, it is more accurately termed positive feedback, meaning that an output signal-or part of an output signal-is fed back into the input of the amp, or an earlier portion of the amp. This output signal is in phase with the input signal and so the overall signal builds upon itself positively until oscillation occurs: thus the squealing sound. Negative feedback, on the other hand, signifies an out-of-phase
or opposing output signal, or a portion of the output suffers. A better approach, and the centerpiece of this signal, fed back into an earlier stage of the amp. In mod, involves replacing the jumper with a 1.5-K-ohm, 1/2-watt resistor. Anything up to 10 K-ohms will work. terms of a Fender amplifier's negative feedback loop, a small amount of signal from the speaker jack passes Be aware that with this configuration the external through a resistor connected to the phase inverter. speaker jack won't function properly. If you still want the jack to function, instead of replacing the jumper, Being out of phase with the incoming signal, it tends to cancel the signal. Because the negative feedback signal disconnect from the jack the wire coming from the ciris small, the cancellation is small, yet large enough to cuit board and place the resistor between that wire and its former connection on the external speaker jack. See cancel unwanted noise. Since cancellation doesn't discriminate, the overall signal is affected and, as a result, the photo on the next page for that wire's location. To make this gain boost technique switch able, use either gain is reduced. An old trick entails disconnecting the negative feedback wire from the speaker to jack to get back gain and, by extension, distortion. While in an older amp such as a tweed model, this might be practical, on a blackface or newer model this technique often becomes a direct path to oscillation. Granted you might not always encounter oscillation; it might come on gradually or even not at all. The amp, though, will undoubtedly feel unstable. I suggest giving it a try and see for yourself, as we will discuss shortly. To retain stability and still retain lost gain, technicians often use larger value negative feedback (NFB) resistors, thus reducing the amount of signal cancellation. This technique is simple and will be our next modification. Note that this mod isn't necessary with Hot Rod or Bassman tweed and Bassman reiss ue models since these am pliTo use this Radio Shack DPDT miniature toggle for a combined vibrato fiers have a presence control that more or and tone stack lift, connect wires from the intensity potentiometer less adjusts the negative feedback. When turning the presence control full blast to terminals A and B; connect wires from the mid and volume potentiometers to terminals C and D. on these amps, you'll notice an increase in background hiss; this is a result of negative feedback being decreased. For other Fender amps, first locate the wire that runs from the speaker jack to the circuit board. On most Fenders this wire will actually be connected to the external speaker jack, located right next to the usual speaker jack. There will then be a small jumper wire between the speaker jack and the external speaker jack, soldered to the same lug as the negative feedback (NFB) wire coming from the circuit board. Refer to the top photo on page 152 for the jumper wire's location. By disconnecting the jumper, NFB will be removed from the circuit. You may be able to run the amp with the To lift the tone stack of a typical two-band Fender normal channel, jumper disconnected but, again, stability disconnect resistor A from ground.
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one of the bright switches or install a SPST toggle switch in place of the external speaker jack. Simply run a wire from each end of the newly installed resistor to each terminal of the switch. The idea is to have the switch, when engaged, function as a jumper across the resistor and thus restore the NFB loop to its original configuration. In Chapter 8 when describing the Dumble-inspired modification, I briefly discussed how the added deep switch could be converted to a gain boost switch. When viewing a typical Fender schematic, you'll see that the
signal coming into the tone stack splits between the 250pF treble cap and the 100-K-ohm slope resistor. Once past the slope resistor the signal splits again between the O.l-uF bass cap and 0.022-uF midrange cap. This mod takes the bass signal from the O.l-uF cap, passes it through a resistor, a switch, and another resistor, and then feeds it into the wiper of the volume control, which, as the schematic shows, is on the other end of the tone stack. Essentially, a large portion of the bass signal bypasses the tone stack and volume pot, resulting in a noticeable low-end boost. We will pick up on this explanation again after detailing the instructions for the mod. The first step is to disconnect the wire and cap from one of the bright switches (whichever one you use least). If you want the boost to be for the vibrato channel, you don't have to disable the vibrato channel bright switch; you can use the normal channel bright switch, and vice versa (normal channel boost with vibrato channel bright switch). Also be aware that besides disabling a bright switch, the tone controls will have a noticeably reduced function. In fact, the bass potentiometer will act more like a level control for the boost than as a bass control. Also note that this is a bass-focused boost; treble will be somewhat diminished. Replace jumper wire A with a 1.5-K-ohm to 10-K-ohm, 1/2-watt resistor Solder a 270-K-ohm resistor to the as a means of reducing negative feedback and increasing gain. To keep wiper (middle lug) of the volume pot the external speaker jack functioning, instead of replacing the jumper, and a 270-K-ohm resistor to the left lug disconnect wire B and install the resistor between it and its original of the treble pot (when viewing it with connecting pointto the external speaker jack. the lugs positioned upward). Make sure
Here, the switchable deep gain boost mod has been installed in the normal channel ofthis silverface.
