Back to the Effects Page

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.

Assembled by R.G. Keen. Original material and assemblage of material copyright 1994-1999 R.G. Keen
Permission refused for local copies or serving from hosts other than www.geofex.com except by written permission.

Most recent revision level is Version 1.20, appx. 11/07/99 - minor updates from 1.10

A fair amount of the commercial information - where to get tubes and their availability, etc. - has become outdated since the FAQ was last updated. This material is being reviewed and corrected.

Contributors - my thanks to the contributors who helped and taught me:

Hundreds of folks who taught ME stuff when I didn't know a triode from a Tri-Axis; I can't remember all of your names, and it all comes out as general knowledge now, but I appreciate it. A few names in that category stand out:

And people who have contributed things that I have included as part of the actual text: 
Dennis O'Neill,   denio@seismo.css.gov
Nathan Stewart,   npstewar@eos.ncsu.edu
George Kaschner,  gckaschner@ucdavis.edu
David Kohn,       kohn@SCTC.COM
Michael Edelman,  mje@pookie.pass.wayne.edu
Len Moskowitz,    moskowit@panix.com
Tremolux@aol.com
Brian Carling,    brian.carling@acenet.com
Eric Barbour      svetengr@earthlink.net
Special thanks to Nathan Stewart who kick started the work converting this to HTML, where it is maintained now. This FAQ was started before HTML and the World Wide Web were commonly available - it's an artifact!

INDEX

Contributors

Back to the Effects Page

  1. *** SAFETY WARNING ***READ THIS FIRST!!!!!
  2. Why is AMP building in a musical instrument building group?
  3. Beginners' items - care and feeding of tube amps
  4. Where can I learn about building tube amps?
  5. Where can I find parts to build/repair amplifiers?
  6. How can I modify my amp to be more powerful?
  7. How can I extend my tube life?
  8. How do I get...
  9. Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet?
  10. Output transformer questions:
  11. What is the easiest way to get tube sound at a good price?
  12. How can I modify my tube amp to ... ? (also see recommended mods, below)
  13. When should I bias my amp and how do I do this?
  14. Amplifier Modifications
  15. Tube Characteristics and Substitutions
  16. Maintenance Issues
  17. Appendix A. Tube Stuff Suppliers
  18. Appendix B. Tube Makers Producing Today (Eric Barbour news posting)
Back to the index...

*** 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

Back to the index...

Why is an amp like a musical instrument

For electric guitars, basses, and possibly other instruments, the amp is as much a part of the final sound as the nominal instrument is, perhaps more. The instrument is relegated to a role of providing a base tone which is profoundly modified by the following effect and amplification stages. The "instrument" is properly the instrument and amp together.

Where can I learn about building tube amps? Back to the index...

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):

Where can I find parts to build/repair amplifiers?

Back to the index... New tube parts and supplies were steadily getting harder to find, but in the last year 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:

Be sure to look at Appendix A for more sources.

Premium Suppliers

Here are "more tube supply sources": George Kaschner notes that parts other than tubes and transformers can be obtained easily from Mouser Electronics (800-346-6873). I have used Mouser and they give good service and prices; $20 min order. another good source is Digi-key for resistors, capacitors, and other general electronic parts. They are not tube oriented, but are also a good general parts source.

Back to the index...

How can I modify my Blender Tweety Bird amp to be as loud as a Marshall Major/AC30/Tweed Bassman/SVT/etc.?

(Alternatively, how can I make my amp twice as loud/more power/ etc.?)

You can't do this in a low power amp, at least not electronically. To put out the power the big amps put out, you need the entire power train to be as beefy as the big amps. This means bigger power transformer, rectifiers, filter capacitors, output transformer, more power tubes, bigger chassis, more ventilation to carry off the heat, lots of things. You can't just add a couple of tubes.

An amplifier is properly thought of as primarily a big power supply that has some extra junk tacked onto it to carefully let a little of the power out to the speakers under special, controlled circumstances.

You might be able to just pull a couple of tubes OUT of a high power amp to make it quieter, under some conditions. O'Connor discusses this in "The Ultimate Tone".

Back to the index...

How can I extend my tube life?

Back to the index...

How do I get...

Where can I find plans for a Belchfire/Maximo/etc. speaker cabinet? Back to the index...

Output transformer questions: Back to the index...

A. How can I tell if my output transformer is live or dead?

There are some simple tests you can run to quickly determine if a transformer is grossly bad. This is much simpler than determining if it will work well and sound "good" for you. The tests of relative "goodness" are also possible, but require a lot of equipment and experience to do correctly. For the quick and dirty tests described here, you'll need a means of measuring AC voltage and current simultaneously, such as a pair of VOMs or DMMs, and a 110/120 to 6.3VCT filament transformer, and either a variac (variable transformer) or a light bulb socket in series with the primary of the filament transformer to limit the power you put into the transformer under test.


