Probably one of the more popular simple tweaks for a guitar amp by guitar players is to get a preamp tube that has a little bit of a different gain factor. However, this is only going to work for those tubes that are plate voltage compatible, so you need to be careful when picking these tubes.
If you are not quite sure what the gain factor is, it’s simple actually. What it does is measure how much it is going to amplify the input signal. For instance, the most common 12AX7 variety tube will have the gain factor of 100 and a 5751 quite often used instead of the 12AX7 which has a gain factor of just 70. What this means is that if you take the 5751 preamp tube and plug it into the socket that is use to a 12AX7 it is going to have around 30% less gain factor. This is going to make the amp much quieter but is often will alter the sound of the amp by making its power section work a lot harder when it is turned up. Often those who use amps for their guitars like reducing the gain factor to create a different kind of sound or to try and calm down their amp to try and prevent feedback.
Sometimes guitar players will want to go the opposite way by increasing the gain factor with preamp tubes. However, this can be a little bit more risky trying to increase the gain instead of decreasing it. The reason for this is that added gain can often be way too much for the rest of the circuitry in the amp. A good example of an up-swap would be found when using a Fender amp that often uses a 12AT7 that has a gain factor of 60 using it as a “reverb driver” tube. You can take a 5751 to increase the gain factor just slightly.
Remember however, when considering using preamp tubes to increase or decrease the gain factor in an amp, the amp you may be using is usually designed for a certain kind and even though tube amps can be forgiving, it could end up with a tone that you may not like. For example, by changing the type of tube not only can change the gain factor but it can also change other things as well and the amp you have may be more picky than other amps. So for a general rule, the subs that were described here more than likely should work but there are always going to be exceptions so it is not a guarantee that the changes will work the way you want.
Even though you can substitute preamp tubes with home audio amps it really isn’t recommended unless you are really having problems finding the right tubes for the amp you might have.