Musical Instrument Tube Amp Building, Maintaining and Modifying FAQ Much of this material applies to building or re-building hi-fi equipment, as well but it was originally intended for musical instrument crazies. Authored, assembled and edited by R.G. Keen, keen@eden.com Most recent revision level is Version 2.00, appx. 1/15/99 -------------------------------------------------------------------------------- Can I replace my own tubes? Preamp tubes - sure! They're in sockets, any replacement tube of the same type will at least not damage the amp. Even preamp tubes that are not exactly the same type can often be substituted as long as they have the same pin connections. For the commonest type, 12AX7, there are many types that have the same pinout and can be put in the same socket for different gains and tone. For instance, you might be able to use 12AU7, 12AT7, 12AY7, ECC82, ECC83, 7025, 5751, 6201, 6072A, 5814A, 12BH7 and others. See the section on tube substitutions for more info. Output tubes are more problematical. You really should have a tech check and if necessary adjust the bias on your output tubes whenever they are replaced. This keeps them from getting too hot. Power tubes are much more finicky than preamp tubes in this way, as preamp tubes do not in general need bias adjustment. You will undoubtedly have heard that you can substitute in other types of output tubes for better tone as well. This is getting into really tricky areas if you don't have the capability to open up the amp and rebias. You can imagine that if you have to rebias when replacing output tubes of the same type, you certainly have to with different tube types. That being said, there are a number of output tubes that have the same pinout. You should consult a competent tech befor doing this, as some of the "compatible" tubes may need minor rewiring or may use more heater current than your amp can supply. It's not a good idea to just swap in different types of output tubes unless you understand the different requirements they have. What things will damage my tube amp, what's safe and what's not? We'll assume that you don't need guidance about the obvious: don't drop it in a lake, or from a helicopter, don't pour it full of soda or beer, and so on. A few more pointed do not's: Never, never, never run the amp with no speaker plugged in. This can cause major damage. Do not flip the power switch off, then back on rapidly. This can cause power supply damage. Never replace a burned out fuse with a bigger-amperage one. Remember - there was a reason the first one burned out, usually protecting something more expensive. Putting a bigger fuse in will just rachet up the power level until something really vital burns out. If the second equal-rating fuse pops, turn it off and get a tech to look at it. Never ignore signs of high heat inside - a wisp of smoke or a burning smell is NOT normal. Your amp produces lots of heat, and will continue to do so even if you block the fresh air vents. Blocking the vents will just allow the amp to heat to the point that you get to buy some very expensive repairs. Never ignore a red glow other than the small orange ends of the filaments. A red glow over a large part of the internal plates of the output tubes means they're about to melt (yes, really melt - heat is our enemy). If you notice this, shut it down and get a tech to help you find out what it wrong. Correspondingly, you can do the following without too much worry: Add another speaker into the "external speaker" jack; a mismatched speaker load won't kill it, while an open circuit (disconnected speakers) may do so. Overdrive the stuffings out of it. Tubes are very forgiving of massive overdrives, unlike solid state stuff. As long as they tubes don't overheat or stay overdriven for long periods, it's not fatal. -------------------------------------------------------------------------------- *** SAFETY WARNING *** READ THIS FIRST!!!!! Working inside a tube amplifier can be dangerous if you don't know the basic safety practices for this kind of work. If you aren't prepared to take the time to learn and apply the right precautions to keep yourself safe, don't work on your own amp. You can seriously injure yourself or get yourself killed. This section is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one. BASIC REQUIREMENTS UNPLUG IT FIRST Pretty self explanatory. Do not, ever, ever, leave the equipment plugged in and start work on it unless you specifically intend to make some live-voltage measurement. Leaving it plugged in guarantees that you will have hazardous voltages inside the chassis where you are about to work. This is like setting a trap for yourself. LET IT DRAIN If the amp has been turned on recently, the caps will still have some high voltage left in them after the switch is turned off. Let it sit for five minutes after you turn it off. SUCK IT DRY When you open up an amp, you need to find a way to drain off any residual high voltage. A handy way to do this is to connect a shorting jumper between the plate of a preamp tube and ground. This jumper will drain any high voltage to ground through the 50k to 100K plate resistor on the tube. To do this successfully, you will need to know which pins are the plate pins. Look it up for the amp you're going to be working on. You'll need to know this for the work anyway. Leave the jumper in place while you do your work ( high voltage electrolytics caps can "regrow" voltage like a battery sometimes. Really. ) Remember to remove it when you finish your work. TEST IT Take your multimeter and ground