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 - has become outdated since the FAQ was last updated.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.
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:
Dennis O'Neill, email@example.com Nathan Stewart, firstname.lastname@example.org George Kaschner, email@example.com David Kohn, kohn@SCTC.COM Michael Edelman, firstname.lastname@example.org Len Moskowitz, email@example.com Tremolux@aol.com Brian Carling, firstname.lastname@example.org Eric Barbour email@example.com
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Old Colony Sound just announced a CD ROM version of this book, apparently indexed, illustrations and all, for $69.95.
(Tom mentions a 2nd and 3rd volume in the distant future - covering low power pentodes & oddball tubes, and Power & Beam Power pentodes respectively.)
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:
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".
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.
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.
Seek experienced help if you have any question in your own mind.
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.
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:
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.
There is a document on exactly this at http://www.wwu.edu/~n9343176/docs/old2new.html The document goes into excellent detail on the in's and out's of building from old tube gear and the possible and useful variations of which stages with how much gain go where in the amp.
This is what the Hughes and Kettner Blues Master and Cream Machine tube preamps did (they've been discontinued). These were entire tube amplifiers with maybe 2 or 3 watts output, a simulated load, and a line level output in addition to the speaker output. They did a VERY respectable job.
George notes "You may already use this in your own amps but I thought we might share it with the rest of the tinkerers - it's especially useful for people that are trying to add extra gain stages. I even use it between the input jacks and the first stage since in most Fender amps it has to traverse the width of the board. (Kaschner)
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.
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.
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.
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."
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.
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...
Currents Per Tube - Class AB1 Operation (most musical instrument amps are designed to run in class AB1)
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.
Nathan points out "I seem to recall one of my Tube Amp Mentors telling me that this is pretty much only the case with the first filter cap after the rectifier, and that the impedance of the power supply was high enough that you could dump hundreds of uf worth of filtering on latter stages (though the only place it's of much benefit is at the power tube plate supply point.)
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:
Preamp and driver tube substitutions:
12AD7* 12DT7 7729 12AU7# 5751* B339 12AU7A# 5751WA* B759 12AX7 6057 CV4004 12AX7A 6681 E83CC 12AX7WA 6L13 ECC803 12BZ7* 7025 ECC83 12DF7 7025A M8137 12DM7* 7494
12AU7[A,AW,] 6189 7730 12AX7* and subs 6670 ECC186 5814[A,AW]* 6680 ECC802 5963 7316 ECC82 6067 7489 M8136
12AT7[many suffixes] 7492 E81CC 6201 7728 ECC801 6679 A2900 M8162 ECC81 B152 QA2406 12AZ7[A]* B309 QB309 6060 B739 6671 CV4024
12AY7(and suffixes) 6072 2082
6267 7189 EF86 6BQ5 7189A EL84 6BQ5WA 7320 N709 6P15 E84L Z729
6L6(many suffixes) 7581(A) 5881 WT6 5932 EL37
EL34 12E13 6CA7 KT77 7D11 KT88
6550[A] 7027A# 7D11 KT88 12E13
These were used a lot in old Ampegs. They are very small and high gain for their physical size, so there may not be a lot of room in a chassis for a larger replacement. The 5881 will work in some circuits, but has significantly lower transconductance.
Rumor Update: The rumor mill on the net says that the Russians will soon be making 7591's soon. Cross your fingers...
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.
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.
Article: 18193 of rec.radio.swap From: firstname.lastname@example.org (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. 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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
AF4K @ W3INK
(The following is the text of a note posted to the alt.guitar news
group by Eric Barbour
Different makers of tubes use different designs. There are six
makers of common audio tubes right now:
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.
Different makers of tubes use different designs. There are six makers of common audio tubes right now:
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.