CAUTION CAUTION CAUTION

Both the filament transformer and the transformer under test will have at least AC line voltage on them, an may well have much higher voltage, several hundred volts on one or more windings. You are therefore in danger of being KILLED if you are not both knowledgeable and careful about how you do these tests.

DO NOT TRY THIS IF YOU DO NOT HAVE THE KNOW-HOW AND EXPERIENCE TO WORK SAFELY WITH THESE VOLTAGES. IF YOU HAVE ANY QUESTION IN YOUR MIND WHETHER YOU CAN DO THIS WORK SAFELY, YOU CAN'T.

Seek experienced help if you have any question in your own mind.

The tests run like this. Identify which wires are which by color code, circuit connection, or by using an ohmmeter to find which connects to which. Label the wires. From the same ohmmeter test, write down the resistances you measured on the windings. Generally, windings with resistances over a few ohms are high voltage windings, either a power transformer primary or high voltage output, or an output transformer primary. Note that it is common for primary windings on power transformers to have from two to six wires, with the wires over two being taps to adjust for various line voltages from 110-117-120-125-208-220-240. Secondary windings on power transformers and primaries on output transformers will have either two or three leads, and secondaries on output transformers will have to to four leads.

Also note if any winding is shorted to the transformer core. Sometimes an internal shield will be deliberately connected to the core, but if a multi-lead winding is connected to the core, this is usually an internal short, and a dead transformer.

Once you have identified the windings, hook up one and only one winding to either 1/2 of the 6.3VCT or to the variac. Try to select a low voltage winding, one that has low resistance from the ohmmeter test. Make sure that no other leads are connected (or shorted together, or touching your screwdriver on your bench or... well, you get the idea). A turn of plastic tape on each wire end you're not using at the moment is a good idea. Set your voltmeter on this winding, and the current meter to measure the current through it, and bring the circuit up. The voltmeter should measure 3 volts AC, the light bulb (if used) should NOT be lit brightly, and nothing should be humming or smoking ;-). There should be little current going through the winding. If the voltage is lower than 3 volts, or you are pulling amps of current, then there is a load on the transformer, internally since you have disconnected all the leads, meaning that there is an internal short. You should try to select a winding for this test that is normally a low voltage winding, either a filament winding in a power transformer, or a secondary in an output transformer.

If all is well, measure the voltage that now appears on the other windings. The voltages will be equal to the ratios of the voltages that will appear on these windings in normal operations.

B. Where can I get a good replacement output transformer for my vintage DoppelBanger amp?

Dixie Sound Works, Gunthersville, Alabama has a great reputation for (re)winding quality vintage re-makes. The company that made the amp may have service parts. The quality is variable from company to company and time to time, though.

There are a number of companies that have entered the transformer market in the last year, so expect that there will be new places to get quality rewinds and replacement transformers

C. I want to make my own power and output transformers. How do I do this?/ Where can I find information about this?

Designing and hand winding transformers is not terribly difficult, but it does require information and skills that are relatively hard to find. You are unlikely to save a whole lot of money unless used or broken parts are cheaply available to you. You may want to do this if you feel that you were selected by some deity to take this on as a life work. First, take a transformer apart. A burned out tube-type power transformer will do. Do this carefully and slowly, imagining how you would have put it together in the first place to get it the way it was. This is an excellent introduction to the manual skills and materials needed to successfully produce one on your own. Learn about how transformers are designed from one or more of the following, in this order:

  1. "Transformers for Electronic Circuits", Grossner (check your library)
  2. "Radiotron Designer's Handbook, fourth edition
  3. "Audio Transformer Design Manual", Wolpert, $36, privately published, available from: Robert G.Wolpert 5200 Irvine Blvd. #107 Irvine CA 92720
  4. "The Williamson Amplifier" D.T.N Williamson, reprint available from Old Colony Sound Labs
  5. Handbook of Transformer Design and Applications by William Flanagan (second ed.)
  6. "rewinding transformers with CAD" by Hugh Wells W6WTU Ham Radio Dec '86 p.83
  7. "Fast Optimization of Transformer Design" EDN Nov '62 by Davis, J. H.
These sources will help. They are NOT a complete cookbook. Note that it is very possible to make a transformer that will operate relatively well, but may break down unexpectedly and KILL you if it is not constructed with safety in mind.

D. Should I replace my stock transformer with a new/old/vintage/purple one for better clean/grunge/grit/etc. sound?

Unless you REALLY know what you're doing and have heard the transformer you'll be swapping in and like it, no.