the (-) lead. Probe the high voltage caps and be sure the voltage across them is down, preferably to less than 10V. BUTTON IT BACK UP FIRST Take the shorting jumper out. Put the chassis back in the cabinet, making sure all of your tools, stray bits of solder, wire, etc. are out of it. You don't have to actually put all the screws and so forth back in if you believe more work might be needed, but make sure that the chassis is sitting stably in the cabinet and won't fall out. At the end of a listening test, either continue buttoning up if you're done, or go back to UNPLUG IT FIRST. -------------------------------------------------------------------------------- Basics of Tube Amps for Beginning Users -------------------------------------------------------------------------------- Where can I learn about building tube amps? Get one or more of the following references (note that these books are mostly old, and highly sought after, and so may be expensive and hard to find): "The Ultimate Tone" by Kevin O'Connor. This the best book on guitar amps I've found. It assumes you know some electronics to start with, so is not a beginner's book. Published by Power Press, which now has a web page at http://www.wwdc.com/~power/ . "The Tube Amp Book" by Aspen Pittman, now in its fourth edition. This contains the majority of guitar amp schematics ever made. Don't believe all the "technical information" as gospel, though. "Electric Guitar Amplifier Repair Handbook" (?) By Jack Darr. Good intro to actually making repairs as well as many schematics. "ARRL Handbook", preferably a late 60's or early 70's edition. Read the sections on construction practice, safety, and tube info. Guitar Player Magazine's article on tube types and operation from a year or so ago Glass Audio magazine, Old Colony Sound in Peterboro NH Mesa/Boogie will send schematics of their amps, call 1-707-778-6565; note however, that these schematics are known to be inaccurate. "Vacuum Tube Amplifiers" by G.E. Valley, Jr. Part of the MIT radiation lab series, originally published by Boston Technical 1964. Reprints are currently available from Antique Radio Classified (P.O. Box @, Carlisle, MA 01741, 508-371-0512) Amplifiers, H. Lewis York. (Evidently part of the Encyclopedia of High Fidelity). Good basic technical ref. Simple math, good explanations. Includes a couple of designs (several use hard to find tubes) and tips on physical construction as well. Radiotron Designer's Handbook, Langford-Smith. Heavy theory, heavy technical. Not coffee table reading, but if you want to know, it's probably in there. This book is perhaps the most highly sought after tube related book, and commonly goes for $75-$100 in good shape. You want the 4th edition. Old Colony Sound just announced a CD ROM version of this book, apparently indexed, illustrations and all, for $69.95. RCA Receiving Tube Manual. Reprints available from several sources, including Antique Electronic Supply & others (Old Colony?) Mostly tube spec sheets & some characteristics charts. The intro is a pretty good technical primer. Electron Tubes, R.G. Kloeffler. little application, but a good easy to digest explanation of characteristics of diodes, triodes, beam power & true pentodes, with the math to go along. Worth reading if you're trying to do modeling. The Audio Designer's Tube Register. Tom Mitchel. 1993, Media Concepts. Volume 1 - Common Low Power Triodes. 144 pages of freshly compiled tube data, some of which was not previously published. Kinda pricey ($18 from Antique Electronic Supply) unless you need the data. Included are plate characteristics, transfer characteristics, constant current curves, mu as a function of grid potential and plate potential, transconductance as a function of plate current and grid potential, and dynamic and static plate resistance as a function of plate potential and plate current. (Tom mentions a 2nd and 3rd volume in the distant future - covering low power pentodes & oddball tubes, and Power & Beam Power pentodes respectively.) Learn about the manual and safety aspects of working on tube amplifier circuits. Read the ARRL handbook, or better yet, get to know a ham radio operator who will give you some guidance and teaching. Do not skimp on the safety aspects. Tube circuits contain deadly voltages. You can - * DIE * - if you mess up or are careless. It is your personal responsibility to learn how to do this safely. Get to know a guitar repair technician, perhaps do some free apprentice grunt work for them in return for some teaching. -------------------------------------------------------------------------------- Where can I find parts to build/repair amplifiers? New tube parts and supplies were steadily getting harder to find, but in the last couple of years this has turned around radically. There are now many companies offering new parts, especially power and output transformers. It is still true that used parts are often nominal cost or free. The hard parts to find in high quality are the transformers. If you're building, I recommend getting your transformers first. If you are getting vintage parts, they are likely to be one-of-a-kind. If you've just ordered new ones, the transformers will have a massive effect on your chassis's mechanical layout. The easiest but most expensive source for parts is at your retail musical instrument store as "repair" parts. Other sources: Musical instrument repair shops will sometimes order parts or sell you parts out of their stock. Amp makers' repair parts departments. Many manufacturers will sell their parts to "repair shops" to