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to keep the existing wires attached to the pot lugs when soldering the resistors to them. Solder a wire from each of the open leads of the resistors (two wires, one for each resistor). Next, solder the other ends of the wires to the open terminals of the disabled bright switch (one wire to each of the two terminals) . The function should be such that when you engage the bright switch, the two resistors become connected to one another. This arrangement causes much of the low-frequency signal from the first-stage preamp to bypass the tone stack and flow directly into the second-stage preamp, thus limiting the insertion loss, at least of the low end. Enough signal will run through the tone stack to render minimal tone shaping control. Mostly, you'll have a heavy, rich, and deep low-mid and bass tone boost, not to mention markedly increased volume. For accuracy, we should call this modification "the switch able deep gain boost mod." Switchable deep gain boost will also work in Fender reissue amps that have a bright switch, although it may prove challenging to work with the pots and switch due to the use of pc boards in these amps. For all reissues as well as silverface and blackface models that don't have a bright switch, you'll need to install a switch elsewhere, such as in the hole for the number 2 input after removing its jack. On amps with pc boards this process will be difficult and nerve-wracking. In that case, definitely reconsider installing this mod. For Hot Rod models, there is no need to install gain boosting modifications as long as you tune-up the overdrive circuit as detailed in Chapter 6.
Adding Gain Stages to the Preamp As we've seen, many different techniques exist for boosting preamp gain. The ones detailed here involve disa bling various circuits or parts of circuits to enable more gain. While guitar amplifiers have a large gain potential on their own, the addition of tone stacks, effects, and negative feedback all tend to reduce that gain potential. Take the typical blackface vibrato channel, for instance-the first triode amplifies the input signal, which then passes through the tone stack to a second triode after which the signal is split, sent through two more amplifying stages around the reverb tank, after which the signal is combined and sent through yet one more triode before entering the phase inverter. That's plenty of triodes. In the normal channel, by contrast, the signal only passes through two triodes, one on each side of the tone stack, before heading to the phase inverter. While these channels aren't quite equal in terms of loudness, the levels aren't that much different. The point here is that due to the addition of reverb and vibrato effects, the vibrato channel uses more than twice the triode stages than does the normal channel. In this section we will approach gain boosting by adding an extra tube stage. Instead of adding an extra tu be, though, we will be redirecting, as it were, the existing tube's function, replacing its designated use with the alternative duty of delivering lots of gain. Since even the addition of one triode has the potential to provide an overwhelming amount of gain to the preamp, we need a means of controlling the delivery of that gain. As indicated at the beginning of the chapter, switches and potentiometers offer the pri mary
For an added gain stage, remove connections from A and B, solder a 220-K-ohm resistor to B, solder the cable from A to the resistor, and solder the cable from B to point A. For full gain potential, liftthe tone stack as detailed in the accompanying text.
