Adjust your amp's sound with a simple, effective tube swap
By Phil O'Keefe
Tone connoisseurs have been experimenting with different brands of tubes for ages in an effort to find the tubes that they like best, and many can argue the relative sonic merits and construction details of Telefunken, Mullard, GE, RCA, Sylvania, Groove Tubes, Sovtek, JJ and countless other new and NOS (new, old stock) tube brands. Opinions vary widely, in large part because what may sound great in one amp, or to one player, may not work as well in a different situation. Because of this, experimentation is pretty much the only way that you can find what works best for you. However, one aspect of tube substitutions is less of a guessing game, and that's the amount of gain the tube provides.
SWAPPING PREAMP TUBES
Did you know that many of the most popular preamp tubes are interchangeable? In fact, the main difference between many common tubes is their gain factor - the amount that they amplify the signal. The plate voltage range, heater voltage, the pin layout, and other basic characteristics of the tubes are the same.
For example, the 12AX7 (Fig. 1) is probably the most commonly encountered preamp tube, and can be found in amps from Fender, Peavey, Marshall, Gibson, Ampeg, and many other brands. It's a good tube, but occasionally you may feel that the amp sounds a bit harsh, or that it "breaks up" too early, distorts too easily, or doesn't stay clean through enough of its volume knob range to suit your tastes. By replacing it with another tube from the same "family", you can slightly - or significantly - reduce the gain, and thereby cause the amp to break up later on the volume control, if at all. You may also notice an improvement in overall sweetness of the amp's tone.
Figure 1: A Tung Sol 12AX7 preamp tube (click on images to enlarge)
Here is a listing of common preamp tubes and their gain factors.
|12AX7 (aka 7025, ECC83)||100|
|12AT7 (aka ECC81)||60|
|12AY7 (aka 6072A)||45|
|12AV7 (aka 5965)||41|
|12AU7 (aka ECC82)||19|
Each of these tubes can be substituted in place of any other tube on the list, with changes in the amount of gain being the main difference between them in this application. You won't harm the amp as long as it calls for one of the tubes on the list, and as long as you stick to tubes from this list for the substitutions.
WHICH SPECIFIC TUBES TO SWAP?
You should refer to your amp's documentation to determine which specific tube or tubes to swap. The idea is to switch out the preamp tubes, but to leave the reverb driver, tremolo and phase inverter tubes alone. You can certainly feel free to experiment with tubes of different gain factors in these spots too, but in general you may be less than pleased with the results in terms of the changes it can make to the sound and operation of the reverb and tremolo, although swapping out the phase inverter is occasionally done by some players, and can make a difference in the amount of "crunch" on some amps. If you'd like to try this, try replacing the stock 12AT7 you'll typically find in the phase inverter position (usually the "small" tube found closest to the large power amp tubes) with a higher gain 5751 or 12AX7. Marshalls typically come stock with a higher gain 12AX7 tube in the phase inverter position, while Fender and many other amp brands use the lower gain 12AT7 tube. Amps with only one power amp tube, such as the Fender Champ, don't have a phase inverter tube. On old single channel Fender amplifiers, "V1" (the preamp tube) is on the far right as you view the amp from the rear. (Fig. 2) For two channel Fender amplifiers, such as the Deluxe Reverb and Twin Reverb, V1 (on the far right) is the preamp tube for the Normal channel, and V2 (the second tube from the right) is the preamp tube for the Vibrato channel.
Figure 2: A Fender Princeton Amp, with the location of the preamp tube indicated with an arrow (metal tube shield removed for clarity)
If your amp's tube chart or manual calls for a 12AX7 in the V1 position, try substituting a 5751 instead. If you really want to clean things up, replace that 12AX7 with a 12AT7, or even a 12AY7. If you use dirt pedals (fuzz, distortion, overdrive, etc.) in front of the amp, such tube substitutions can have a noticeable effect on the way the amp responds to the pedals - often for the better. Remember, always stick to substitutions that are known to work - don't assume that a tube will work just because it has the same number of pins and "fits" the socket! Also use caution whenever swapping out tubes. The glass can get extremely hot, so either use an oven mitt, or better yet, wait until the tubes cool before making the switch, and always power down and unplug it from the AC outlet whenever you are working on your amp.
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.