Q: How compatible are different tubes in an amplifier? I mean, can I swap out pretty much any preamp tubes that I want?
A: Not all tubes are direct equivalents. We highly recommend that you contact the manufacturer of your amplifier or a knowledgeable technician before substituting one type of tube for another.
Having given that disclaimer and warning, here are a few guidelines:
You should be able to swap preamp tubes of the same type without problem. For example, any 12AX7 should be compatible with another 12AX7.
With power tubes, you can swap different brands of the same tube. (For example, 6L6 tubes from different manufacturers.) But do not swap different types of power tubes unless you are absolutely sure you know what you are doing — damage may result. Amps are designed to use specific power tubes (for example, EL34 or 6V6) and are not compatible with other types of power tubes without serious circuit modifications.
You should have the amp re-biased when swapping power tubes (in many cases it’s a good idea to have the amp re-biased when changing power tubes regardless of whether you are changing tube manufacturers or not). Also, remember that in many amps, the power tubes are designed to work in pairs. It’s best to replace both tubes in this case.
In some cases, “families” of tubes, mostly preamp tubes, are compatible, such as the “12A” family of preamp tubes — 12AX7, 12AT7, 12AU7, 12AY7. These tubes are largely the same, but feature different gain factors, so you can use them to custom-tailor the response and tone of your amp.
Different tubes can noticeably change the tone of an amp, so it can be worthwhile to experiment with various tubes to find the ones that your amp “likes” the best. Just make sure the tubes are compatible before you proceed!