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methods of controlling gain. To that end, our first mod uses one of the amp's existing volume controls to vary gain, while the second mod makes use of the multiple input jacks to provide a high-gain input in addition to the standard input. Finally, the last mod outlines the procedure for incorporating a switch or relay to flip between normal gain and high gain. Adding a gain stage for overdrive to a dual channel silverface, blackface, or blackface reissue isn't as difficult as it sounds. Be aware, though, that the amp becomes a single channel due to the disabling of the normal channel's functionality (although the normal channel's input jacks become the only functioning input). Moreover, the larger the increase in gain, the greater the accompanying background noise. Expect your amp to have some added interference. First, locate the shielded cable coming from normal volume
from pin 7 of the first tube on the right (tube VI). On a reissue this will be a wire that runs from the circuit board to pin 7 of VI. Unsolder this cable or wire from pin 7. Next, locate the shielded cable that runs from the vibrato channel's input jacks (actually the input jacks' resistors) to pin 2 of the second tube from the right (tube V2). Again, on a reissue this is a wire that comes from the circuit board. Unsolder this cable or wire from pin 2 of V2. Next, solder a 220-K-ohm resistor to pin 2 of V2. The resistor value isn't critical but should be close to 220-K-ohm. In fact, you can go quite a bit lower for a little more gain boost; however, the resistor functions to reduce parasitic oscillation as well as to properly couple the triodes. Now, solder the volume cable or wire that was on pin 7 of VI to the other end of this resistor, and install the cable or wire that was on pin 2 of V2 to pin 7 of VI. The reason for
Adding an extra gain stage to a blackface reissue is essentially the same as adding one to an original blackface. Remove the wires from A and B, solder a 220-K-ohm resistor to B, solder the wire from A to the resistor, and solder the wire from B to point A. Lifting the tone stack on a reissue requires more work than lifting a blackface's tone stack. See the accompanying text.
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To lift the tone stack in a dual channel blackface reissue requires flipping the tone control pc board around. Unsolder one lead of normal channel's midrange resistor (usually designated as R9) and remove the lead from its mounting hole.
making this last connection on VI is to ground the grid of the unused triode of VI; otherwise the tube will generate noise and could go into a runaway condition, which is definitely not good for the tube. As long as the vibrato channel's input jacks stay ground-that is, unused-the grid will stay ground. Finally, to get the full gain potential from the added triode, consider lifting the normal channel's tone stack. Perhaps try the amp with the tone stack connected and hear if you like it. The amp is still going to have plenty of added gain with the tone stack engaged. Furthermore, a functioning treble control on the normal channel acts like a high-cut filter that helps eliminate background noises. Lifting the tone, as you might recall, is fairly easy on a silverface or blackface model. Simply unsolder either lead of the resistor attached to the left lug of the bass potentiometer. Also, consider converting the normal channel's bright switch into a tone stack disabling switch as detailed earlier in this chapter. On a blackface reissue, lifting the tone stack proves more difficult since the pc board to which all the pots and jacks mount has to be removed. Removing the board involves removing all the knobs and all the pot and jack mounting nuts. The board doesn't need to come all the way out, though; it just needs to come out far enough to flip it downward. Locate the normal channel's midrange resistor (usually
designated R9, but check the schematic, as I've also seen it designated RS) just below the bass potentiometer, unsolder one of the leads, and then lift the lead out of its mounting hole. After considering these procedures, you might decide to keep the tone stack engaged. The extra work may not be worth the extra gain with the extra background noise. After the mod's completion, plugging a guitar into the normal channel input will give you the full preamp chain. Use the normal channel volume control as a gain control or preamp volume control (both mean the same thing here). The volume control of the vibrato channel, on the other hand, now acts like a master volume or second preamp volume (again, the same thing). If you didn't lift the tone stack, both of the channels' tone controls will function, with, again, the treble control making a handy high-cut filter. Turning the normal channel volume to near full and using the vibrato channel volume at a lower setting to set overall volume will give the overdrive effect at lower volumes, or vice versa, meaning either volume control can be used to control the overall volume. Conversely, turning both volume controls to a high setting gives a huge gain boost. Be aware that if you plug a guitar into either jack of the vibrato channel, you will be plugging into a triode with no volume control, meaning the signal will
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For the next modification, locate the vibrato channel's number one input jack. The connection between the jack's grounding ring, A, and its shorting blade, B, needs to be opened. The 68-K-ohm resistor, C, for input jack number two also gets disconnected.
