One simple tweak that is favored by guitar amp users is to plug in a pre-amp tube with a slightly different gain factor. (Important: this only works for tubes that are pinout and plate voltage compatible.)
The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (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. 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.
The tubes we list here also have several alternate names, which we list below. Note that the alternates do not have significantly 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
Before you discover the new possibilities in your amp, let us give a disclaimer. 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 we describe here should work well but there are exceptions and we don't warranty any problems that result from trying mismatched tube types. For the best results, findotherpeople on the net who have experience swapping tubes with the amp you use.
These substitutions can also work in home audio amps, but we don't recommend trying this unless you are having trouble finding the correct tube type for your amp.