Just a few things you can do without taking the chassis out, if only to keep an amp going through a gig.

1. The effects loop cutout issue. If your amp is intermittently or permanently silent and it has an effects loop, get a spare guitar cable to use as a patch lead and plug it into the send and return jacks. The lead bridges the loop. One of the jacks, usually the return, has a cutout on it on most loops, and the contacts start to fail. If the amp comes back with the cable plugged in, it’s a loop problem. Now you might need a new jack fitted anyway, but you could try some switch cleaner squirted into the jack hole. Even a bit of WD40 in an emergency. It is fine to play the amp with the patch lead in.

2. Preamp valves. These are the little valves. They can cause various problems, usually pops, crackles and hisses, but they can cause cutouts too. Pull out the small preamp-type valve nearest the big power valves; 10 to 1 this is the phase invertor (PI), which is in fact the first power amp stage, linking preamp to power amp (all the signal goes through it). If all goes quiet, you have a preamp problem. Stick the PI valve back in, get a spare valve and replace each preamp valve with it in turn. If the problem stops, leave the valve there and you’ve cured it. They can be tricky to get back in – use your eyes and if it don’t fit, don’t force it as the old song says. They can get hot but not usually skin-damagingly so. It is ok to take them in and out with the amp on. Don’t stick your finger in the socket, obviously.

3. Power valves. These are the bigger hotter valves. Often they short and blow fuses. If you have a fuse the same value (and type – slo-blo fuses have a T next to the value usually), you can replace it and plug in some new valves. If the plates don’t glow red you can play the amp ok. Sure it ought to be biased but if it plays ok and sounds ok, and the valves don’t get red-hot and melt, that’s ok.

Sometimes a power valve gets noisy. When you pull out the PI the noise continues – and you know you have a power amp problem. You can swap in a new valve to try to get rid of the noise. If there’s not too much hum with the PI out you can assume you have a good balance. Watch the heat on these valves, they get well hot enough to burn skin. And the sockets have high voltage on them, often 500 volts or more.

It is well worth remembering that unbalanced power valves (which can happen if one is failing or failed) cause power amp hum, a hum which persists even with the PI valve out.

4. Microphonic valves. If your amp has a tendency to squeal like a little piggy or indeed a distressed warthog, or if you can hear it through the speakers when you knock on the case, you may have a microphonic valve. Tap each valve to find it, and replace the one that picks up too much sound from the tapping. Remember though that early stage preamp valves in higher gain amps will always be a little microphonic, as they are being amplified so much through subsequent stages. The first stage preamp valve is almost always the one nearest the inputs.
Caveat: Valve problems might have also caused internal damage, but you will know by the amp’s poor performance if you have such an issue.