fix their amps. Some of them are better than others about this, so be polite and businesslike. Antique Electronics Supply, Tempe AZ. They stock tubes, some transformers, some capacitors, tube sockets, etc. Antique Electronic Supply recently added several steel and a few aluminum chassis boxes to their line. I was told this is to be a continuing trend, and not just a one time buyout of a couple of boxes. Mouser and SESCOM (don't have the address/phone) sell various rack enclosures. Mouser has a couple which would house a healthy size tube amp project. old, broken, or unloved equipment. This may be free, or units-of-dollars. You get transformers, sockets, tubes, and chassis in the deal. May require cruising garage sales or diving in dumpsters. Trash every part except the tubes, transformers, sockets and chassis. I got a 15 Watt mono amp/preamp intended for mono hi-fi music for $20 at a local garage sale. Needs only some tweaking to be a Studio .22 or an AC-15. Be sure to look at Appendix A for more sources. Premium Suppliers Fender Musical Instruments - call 800-854-6230 for a list of dealers Richardson Electronics - 800-348-5580 for dealer list RAM Tubes, 805-962-4445 VTL, 714-627-5944 Here are "more tube supply sources": Triode Electronics, 312-871-7459 Elmiria Electronics 800-847-1695 Antique Electronic Supply, oriented to radio collectors, 602-820-5411 Antique Audio, oriented to radio collectors, 512-467-0304 New Sensor, mostly imported tubes (here's the source of Sovtek), call Mike Mathews, 212-980-6748. Min. order is $50.00. ARS Electronics, 800-422-4250 Department of Defense surplus auction. DRMO-Tobyhanna Army Depot, Building 16, Tobyhanna, PA 18466 is the gummint storage facility for communications gear and is said to have good stuff. Also, get "How to buy...Surplus Personal Property from the Department of Defense", free from DOD Surplus Sales, PO Box 1370, Battle Creek, Michigan 49016. Surplus electronics dealers - see the Telco yellow pages call everybody in the yellow pages under Electronics, TV-Repair, Communications, and any other promising category. Hamfests Angela Instruments, 8600 Foundry St. Box 2043, Savage, Md. 20763, 301-725-0451. Tube Amp Service in San Francisco,run by Tom Balon; call 415-334-5200 PST. (O'Neill) -------------------------------------------------------------------------------- How can I extend my tube life? Modify the power on switching to heat the filaments first, let them warm up for 30 seconds, then switch on the high voltage plate supply. Add more ventilation to the amp chassis, perhaps with a small fan. Modify the tube operating conditions so the maximum cathode current is not exceeded under even maximum warp drive conditions. Exceeding max cathode current causes cumulative emission losses and early tube death. This requires a somewhat deep understanding of the design of tube amps to do, unfortunately. -------------------------------------------------------------------------------- How do I get... Blues distortion? Made by overdriving preamp and power tubes a little, enough to just start compressing the peaks of the waveforms, and not very much high frequency content, by electronically cutting highs or running the signal into a speaker cab that acoustically cuts highs. Guitar Player magazine ran a construction article on this very topic, modifying a Fender Bassman to be the "Ultimate Blues Machine". The article ran in 1995, authored by John McIntyre. A recently voiced although intuitively applied idea in distortion is that tube distortion sounds best when each successive distortion stage is overdriven by less than about 12db. This has the effect of keeping the tubes inside the area where the signal is more compression-distorted than clipped. That is what those resistive divider chains between distortion stages are for inside those distortion preamp schematics. Mesa's distortion preamps are another good example. Overdriving a tube stage too much gives you harsher clipping, not the singing, sweet distortion we want. To really get sweet, crunchy distortion, keep each stage that goes into distortion no more than 6-9db into distortion. Marshall/metal/Boogie/etc. distortion? Made by massively overdriving preamp tubes until the original waveform is massively compressed and clipped. Usually followed with a moderate amount of high frequency cut to remove some of the "insect attracting" overtones generated in the clipping process. There is commonly some output tube overdrive in this process, too. Good distortion at low(er) volumes? overdrive preamp tubes until you get the clipping you want, then feed a limited amount of this to a power amp stage to get the loudness you want. This is how master volume controls work. -------------------------------------------------------------------------------- Output transformer questions: Q: Do output transformers sound different from one another? If so, why do they sound different? Can you for instance wind a transformer to get an intentional frequency response? Like, you think your Marshall has too much midrange, so you put a new OT in that has like a 6-9dB cut around 800hz to get a natural "scooped" sound? A: These things cause transformers to sound different: High and low frequency roll-offs caused by the details of the iron, copper, winding and manufacturing processing. Inherent distortion caused by the magnetic properties of the iron and the driving impedance Excess distortion caused by lack of coupling between sections of the windings - literally where all the windings are in relationship to one another - in a Class AB or