be at full blast. Yet, this full-throttle half-tube channel won't be as loud as a full-tube channel. It will have an interesting compressed sound to it, but with a fair amount of background noise. Give it a try for the heck of it; the guitar's volume and tone controls will be the only controls over the half-channel. As an extra precaution, though, consider grounding pin 7 of VI (the unused triode's grid). One way to do this is to unsolder the other end of the cable you connected to VI from the two 68-K-ohm resistors at the vibrato channel's input jacks, and solder it to the input jack lug to which its shielding attaches. In other words, solder the center conductor to ground. On a blackface reissue, unsolder the other end of the wire you connected to VI from the circuit and solder it to ground, such as the grounding lug for the footswitch jack or the upper (negative) lead of one of the cathode capacitors, such as C6, on the circuit board. You might have to extend the wire if it won't reach. Note that now the vibrato input jacks will essentially be useless since they are disconnected from everything. Rather than leaving the second triode of VI unused, it too can be added to the chain of preamp stages, the
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result being, not surprisingly, an immense amount of gain with an annoying amount of noise. I advise against adding this second triode since making it function properly requires a major amp overhaul. Without changing key components, replacing potentiometers, and rewiring the controls, adding both triodes as they are configured will not have desirable tone and, in fact, might be quite noisy. A variation of the previous configuration for adding an extra gain stage allows you to use the normal channel input as a high-gain channel and the vibrato channel input as a standard gain channel. Because blackface reissues have their input jacks attached to a pc board, the modification is meant only for silverface and blackface models. The setup works basically the same as above: A triode from the normal channel becomes the first preamp stage. Yet, whereas with the previous configuration the extra preamp remains continuously engaged, this configuration makes use of the input jack's shorting feature (refer to the photo above). Normally, when a guitar is not plugged in, the input blade of the jack contacts another blade that connects to ground through the jack's grounding ring. Once a guitar is plugged in,
As detailed in the instructions, after removing the cable from pin 7 of Vl, the empty pin should be grounded to either the reverb jack ground, orthe cathode resistor and capacitor ground. Here, the reverb jack ground is indicated atA.
the input blade's connection to the grounded shorting blade opens, allowing the guitar signal to enter the preamp. This arrangement keeps the grid of the first triode grounded when nothing is plugged into the amp and, by extension, keeps the amp stable and quiet. If we disconnect the shorting blade and instead connect it to the output of an extra preamp stage, when nothing is plugged into the jack, this extra preamp will feed into the input of the existing preamp. Conversely, if we plug a guitar into the jack, the extra preamp disengages from the jack and therefore from the existing preamp, allowing only the guitar signal into the existing preamp. Referring to the photograph above, locate and disconnect the shielded cable from pin 7 of the normal channel's preamp tube, the first tube on the right (tube VI). Referring to the previous photo, locate the number 1 vibrato channel input jack and disconnect or cut the wire between the grounding ring lug and the shorting blade lug (the wire is the lower lead of the I-M-ohm resistor that bridges the lugs; the shorting blade lug is the uppermost one). Also disconnect the 68-ohm resistor from the number 2 vibrato channel input. Next, solder the cable you unhooked from pin 7 of VI to the shorting blade lug of the number 1 vibrato channel input (the lug that you disconnected from the grounding ring). Finally, solder a 5- to 6-inch wire from the now-empty pin 7 of VI to ground. Good grounding spots include the grounding lug of the reverb out jack (where the black wire from the reverb transformer connects) or the ground junction of the cathode resistors and capacitors from VI and V2. When plugging into the normal channel input, the normal channel's tone and volume controls function with the first gain stage (the normal channel) while the vibrato channel's tone and volume controls act somewhat like master controls. On the other hand,
when plugged into the vibrato channel input, the amp works as usual. Note, that the number 2 vibrato channel input will be inoperative. As guitar amp technicians have known for decades, the tweed-era Bassman is especially suited for converting one of its two input preamp stages into an extra gain stage. Whereas later dual channel Fenders have more-or-Iess two distinct channels, the tweed Bassman essentially has two slightly different input channels, meaning that a dual triode is split, with one triode for each input. The resulting signals are then mixed by the volume controls and sent to the second preamp stage. One glance at the schematic reveals that the only difference between these two input channels, one labeled "bright" and the other "normal," is the presence of a IOO-pF capacitor across the bright channel's volume potentiometer. Just like the bright switch capacitor on later Fenders, the Bassman bright cap allows high tonal frequencies to pass around the volume pot, giving this channel its name. One of the beauties of this arrangement, at least in my mind, is the ease at which one of the input triodes can be staged in front of the other, allowing for the mid-heavy crunch of this amp to really come through. Now, I realize that I could be drawn and quartered for suggesting that a tweed-era Bassman should be modified in such a way. I assure you that the mod is nondestructive and the amp easily restored, as long as you have decent soldering skills and don't burn up wires while you're poking around in the chassis. Yet, if you or someone you know owns an original tweed Bassman and you want to leave it alone, by all means don't do this mod. On the other hand, chances are fairly slim that you'll find a tweed Bassman that hasn't had some sort of work done on it, possibly even have been the victim of a butcher's soldering iron. The good news is,