There are a huge number of variables in the "sound" of a transformer, and you should exhaust other means first. You might not get that magic sound after all that work unless your ears - and amp tech - are really good.

What is the easiest way to get tube sound at a good price? Back to the index...

How can I modify my tube amp to ... ? Back to the index...

(also see recommended mods, below)

When should I bias my amp and how do I do this? Back to the index...

A. What is "bias"?

"Bias" in this context refers to the amount of voltage held on the grids of the output power tubes. This controls the amount of current the output tube(s) conduct exclusive of the signal current, or, looking at it another way, the amount of overlap where both tubes are conducting simultaneously.

I will talk about the output tube current since the terms "underbiased" and "overbiased" are confusing with tube amps. A technician who works with only tube amps will usually refer to the voltage which sets the operating current in the tubes. In these amps, the bias is a negative voltage, so "overbiased" to such a technician would mean that the tubes are held in a condition of too little current, just backwards from the solid state terms most of us are familiar with. "Underbiased" would mean that the tubes have too little negative voltage on their grids and are conducting too much current simultaneously.

The idle current in the output tube and the degree to which the output tubes overlap in conduction is what you're trying to adjust, not how many volts go on the grids; you just have to use the grid volts to change the current and conduction angle.

The whole topic of bias is tied up with the "Operating Class" the power amp is designed for. There are only three classes useful to us in tube amps, Classes A, AB1, and AB2. Class A means that the output tubes are biased so that both tubes are always conducting. Even on maximum signal peaks, the tube driven most "off" will still be conducting some current. In both class AB's, the bias is set so that on a signal peak, one of the tubes can be driven completely off for some part of a signal cycle. In class AB1, no grid current flows into the grid of the tube, and in class AB2 some grid current is driven into the grid of the tubes. There is a class B, where both tubes never conduct current at the same time, only alternately.

The point of all this is this: The Class of the amplifier is determined by how much bias current is present. If there is a lot of bias voltage, the grids are held 'way negative, then only the tube which is driven by the positive going half wave of the signal at any moment is conducting. This is class B. It sounds ugly because the point where the signal crosses over from positive to negative and begins to drive the other tube is not reproduced cleanly, and creates [surprise!] crossover distortion. You can look at the output signal with an oscilloscope and see crossover clearly as you make the bias voltage too negative for both tubes to conduct at the same time. As the bias voltage is made less negative and allows both tubes to conduct a little, the crossover notch diminishes swiftly, and you are in class AB2; a little less negative, and they both conduct more, and you have class AB1. If you go further, you get to the point where both tubes always conduct, making the amp work in class A, which has the least crossover distortion of any of these operating conditions.

Too little simultaneous conduction in the output devices puts them in the most nonlinear region of their transfer characteristic, so crossover distortion is high; but as you increase the amount of simultaneous conduction, the power used and dissipated by the outputs goes up, perhaps to a disastrous degree. You are trading standby current and power dissipation in the output devices off against distortion. If both outputs are biased almost totally off at idle, crossover distortion is very bad. As the simultaneous conduction is increased, crossover goes down rapidly, until it gets smaller than the residual THD of the amp itself, and becomes much less audible. There is a fairly broad sweet spot where the crossover distortion is comparable to the THD and the idle current and idle power dissipation are reasonably low. This is the region you're looking for.

Lots of bias, both tubes conduct all the time - and eat a lot of power, get hot, other Class A kinds of things. Little bias, both tubes overlap less, get less hot, put out more total power - and produce crossover distortion, which sounds especially unpleasant.

Power tubes individually have slightly different DC gains, so the same bias voltage on two different tubes produces two different current levels. "Matched pairs" are two tubes selected to be close together. Groove Tubes grades tubes from 1 to 10 so that any two "3"'s for instance are close enough to sub for any other "3", so you don't need to rebias if you keep buying the same number from them.

Note that you may not want matched pairs, depending you your taste. See section D. below.

B. When should I bias my amp?

You should re-bias the amp whenever you change power tubes or modify the power amp circuits.

Each power tube needs a certain bias current to keep it operating at the point where the amount and type of distortion under normal conditions is well controlled. Individual tubes vary widely in the grid bias that sets the correct idle bias current. If you change tubes or tinker with the circuit, you need to make sure the tubes are set back into operation in a way that sounds good and does not cook the tubes.

Amps typically provide only one adjustment point for bias, assuming that you will have bought matched sets of power tubes.

It is possible to modify your amp to "match" unmatched tubes by setting the bias voltage and AC drive level of each tube individually. This may require some serious soldering, though. See section D. below for a discussion on matching, and the mods section for what you have to change.