B biased amplifier. High and Low Frequency Roll-offs: Transformers all by themselves have a high end rolloff point and a low end rolloff point, and a broad flat region between the two. The exact frequencies where the highs and lows roll off are a characteristic of the iron, copper, and how they're wound and stacked together and treated or mis-treated. It is possible to get a resonance or two between the two ends, but most often there is only a self-resonance point above the high frequency end. Usually any reasonably well-made output transformer has no oddities within the high and low frequency roll-off points. Given enough info about the transformer and the use (power through it, temperature, DC bias current, etc,) you can use the parasitic parts of the transformer's response, the unavoidable self capacitances and leakage inductances, with other components outside the transformer to shape the frequency response inside the relatively flat pass band; this is more like shaping it externally than designing the response into the transformer, though. The parasitics are largely what cause the high and low frequency cutoffs, so it's not easy to move their effects out into the passband with simple circuits. You can design in a high and/or low frequency response point, and you may be able to tinker inside the passband a little, but even an experienced designer can't easily just design in a scoop of arbitrary depth at a given frequency with only the transformer windings to work with. Inherent (or Iron) Distortion: It's not widely understood, but transformers have their own distortion generating mechanism. The iron in a transformer has to be driven to a certain magnetic flux level to couple signals to the secondaries. The output tubes must supply this magnetizing power as well as the power to the secondaries. A well designed transformer can make this power very small. However, the nature of the iron itself is that the magnetizing power is non-linear. It takes proportionately more or less magnetizing power at different magnetization levels, so the iron siphons off more or less of the signal. This depends on the core material linearity and magnetic softness, and how close the transformer is driven to saturation. The amount of distortion depends, among other things, on the source impedance driving it, as the distortion comes from the nonlinear shunting effect of the transformer's primary inductance. A voltage source driving a transformer will be able to be distortion free on the secondary. A high impedance drive source (like a pentode plate, unfortunately) will not be able to provide all the current the core needs to keep the voltage linear, and so some distortion will show up on the secondary as a result. This distoriton is relatively small, probably 2-5% below the beginning of saturation, and is primarily third harmonic. Excess Distortion: Only in push-pull Class AB or B amplifiers (that is, most guitar amps) when the crossover point where one tube turns off and the other tube carries all of the power load, EVEN IF THE BIAS FOR THE TUBES PROPERLY ELIMINATED TUBE CROSSOVER DISTORTION the sudden change from two half-primary windings conducting all of the power to the the secondaries to only one tube supplying the secondaries, if the magnetic coupling from both half-primaries to the entire secondary is not excellent, there will be a glitch in the output waveform caused by the change in current in the leakage inductance in both the half-primary that is turning off and the one turning on. This crossover distortion can not be biased away. Cheap, non-interleaved output transformers often have this kind of distortion and sound "harsh". Q: Obviously, output transformers have tolerances. Are the impedance tolerances the reason one OT may sound better than another? A: The impedance ratio(s) of a transformer are fixed by the number of turns on each winding, and it's unusual for modern winding machines to forget or miss a turn. Can happen, but it's rare; so the impedance ratios are not what are the biggest contributors to tone differences. Q: Are paper bobbins really superior to plastic bobbins in output transformers? How about power transformers? A: No. Both paper and plastic are non-magnetic and non-conductive. The only way they can affect a magnetic field is by taking up space and therefore excluding either a conductor or a magnetic material from that space. As long as the winding window is reasonably filled with turns of copper wire, the effect of one bobbin material versus another is essentially impossible to find even with sensitive lab instruments designed for work on such things. The business about paper bobbins being thinner and letting the copper wires be nearer the core is nonsense. The entire iron core "conducts" the magnetic flux. Moving a turn of copper a tiny fraction of an inch closer to the center leg moves it that same fraction further from the outer leg. As long as the dimensions for paper and plastic bobbins are not grossly different (and the differences between old paper -actually glued-up cardboard- bobbins and plastic bobbins are tiny) then there is no reason that the magnetics should be affected at all. There is some reason for this myth being started, though. The output transformers from the "golden age" were pretty much all hi-fi output trannys. This meant they were, among other things, carefully interleaved and wound. The earliest plastic bobbins were wound not to get good sound, but because the MBA's that had infiltrated the guitar companies were trying to save a buck. A quick and dirty, lowest-bidder