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this mod can also be done on a' 59 Bassman Reissue, even without replacing its printed circuit board (although, I think it's always a good idea to have it replaced with a point-to-point-style epoxy board or tag board). First, the sacrilegious suggestion: On a tweed Bassman, remove the wire that runs from the center lug of the normal channel's volume pot to the board. Solder one lead of a 220-K-ohm, 112-watt resistor to the volume pot's center lug and the other lead to the lug on the bright channel's number 1 input jack, which connects to the lug of the number 2 input jack (the 1-M -ohm resistor and a wire going to the circuit board also connect here; leave those in place). That's all there is to it. If the leads of the resistor aren't long enough, just add an extension wire to one end. Furthermore, make sure to cover the exposed resistor leads with heat shrink tubing or, at the least, electrician's tape. Now, plug your guitar into the normal channel. The normal channel volume control sets the gain level of the added triode stage while the bright channel volume control sets the overall preamp volume . I would suggest not plugging anything into the bright channel inputs as things will sound bad . In fact, I strongly recommend a more professional way to perform this mod: Unsolder the wire from the number 1 bright channel input jack and instead of soldering the 220-K-ohm resistor to the volume wiper, solder it right to this wire, completely bypassing the input jack. This will ensure that if anyone plugs into the bright channel number 1 input, nothing will happen. For the '59 Bassman Reissue the operation is only slightly more complex. Because the potentiometers on the reissue are soldered to a printed circuit board rather than having wires run from them, we can't use the same approach as before. (It is possible that your reissue might have had the pots replaced without using the pc board, but not likely.) There are several different approaches that can be used. The one I favor is to disconnect the wire from pin 7 of the first tube on the right (tube V1). This wire brings the input signal to the grid of the bright channel's input triode. Either remove this wire completely (preferred method) or tape the exposed end and tie the wire away. Next, on the right side of the circuit board, locate R12, the 270-K-ohm resistor right next to C24, the large 22-uF electrolytic capacitor, which is just to the left of another large 22 uF electrolytic capacitor (C24 being the second electrolytic cap from the right). Lift the lower lead of R12 by carefully heating it with a soldering iron while simultaneously either pulling the lead up gently with small needle-nose pliers or lightly prying it upward with a small screwdriver between it and the board. Take your time, remain patient, and be limited with the application of the soldering iron. Once the lead has been removed from its mounting hole, solder one end of a wire to it and the other end of the wire to the empty pin 7 of Vl. The volume controls will
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function as they do with the tweed Bassman mod above; however, you won't need to take precautions regarding the bright channel inputs since they become completely disabled when the first wire has been removed from pin 70fVl. While adding an extra gain stage from one of an amplifier's existing tubes may not be a difficult procedure, adding a switch that can enable and disable the added gain stage or stages proves to be more complicated. While finding a location for the switch may not present a challenge-a DPDT miniature toggle switch could replace a bright switch, or a number 2 input jack might be sacrificed for standard size DPDT togglerunning numerous wires that carry signals does pose problems of potential buzzing and hissing. The potential introduction of noise necessitates the use of shielded cable and using the shortest runs as possible. Finally, because cables are thicker and less flexible than wires, and because space is at a premium behind the front panel of a typical silverface, soldering cables to the back of the switch can be challenging, as can be preventing the surrounding wires from being burned by the soldering iron. One solution entails leaving the switch loose, unmounted until after you attach the cables. Of course, the easiest solution involves forgoing the use of switches entirely. Indeed, with the previous modification of adding separate high-gain and standard inputs, the use of a switch is unnecessary. And certainly, adding a switch to a vintage tweed Bassman seems down-right wrong; whereas, adding one to a Bassman reissue presents unnecessary challenges. That leaves our first modification of this section. However, be aware that the large sound level difference between normal gain and gain boost makes flipping in and out of gain during a song impractical, unless of course you're aiming for a volume boost, such as during a lead. The gain modification procedure we'll use when incorporating the switch differs slightly from what we previously did. Specifically, rather than switching the cable at the tube sockets, we'll switch them at the pots and jacks, close to the location of the switch. The terminal numbers in the following instructions refer to the number designations for the terminals in the diagram. First, determine whether to replace the normal channel bright switch or the number 1 or 2 input jacks of the vibrato channel with a double-pole, double-throw switch. Next remove either the bright switch or jack. Note that it will be easier to wire the DPDT switch before mounting it to the chassis. Solder a jumper wire from terminal 2 to terminal 5 of the switch. Run a wire from terminal 1 to ground, such as the left lug of either volume pot. Unsolder the cable from 68-K-ohm resistors of the normal channel's input jacks and solder the cable to terminal 3 of the switch.