C. How do I bias my amp?


CAUTION CAUTION CAUTION

Keep in mind that tube amps use high voltages, and they can *kill* you if you don't know what you're doing. So, if in doubt, leave the job to a qualified technician.

How do you correctly bias an amp? There a few different approaches but first hook up a speaker or a passive load to the output and remove any input signals; tube amps need to have a load or they can sometimes become unstable. Check and make sure the proper size fuse is installed.

Output Transformer Shunt Method

The most common and simplest procedure is to hook a current meter from the plate (anode) across half of the primary of the output transformer; this is called the "output transformer shunt method." The idea here is that milliammeters commonly have a very low series impedance so that when placed in parallel to half of the primary, almost all of the current flows through the ammeter. When you hook things up this way, your meter is floating at the voltage level of the plate, which is typically hundreds of volts -- be very careful! You could open the wire from each plate to the output transformer and hook in a meter in series with the plate temporarily, but that is a terrible amount of work for the small gain in accuracy.

Adjust the bias pot so that the current reading is the appropriate value for the type of tube (see the table below). Let the amp warm up and note if the bias changes significantly. If so, select a compromise bias point.

Keep in mind that if your circuit uses more than one tube per side, the bias current you're reading is multiplied by the number of tubes (e.g., if you're reading 60 milliamps and there are two power tubes per side, if the tubes are matched each of the two are getting nominally 30 milliamps). Check the other side of the circuit to confirm that the two sides are close (within 5 milliamps) to each other.

If your ammeter has too high a series impedance, the shunt method won't work because the bias current gets significantly split between the meter and the transformer; the meter has no idea how much current is going through the transformer. You'll know it's not working because the current values you'll be reading will be much too low no matter how far you adjust the bias pot, the tubes will be glowing hot, and when you note that you'll reach quickly for the power switch! If you don't reach it quickly enough, you might blow a fuse. Don't despair: you can use another method called the "cathode resistor method."

Cathode Resistor Method

If the circuit already has a resistor in-line between the cathode and ground, use it. If the circuit has the cathode hooked up directly to ground, insert a low value resistor (say 1 Ohm/1 Watt) [even 10 ohms will work well, as the currents in a tube circuit will cause only a volt or so max across a 10 ohm resistor, not enough to change the circuit operation a lot.] in between the cathode and ground. This doesn't have to be a permanent change to the circuit; you can make a little adapter that fits between the tube and its socket that runs all the signals straight through except for the cathode lead -- that path gets the low value resistor in-line. If you make the adapter, you don't even have to drop the chassis from the amp to set the bias. Just pull a tube, install the adapter, and adjust.

Hook up a voltmeter across the resistor and measure the voltage. For a 1 Ohm resistor, if you read 30 millivolts Ohm's Law says that you have 30 milliamps running through it. If you have some other value resistor, make the appropriate calculation. Easy! But since the current at the cathode is the sum of the bias current and some other leakage currents, you need to compensate the reading a bit, typically 5 to 10 milliamps.

What's nice about the cathode resistor method is that you're not dealing with high voltages. The cathode sits very close to ground so the chance of a dangerous mistake is lessened. You're also reading each tube's bias current individually.

Other Methods

Some of the manufacturers say to set the bias voltage to some specified voltage, without any other measurements. Presumably some designer somewhere decided how much was good for you and wrote down "Set the bias to xx volts" as a good compromise for all the tubes s/he expected. This method ignores the variability of transconductance in output tubes, and only gives good results for matched sets that happen to be exactly like the "typical" ones the designer thought they'd get. Note that Gr@@ve Tubes tries to help by providing matched tubes with a bias number from 1 to 10. If you have GT's with a "4" bias number, and you replace with a GT "4" set, they will have selected only tubes that are properly biased at that level, and no rebiasing will be necessary. Of course, GT expects to be repaid a fair profit for this service to you...

Another way to set bias is to use a test signal, typically a sine wave. Monitor the output waveform on an oscilloscope and adjust the bias for minimum crossover distortion. The obvious problem is when has it "just disappeared"? Most folks do just a bit more than "just disappeared" and get their outputs too hot causing shortened tube life and overheating. Not very accurate or repeatable.

You can also use a special purpose instrument that nulls the input signal out of the output signal so that you can monitor just the distortion products. You then adjust the bias to get the distortion to a realistic minimum without making it dramatically less than the residual THD. This is the premium method, but requires a distortion analyzer - big bucks.

These methods can be more accurate than the first two methods but they require expertise and tools that most folks don't have.

