Try changing the guitar preamp tube type for a different sound.
12AX7 |
100% |
5751 |
70% |
12AT7 |
60% |
12AY7 |
45% |
12AV7 |
41% |
12AU7 |
19% |
One simple tweak that is favored by guitar amp users is to plug in a pre-amp tube with a slightly different gain factor. This only works for tubes that are pin out and plate voltage compatible, such as the six types listed here.
The tubes listed here also have several alternate names, which are listed below. Note that the alternate names do not have different gain factors, they are simply a different name for the same thing.
12AX7 - ECC83, 7025, ECC803, E83CC, 6681
12AT7 - ECC81, 6201, 6679
12AY7 - 6072
12AV7 - 5965
12AU7 - ECC82, 5963, 5814, 6189
The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 type has a gain factor of 100%, while a 5751 (which is often used in place of a 12AX7) has a gain factor of 70%. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain. Not only will this make the amp quieter, but it can also alter the sound by making the power section work harder when you turn it up. Power amp distortion will sound different than preamp distortion. Many guitar amp users (particularly harmonica players) like to reduce the gain to get a different sound or calm the amp down to prevent feedback.
In some cases, you can also go the other way and increase the gain factor. It is generally more risky to increase the gain factor rather than decrease it as the added gain may be too much for the rest of the amplifier circuit. An example of an acceptable up-swap can be found in Fender amps that use a 12AT7 (with a gain factor of 60) as a "reverb driver" tube. You can sub in a 5751 to increase the gain by just a touch.
If you’re looking for a different sound, first try a tube with the same gain factor, just a different brand. If your amp was originally designed for a certain type, and although tube amps are often forgiving, it may not have a tone that you like with a different tube type. For instance, changing the tube type doesn't only change the gain factor; there are other variables as well and your amp may have a circuit that is more particular than others. As a general rule, the substitutions described here should work well but there are exceptions. For the best results, find other people on the net who have experience swapping tubes with the amp you use.