Double-Pole, Double-Throw Switch Terminals
To Ground
To Pin 2 ofV1
To Normal Input
-+- Jumper 2 to 5
0 4_ +- To
Pin 20fV2
From Normal Volume Pot
This diagram shows the connections to the terminals atthe rear of a double-pole, double-throw switch.
Unsolder the cable from the 16 THS ' 1 2 middle of the vibrato channel's volume pot and solder it to terminal 4 of the switch. Run a new shielded cable from the 68-K-ohm resistors of the normal channel's input jacks, the same place from where you removed the cable in step C to terminal 5 of the switch. Ground the shielding to terminal 1. Refer to Chapter 6 for instructions regarding the preparation of shielded cable. Unsolder the cable from the A low-voltage, physically small relay suitable for use as a remote gain switch. middle lug of the normal channel's volume pot and solder it to the left lug of the pot, effectively grounding it and, in turn, the and then 6 feet back across the floor and back into the grid of the unused triode of the normal channel. amp. The length and twists of the previous sentence demonstrates the stretch the amp signal would have Run a new shielded cable from the middle lug of the normal channel's volume pot to terminal 6 of the to take, and besides being weakened by that obstacle switch. Again, ground the shielding to terminal 1. course, the signal would invariably pick up all manYou now have switch able gain. The normal channer of interference. If one follows that course, I assure nel's input jacks now become the only functioning you, there will be anguish. The solution, then, involves jacks. Flipping the gain switch will switch a triode in some form of remote switching, a way to keep the signal in the amp while being able to engage and disand out of the first preamp stage position. Enabling high-gain at the flip of a switch is fine if engage gain from a distance. The most common device you aren't planning on using it in the middle of a song, suitable for this purpose is a small relay consisting of a in which case, the presence of a switch seems unneclow-voltage winding, which, when engaged, pulls a set of internal contacts closed. Furthermore, the contacts essary. A better plan would be to use a footswitch so that you can flip in and out of high gain without need to be configured in a double-pole, double-throw your hands leaving the guitar. It's extremely unwise, (DPDT) arrangement, just like our switch. In fact, the not to mention impractical, to run signal wires out configuration is exactly the same, except instead of of an amp, 6 feet across the floor, into a footswitch, flipping a toggle to move the switch contacts, a relay
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uses electromagnetism to pull them open and closed. Additionally, a small relay can be mounted in the amp's chassis with a dab of silicon adhesive. When properly connected to a power supply, low voltage will be present in the winding at all times and when we engage a footswitch it will ground the opposite end of the winding from where the voltage enters. In this way only a small voltage surrounded by grounded shielding passes through the cord to the footswitch, which grounds the voltage to the shielding, allowing the winding of the relay to move the contacts. Several different ratings determine the uses of a typical relay, for our purposes the most relevant being the voltage rating of the winding. Since the signal wires going to the relay contacts draw a miniscule amount of current and carry low voltages, the contact ratings aren't as important. The winding, on the other hand, should be rated at 5 or 6 volts so that the lowvoltage tube filament and lamp supply can be used to power the relay. Note that the voltage requirement for the winding will be for DC voltage. Also note that the filament voltage supply is AC. Batteries have been
used successfully to power small relays and can be mounted in the cabinet with wires running to the relay mounted inside the chassis. A more permanent solution consists of building a small voltage rectifier for the relay. Of course things get much more complicated here, and we're pushing the limits of the scope of this book. I recommend visiting Doug Hoffman's website, www.el34world.com. where he provides detailed instructions on not only how to install and wire a relay in an amp, but also how to build a power supply for the relay. Personally, I usually don't bother with relays but do occasionally incorporate a switch in the chassis. For the most part, my approach for boosting the gain of a Fender isn't so much to provide an additional overdrive distortion effect-pedals work great for that. Rather, I consider boosting gain in a Fender to be an enhancement measure, something that adds bolder, thicker tone with overtones of distortion, such as the procedures at the beginning of this chapter. Yet, on the other hand, it's important to explore many of the potentials that can be unlocked in a single amp.