If you are a circuit hacker, and live on solder fumes and cold coffee, you can modify the amp with solid state servo bias adjusters that twiddle the bias to each output tube on the fly on a continuous, real time basis to keep each tube -* exactly *- where it ought to be. Only recommended for real wiring fanatics...

GENERAL BIAS GUIDELINES (from Tremolux@aol.com)

Currents Per Tube - Class AB1 Operation (most musical instrument amps are designed to run in class AB1)

Class A currents will be higher. Example is 50 ma for a 6L6. Don't try to run an amp designed for AB1 in pure class A, it will overheat and probably blow. To handle the higher idle currents, Class A amps usually run at lower plate voltages.

D. Matched output tubes - do you need them?

Do I always have to buy matched pairs of output tubes? The issue of "matching" output tubes, either by buying carefully matched pairs or by tweaking the bias levels and drive signals per output tube is not a settled one. It used to be common wisdom to simply buy matched tubes. A few people noticed, however, that they had a favorite pair of output tubes, which made their amp sound much better than others. The common assumption was that these tubes were better matched somehow. When these tubes get measured, though, it usually turns out that they are NOT matched, at least not matched for AC gain characteristics.

The concept of matched output tubes comes to us musical amp types from the hifi community, where they are trying to get the lowest possible distortion. This was true from the start, when Fender was trying to build low distortion amps and copied hifi circuits. The concept has simply clung to us, largely through inertia. It is relatively well accepted even in the hifi circles now that even-order distortion is euphonic, sounds good to our ears. It is very likely that the even-order distortion produced when mismatched output tubes are used sounds better than perfectly matched tubes.

If you have modified your amp so you can independently set the DC bias and the AC drive signal, you can tune almost any pair of tubes into AC and DC matching. You can also tune in a selective amount of AC drive mismatch to experiment with the selective mismatching sound.

There are technical reasons for matching. Getting enough turns of wire on the primary of an output transformer to get the right primary inductance and still using as little iron and copper as possible to do the job properly is an engineering problem that almost always results in Class AB output transformers being smaller for proportional power outptu than a Class A output transformer would be. The (relatively) smaller transformer and wire size makes a class AB (most guitar amps) output transformer susceptible to burning out if one of the half-primaries carries too much current.

If one side of the transformer carries significantly more current (like double) than it would otherwise in "normal" operation, it is possible it will overheat or open, effectively killing the transformer. Tubes that are so mismatched that to get the right total current for a pair means that one is carrying more than 50% over the nominal DC current for a matched pair is getting into the region where you ought to worry about output transformer damage.

If you mismatch, try to get the DC current the same in both sides of the output transformer, and an imbalance in the AC gain of the tubes. The logical limit of this AC mismatching is to remove all the AC drive from one output tube, which is a technique used by at least one commercial amp maker. This effectively keeps the output transformer happy with respect to DC, and gives you a single ended output stage; this also costs you a large amount of your available output power, but, hey, we're after tone, right?

Note that the commercial tube suppliers have good reason for wanting to sell us matched sets at a premium. I would expect their opinion to be that matched sets are absolutely crucial. As in all musical matters, let your own personal ears be your guide.

If you have a set of tubes you know are not matched, or if you have modified your amp to be able to set the bias and drive levels on each output tube separately so you can either match or not match the tubes at will, you might want to try un-matching them and see how it sounds to you.

Amplifier Modifications Back to the index...

A. OK/Recommended amp modifications

Read the SAFETY WARNING first, before you put your hands - or other personal parts - into a tube amp.

C. NOT Recommended amp modifications

These are likely to be just plain bad, either grossly (it dies soon) or subtly (it dies slowly, eats tubes, or other sicknesses). Don't do these or let a tech do them to... er... for you.

Tube Characteristics and substitutions Back to the index...

Some quick and dirty subs and some tube data such as recommended bias current and appx voltages. These subs are all taken from the Tube Substitution Handbook sold by Antique Electronics Supply. or provided from the net.

A (short) catalog of tubes you are likely to see in a guitar amp:

Substitutions:

* means appropriate for parallel filament circuits
# means may not work in all circuits

Preamp and driver tube substitutions:

Power tube substitutions:
 Cautionary Tubes - these are very hard to find

Maintenance Issues Back to the index...
Cap Jobs - Do I need one? How often? Why?
What's a cap job? A technician may recommend you have a "cap job". This means that he will replace every single electrolytic capactor in the amp, from the power supply right down to the cathode bypass caps.

This is because electrolytic (polarized) capacitors have an inherent wear-out mechanism and will eventually die even if you don't play death/metal/country/barbershop through them every day - in fact they may wear out sooner if you leave it sitting in the attic. Here's why.