transformer wound without interleaving on a plastic bobbin and laminated with cheap, thick iron costs less than a good on from an old-line hifi maker, so that's what they started making. The plastic was not the problem, but it was all that the uneducated user could see, so plastic got an undeserved bad rep. Let me be clear - IT'S NOT THE BOBBIN, IT'S THE DESIGN AND ASSEMBLY. Q:Are differences in transformer sound in the pattern of the windings? Or something else? A:Having pooh-poohed the idea of paper bobbins being better with the "a turn's a turn" argument, I hesitate to get into this, but I guess I should. The thing that makes for an OT tone is the sum total of the contributions of all those parasitic elements in the transformer - the series wire resistance, the leakage inductance in the magnetic path, the turn-to-turn capacitance, the layering and distribution of layers in the window, the depth of the core stack, the thickness of the iron in the laminations the -*magnetic softness*- of the iron in the laminations and how carefully they have been interleaved and butted together, and how free the laminations are from electrical contacts. Paper bobbins are one of the few things that DON'T affect a transformer's tone, all other things being equal. It is really a no-brainer to copy a transformer, but you have to know about and be able to copy all of the details about how it's made. Once you do that, you get fairly repeatable results, with the exception of the handling of the iron. Transformer iron is annealed to be physically and magnetically soft (which go together), and an oxide layer is grown on it to prevent the layers from electrical contact, which affects how much core loss happens. If the iron is roughly handled, it gets hardened and has more core losses and higher distortion, as well as having the oxide scraped and or punctured, which increases eddy current losses. These will have an effect. You can detect the changes in magnetic characteristics of transformer iron by testing the iron into a test coil, then striking it sharply EVEN ONCE. I've performed this test myself on occasion. Care in stacking the iron and jogging it together also has an audible effect. If the laminations are carefully jogged together so the joints are as tightly fitted as possible, it runs the primary inductance up and the low frequency rolloff down, making for a better bass response. All this sounds obscure, but is really much easier than learning to play guitar on an intellectual scale. It's just another special language to learn. Most people just don't go there, so it gets shrouded in myth. Q: I remember reading Matchless's (or others...) ad stating they copied the OT of an incredible sounding amp and that was the key ingredient of their sound. What was it they copied, impedance or winding pattern? A: They had to do both to get a good copy - as well as having to duplicate or approximate iron composition, thickness, and stacking. Q: Is hand winding superior to machine winding for output transformers? When a known "good sounding" vintage or modern transformer is duplicated, will modern equipment should be able to consistently reproduce them without varying results? Is hand winding being superior to machine winding is a myth? A: The devil is in the details. Machines can produce goods with a consistency that a human hand/eye/brain simply can't, so where pure replication is required, bet on the machine. Whenever the materials/situation/adjustments, etc. require judgement and on-the-spot compensation, bet on the human - IF the human is an expert at whatever the situation is. I would say that historically, the real situation is that cheaply produced and poorly designed (for instance, non-interleaved) machine wound transformers were accurately reproduced, and sure enough, the sound was reliably bad. A good hand winder repro-ing a carefully stacked interleaved OT could, in spite of the inevitable slight variations and flaws produce a much better sounding transformers - at a much higher price - in low quantities. A myth gets started - "I used so-and-so's hand wound OT, and it ran rings around the machine-wound replacements," which is demonstrably true, but then the leap to myth is made "therefore only hand wound transformers are good." This train of reasoning is so seductive in the "vintage-is-god" environment that it almost instantly becomes common wisdom, putting the merit on hand winding when in fact the differences were in different designs. For duplicating windings, a well made, well adjusted and well maintained machine will produce more consistently identical windings than a human. Note that I put a number of caveats on that statement. An expert human with little or no equipment will produce better, more consistent goods than a bored, ill-trained, poorly paid operator coping with the wrong wire size on a poorly adjusted forty year old machine, which is how some (especially the cheaper) transformers get made. So - is the notion that handwound transformers are ALWAYS superior BECAUSE they're hand-wound a myth? Yes. Are hand-wound transformers SOMETIMES superior to machine wound? Yes, especially where the hand-winder is producing a different design from the machine. I'd call the quality pretty much the same for hand-wound and machine wound transformers where the hand-wound ones are produced in low quantity by experts (a caveat that excludes fatigue and skills flaws) and the machine wound ones are made on modern equipment with skilled operators in large quantity (which allows adjusting the machines for consistency).