Variable Gain Modification While this modification doesn't add an extra gain stage, it does provide a way to switch between a lower preamp gain (for clean guitar playing, for instance) and the regular preamplifier gain. As discussed in Chapter 3, the gain of a triode is partially determined by the cathode bypass capacitor. Because a capacitor passes AC voltage, portions of the signal that collect at the cathode are shunted to ground . Withoutthis shunting, the signal will act as negative feedback that reduces the gain of the triode while also quieting background noise . The standard Fender value of 25 uF allows a relatively high amount of gain and doesn't really differ from the more contemporary value of 22 uF (which is essentially the same as 25 uF when typical 10 percenttolerance range is taken into account) . Marshall, though, sometimes uses a 250-uF as a cathode bypass capacitor on the first-stage preamp. Atfirst glance, the gain difference between a 25-uF and 250-uF might seem rather pronounced . Yet a more dramatic sonic contrast exists between a 25-uF capacitor and none at all. With no cathode bypass capacitor, the preamp volume level will be reduced and the tone clean, making the amp well-suited for an acoustic guitar pickup or general rhythm guitar. If you find the gain too low with no capacitor, you can instead use a 0.47- or 0.68uF film capacitor. For a little more gain a 2.2-uF electrolytic capacitor will also work. Even though
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The normal channel on this silverface has switchable low gain feature through the normal channel bright switch.
the reduction in gain makes an amp quieter, it allows a broader range of the volume control and encourages more harmonic distortion from the outputtubes. By converting a bright switch away from its usual function of passing hightreble frequencies, you can flip the switch from normal gain to a clean, quieter low-gain effect. There are basically three places where you can provide a normal gain to low gain switching option: the first stage of the normal channel, the first stage of the vibrato channel, or the combined second stages of the normal and vibrato channels. The later position is possible because the second-stage triodes of both channels use a linked cathode design with shared cathode resistor and bypass capacitor. The tonal distinctions among these positions are slight, so the primary decision should be based on which channel you wantto have the low-gain option. The procedure is simple. Unsolder the lead atthe negative end of the bypass capacitor, solder a wire to the disconnected lead, long enough to reach one of the bright switches, and cover the lead with heat shrink tubing or tape. Because only one lead of the capacitor is left connected, the mounting may be a little unstable. A small drop of silicon adhesive between the cap and the board will keep it secure. Unlike glue, silicon adhesive is flexible Here the normal channel switches between standard gain and is fairly easy to remove. Be aware that and a lower gain due to the lower value cathode capacitor one of the bright switches will be disabled. If added to the circuit. The SPST mini-toggle switch is shown your amp doesn't have a bright switch, you can install a switch in the spot normally occupied unmounted to reveal the terminal hookup. by the number 2 input jack after unsoldering the wire and resistor from the jack and removing it. If using the brightswitch, unsolder the existing wire and capacitor from it and solder a 100-K-ohm or larger (up to l-M-ohm) resistor across the switch (one lead to each ofthe switch terminals). The purpose ofthe resistor is to eliminate any "popping" sound when switching the bypass capacitor into the circuit while the amp is running. Next, solder one end of a wire to the volume potentiometer's ground lug (the left one) and the other end to one of the switch terminals. Finally, solder the free end ofthe wire from the bypass capacitor lead to the other terminal of the switch. If adding a switch in place of an input jack, follow the same procedure, making sure to solder the shielding from the cable to the ground lug of the number 1 input jack if you unsoldered it from the number 2 input jack. Also, solder the lead from the 68-K-ohm resistor, originally connected to the number 2 input, to the same lug of the number 1 input to which the other 68-K-ohm resistor is soldered . This puts the resistors in parallel and therefore replicates the original jack hookup. To enable switching between the 25-uF bypass resistor and an added 0.47-, 0.68-, or 2.2-uF capacitor, replace the bright switch with a single-pole, double-throw (SPOT) switch. A OPOT can also be used, but you'll only use one side of it. Mountthe additional capacitor nextto the existing cathode capacitor by soldering its lower lead (with the chassis oriented with the tubes at bottom) into the same mounting hole as the cathode resistor's lower lead. Rememberto observe polarity if using a 2.2 electrolytic capacitor. Use a small amount of silicone adhesive to secure the cap to the board. Follow the steps above for installing and wiring the switch, except connect the middle terminal ofthe switch to ground and solder each wire from the upper leads of each capacitorto the top and bottom terminals (one wire to each terminal). Now when flipping the switch the gain will shift from normal gain (using the 25-uF cathode resistor) to a lower gain (the additional capacitor), but higherthan if no capacitor is used.