A capacitor is essentially two conductive plates separated by an insulator. The bigger the plate area and the thinner the insulator, the higher the "capacitance" is. Electrolytic capacitors get a very thin insulator by "growing" an insulating layer of aluminum oxide on the outside of a rolled up piece of aluminum foil.

The oxide layer is "formed" at manufacture by feeding the aluminum foil a very small and carefully controlled amount of current. The current causes a chemical reaction between the foil and the water solution (electrolyte! ... hey... is that where they got the name?? yep.) which makes an oxide layer grow. As the layer grows, they use higher and higher voltages to force the same small current through the layer, which gets thicker and more resistive with time. When they have to use the full rated voltage to get the forming current through, the cap is fully "formed" and ready to ship.

If the capacitor is used regularly, has voltage applied to it, and does not get too hot, the oxide film lasts up to a few decades. If the capacitor is not used much, or gets too hot, the oxide film slowly un-forms, the leakage current goes up, and it will eventually short.

Electrolytic caps are designed to last ten years. It is a tribute to the quality of manufacture that they often last three, sometimes four times that.

Old amps, particularly if they have not been used regularly need to have every electrolytic cap replaced. This cap job may be needed every ten or so years.

Non-electrolytic caps do not have this wear out mechanism, and do not need replaced for this reason. Modern capacitors can in some circumstances be much better than old ones, and you can sometimes get a clearer, more sparkly tone by changing the non-electrolytic caps - assuming that is something you want to do.

Do new caps need to be formed?
There's a lot of controvery on "reforming" replacement caps. Here are a few answers.
Manufacturers of caps design their caps for a ten year working life, and a five year shelf life. That means that the stresses and heat of working in equipment will leave the vast majority of caps functioning OK after ten years of normal operation. After that, it's gravy to the buyer.
They also design them to work OK after sitting on a shelf unused for five years, meaning that the cap should not fail if it's put into operation at rated voltage after sitting unused for five years. As noted above, the caps do slowly un-form without regular use.
If the electrolytic caps you use to fix your amp are over five years old as determined by the date code on them, you ought to at least worry about forming them, and if they're over ten years old (like NOS multisection cans), definitely re-form them. Other than that, put them in and turn it on.
How do I "re-form" electrolytic caps?
You'll hear folks talk about "bringing an amp up slowly on a variac"; this can work but is not particularly good for your tubes. A better way is this:
  1. Pull out all the tubes.
  2. if your amp has a tube rectifier, solder in temporarily some high voltage silicon diodes across the tube lugs to be a rectifier that does not depend on the filament voltages. If your amp has silicon diodes, you can skip this.
  3. open up the wire that goes from the rectifier tube (or solid state diodes) to the first power supply filter stage and solder in series with the wire a temporary 100K 2- 5W resistor. This resistor will limit the current that can flow into the caps and the amount of voltage that is applied to them to safe values that will cause the insulating layer to re-form.
  4. clip your voltmeter across the resistor
  5. button it up. Turn it on (no tubes in it, remember). Watch the voltmeter.
  6. when the voltmeter reading drops to less than 20-30VDC, your caps are formed.
  7. open it back up and pull out those diodes and resistor, putting it back in original shape.
The forming could take hours to days.
Sockets
Sockets get dirty, corroded, broken, and "arced" To recondition them, get a can of spray contact cleaner, the kind that says "no residue". Squirt some in each socket hole, then insert that tube in the socket, wiggle it around, and remove it several times to get the crud off. Take a thin tool like a jeweler's screwdriver or ice pick and gently bend the contacts inside each hole so they hold the pins better. If the socket is cracked, or has blackened lines from pin to pin (where an electrical arc has actually burned the socket into a carbon material that conducts electricity), replace the socket.
Dirt
The dusty, hairy, oily layer that collects on the chassis can conduct electricity as it absorbs humidity from the air. Vacuum it away periodically.
Other Issues
Lots of good info is contained in Jack Darr's "Electric Guitar Repair Book", if you can find a copy (it's now out of print) and in Pittman's "The Tube Amp Book" and Webers "Desktop Reference...". Look for: - checking for capacitor leakage
From watching a tech work on Fenders, I picked up a nice tidbit. The eyelet boards in Fenders have most components mounted across the eyelet board. A very few parts run along the length of the eyelet board. Because the eyelet board flexes, there is a lot of stress on the solder joints at the end of these lengthwise components and the joints often crack. Every time you open up a Fender, take a look and maybe a soldering iron to these joints. If it's your personal amp, you might want to get a new part for these positions with long leads and bend a loop in the leads so that the leads can flex and not put stress on the solder joints.

Appendix A. Tube Stuff Suppliers

Back to the index...

The following article appeared in rec.radio.swap. Some time ago, and is now badly out of date. My apologies, I'm working on an update.

Gang: The enclosed is a bit long but it's the most complete list of tube sellers I've seen presented on the Internet. You'll want to send this list to your printer! I didn't edit it down like I usually do since that would have made it quit difficult to read. - Jeff NH6IL 
  Article: 18193 of rec.radio.swap
  From: brian.carling@acenet.com (Brian Carling)
  List of suppliers and sources for finding vacuum tubes:
  Adkins:Charles P. N8QXP       (313) 382-0272
  1821 La Blanc.
  Lincoln Park,MI 48146         Tubes

  Allied Electronics: 800-433-5700 Located in 36 states, 83 branches.
  7410 Pebble Drive      Call for nearest branch. Call for Catalog.
  Fort Worth, Tx. 76118: Electronic components and tubes. Min.Prepaid
  order $25.  Min. Credit Card order $50 as well as COD is $50. Min.

  Antique Audio                 512-467-0304
  5555 N. Lamar, Bldg. H-105
  Austin, TX 78751              Tubes, parts, books, kits

  Antique Electronic Supply Co. 602-820-5411
  6221 S. Maple Avenue
  Tempe, AZ 85283               (Tubes & other components)

  Antique Radio Classified    508-371-0512   Write for free sample.
  P.O. Box 802             Magazine. You'll find almost ANYTHING here
  Carlisle, MA 01741       for older radios, obscure parts, tubes etc.

  Arlen Supply Company         (610) 352-9311 / -9388 = FAX
  7409 W. Chester Pike
  Upper Darby, PA 19082         Tubes. 1 million stocked. Minimum $150.00!

  A.R.S.                   602-820-5411
  Arizona  Need address           Tubes

  David Ask                     No phone number given
  RR 2
  Houston, MN 55943             Tubes 4, 5 and 6 pin tubes. Send $1 for list

  Mel Brooks, K5DJB             No phone number given
  932 Macklyn Lane
  Bartlesville, OK 74006        Tubes, parts, schematics (incl. antique)

  Bauman:Jeff, WB5KZW. 313-435-9922: 313-661-0202 Jeff had 1500 Tubes
  6647 Stonebridge East,                   for sale as of December,1993
  West Bloomfield, MI 48322

  Cable:John,  619-258-7931
  Address needed. Tubes from 1941 to 1960, new.

  CeCo Communications. 800-221-0860: 212-646-6300
  2115 Avenue X            Vacuum tubes
  Brooklyn, NY 11235

  C & N Electronics             800-421-9397: 612-429-9397
  6104 Egg Lake Road            FAX 612-429-0292
  Hugo, MN 55038                Buy & sell tubes

  Daily Electronics    800-346-6667: 206-896-8856: FAX 206-896-5476
  10914 N.E. 39th Street
  Vancouver, WA 98682           Tubes, all types.

  Davilyn Corp. 800-235-6222 xct.CA: 818-787-3334 CA: FAX 818-787-4732
  13406 Saticoy St.     Electronic Tubes. Good Prices, Call for Catalog
  North Hollywood, CA 91605-3475   Also Surplus Electronic Gear.

  DH Distributors               316-684-0050
  P.O. Box 48623
  Wichita, KS 67201             Tubes, radio, TV, industrial

  Electron Tube Enterprises     802-879-0611
  Box 311
  Essex, VT 05451               Tubes

  Electronic Bits 'N Pieces     303-361-6530
  P.O. Box 31654
  Aurora, CO 80041              Tubes, transistors, diodes, chips

  William Erickson, W4UIL       No phone number given
  3905 Cherrywood Lane
  Annandale, VA 22003-1901      Tubes, older radios etc.

  E.S.R.C.                      (407) 735-3397
  P.O. Box 1192
  Delray Beach, FL 33447-1192   Buy, sell, swap tubes of all kinds

  Fair Radio Sales. 419-227-6573:419-223-2196: FAX 419-227-1313
  1016 E. Eureka   Box 1105     Parts, transformers, power supplies,
  Lima, OH 45802                Surplus and gov't surplus radios etc.

  Fala Electronics              (No number listed) send S.A.S.E.
  P.O. Box 1376-1
  Milwaukee, WI 53201           Vacuum tubes

  Melvin Heineken, K5MNJ        No phone number given
  2204 Spruce Needle Rd. N.E.
  Rio Rancho, NM 87124-6308     Tubes. New unboxed. Other parts.