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chapter
10
DUAl CHANNfl MODIfiCATIONS The modifications in this chapter are advanced and meant for those with a decent grasp of electronics and more than basic soldering skills. However, the information in this chapter should prove instructive to anyone wanting to move beyond basic Fender modifications. There are only two modifications in this chapter, one for installing a classic Marshall-style preamp and one for installing an early Vox-style preamp. These are followed by a brief discussion of the Bassman channel-bridging technique and the ways in which it can and cannot be used with these modifications. Since most blackface and silverface Fenders have two channels, the normal channel preamp can be converted into a Marshall or Vox preamp and the vibrato channel can retain its Fender preamp with reverb. It will be somewhat like having two amps in one cabinet. I say "somewhat" because the output stage remains the same. Another option might be to convert the normal channel preamp to a Vox and the vibrato channel preamp to a Marshall; however, as will be mentioned later, due to its reverb effect, the vibrato channel won't have that rich, mid-heavy tone. In fact, the mid-frequency boost tends to muddy the reverb, which ultimately responds better to the treble-focused, mid-scooped tone of the standard Fender preamp.
Marshall Channel Modification First of all, this modification involves the preamp and not the power amp, meaning this will not make your Fender sound like a vintage Marshall. The output remains clearly Fender. First of all, in a Fender dualchannel amp, two preamp channels share a single output stage. But beyond that, even if a dual channel had two separate output stages, replacing the Fender output with a Marshall output wouldn't be worth the cost or effort. A quality output transformer alone costs more than $100, and to make it fit means drilling new mounting holes in the chassis. Now that we've gotten that out of the way, let's point out what will change. Converting one of the
Derek Trucks' blackface Super Reverb. Derek Trucks
Collection/Rick Gould
preamp channels to a Marshall configuration adds more mid-rich gain. In fact, the amp will sound more like a vintage tweed Bassman than a Marshall, since the preamp of a vintage Marshall is basically the same as that of a tweed Bassman. The following instructions focus on converting the normal channel of a dual blackface or silverface model and will not work on a blackface reissue unless it has had the pc board swapped with what I've been calling a Hoffman board. The normal channel configuration brings us closer to the classic Marshall/Bassman sound than does that of the vibrato channel with its additional reverb and vibrato effects circuitry, neither of which belongs to a classic Marshall or Bassman. However, you can easily adapt these instructions to the vibrato channel in which case I recommend using an added cathode follower (to be explained shortly) after the second triode, prior to the signal split into the reverb circuit. In this way, the signal will have its tone shaped before being run through the reverb tank. While it doesn't make a world of difference, I feel that the tone sounds better when shaped prior to reverb, which tends to complicate frequency separation. An obvious disadvantage to using the typical normal channel involves the two-band tone stack. Since Marshalls have three-band tone stacks, the vibrato channel seems the better choice for conversion, yet, again, the inclusion of reverb and vibrato moves us further away from Marshall territory than does the lack of a midrange control. Moreover, I originally intended this modification for the silverface Fender Twin, which does have a three-band tone stack for its normal channel. The following rendition of the Twin mod attempts to universalize the mod, adapting it to the typical blackface and silverface two-band normal channel. As discussed in Chapter 8, a primary difference between the preamp of a tweed Bassman, and by extension a classic Marshall, and that of a blackface or later Fender entails both the location of the tone stack and the use of a cathode follower between it and the previous second-stage triode preamp circuit. As pointed out, even though the cathode follower contributes no gain to the signal, its design allows for less insertion loss from the tone stack and therefore allows more gain to be retained and passed on to the output stage of the
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Standard Blackface Fender Twin Channel
VIA V212 AX7
VIB V212 AX7
r------------r-----100K
Bright
100K
120pf 250pf
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