  Henry Radio Co.               (310) 820-1234
  2050 S. Bundy Drive
  Los Angeles, CA 90025         New tubes

  International Components Corp.800-325-0101: FAX 503-336-4400
  1803 NW Lincoln Way         Cabinets, components & vacuum tubes
  Toledo, OR 97391

  Jolida Tube Factory           800-783-2555
  10820 Guilford Road           Vacuum tubes
  Annapolis Junction, MD 20701

  KB5QOH (No name given)        No phone number given
  667 Nine Mile Hill Road
  Fairbanks, AK 99712           Tubes, parts, used amateur gear

  Kirby                         No number listed
  298 W. Carmel Drive           Tubes, new up to 90% off
  Carmel, IN 46032

  Cliff Kurtz, N6ZU             No phone number given
  6727 N. Pershing Avenue
  Stockton, CA 95207-2522       Tubes. Minimum order $10.00

  Robert Lang AA2EO             (212) 877-0980
  120 W. 70th Street Apt. 7-A
  New York, NY 10023            Tubes, vacuum variables, xfmrs etc.

  Madison Electronics           (800) 231-3057
  12310 Zavalla
  Houston, TX 77085             Tubes, meters etc.

  Rex Mason                     (704) 392-0359
  100 Honeywood Avenue
  Charlotte, NC 28216           Tubes, antique parts, amateur, TV, VCR

  New Sensor Corp. 800-633-5477: 212-529-0466: FAX 212-529-0486
  133 Fifth Avenue.   Vacuum tubes galore! Call for list.Min.Order $50.
  New York, NY 10003   Std. test= $0.75/Tube. Premium Match $2/Tube

  No name (SHY?!)               No phone number given
  5150 Merritt Road
  Black Hawk, SD 57718          Tubes. S.A.S.E. for list

  P.E.M. Tubes                  (916) 383-9107
  7392 French Road              Tubes, radio, TV, transmitting, CRT
  Sacramento, CA 95828

  Pride Tubes                   800-638-3925: 205-650-5522: FAX 205-880-8077
  8200 South Memorial Parkway   (800) 456-5642 100% RF Tested Tubes
  Huntsville, AL. 35802

  Rauchwerger, Lawrence         217-352-6195
  1610 1/2, W. Union St
  Champaign, IL 61821           Tubes. S.A.S.E. list

  R.F Parts.  To Order 800-737-2787:619-744-0700 or 0750 for Tech info
  1320 Grand Avenue     FAX 619-744-1943
  San Marcos, CA 92069  Diamond Antennas, RF Power Transistors & Tubes.

  Richardson Electronics        (708) 208-2200 / (800) 235-2143
  40 W. 267 Keflinger Road
  La Fox, IL 60147              Tubes, RF parts

  Steinmetz Electronics         219-931-9316
  7519 Maplewood Avenue         Tubes
  Hammond, IN 46324

  Svetlana Electron Devices Co. (415) 233-0429 / - 0439 = FAX
  3000 Alpine Road
  Portola Valley, CA 94028      Tubes, RF power etc.

  Turner Electronics            No number listed
  16701 Main Street Suite 121
  Hesperia, CA 92345            Tubes, capacitors, S.A.S.E. list

  Unity Electronics             No number listed
  P.O. Box 213                  Vacuum tubes
  Elizabeth, NJ 07206

  C. Verderber                  No phone number given
  2266, Route 9G
  Rhinebeck, NY 12572           Radios & tubes

  Carl R. Warren, W0KWS         (417) 869-4738
  MPO Box 567
  Springfield, MO 65801         Tubes & parts. Also repair service

  Wayne (no last name given)    (301) 963-4619
  No address given
  Gaithersburg, MD              Tubes, equipment, parts, books

  Westgate Co.                  (800) 213-4563
  Need address!                 Tubes & transistors

This list was prepared by AF4K, Brian Carling Please send additional sources for inclusion in this list. If you go to a hamfest and see someone selling tubes, get a card please and send me their name, address and phone number.

AF4K @ W3INK
(301) 990-6070
brian.carling@acenet.com

Appendix B. Tube Makers Producing Today (Eric Barbour news posting)

Back to the index...

(The following is the text of a note posted to the alt.guitar news group by Eric Barbour it is also badly out of date, and will be upgraded soon.)

Different makers of tubes use different designs. There are six makers of common audio tubes right now:

That's ALL there are right now. That's it. Any NEW tube you buy is from one of the above.

For your guitar amp, I would recommend the "Sovtek" 5881, it's a really nice, rugged and smooth-sounding tube. It was a military type used in servo amps in jet aircraft, so it has to be good. If you have a Marshall or other EL34 amp, the Sovtek 6CA7 imitation (recently released) is probably most rugged. If you want more distortion and a more bluesy sound, you want the skinny EL34s. The Svetlana EL34 will be a skinny type, it should be very good.