Most tube amp problems relate to the tubes themselves.  After all, tubes are not the cutting edge of technological advancement.  Tube problems are not infrequent occurrences.  But before you have to worry about shipping your amp for repairs, learning to diagnose a tube problem can keep you rockin’.  

I can not stress enough the importance of changing tubes to keep your amp running optimally.  But even with regular maintenance, you can still have tube problems.  If you are playing out regularly, I suggest you keep a spare set of power tubes in your gig bag and maybe a few preamp tubes just in case.

Crackling, squeals and feedback, excessive noise and muddiness or low output are all evidence of tube problems.  

Power tubes.  The two main symptoms of a power tube problem are a blown fuse or a tube that begins to glow cherry red.  Either are typically indicative of a power tube failure.  Some failures can be an intermittent short and some can be a tube failure.  If you blow a fuse, replace it.  If it happens again, replace your power tubes.  If the tube glows cherry red, shut the amp off immediately.  As power tubes are wired in pairs, a failure of this nature in one tube can cause the other tube in the pair to fail with it.  If you encounter this problem, as noted, shut the amp down immediately.  Wait a few minutes and fire it back up.  If it happens again, replace the tubes before using the amp again.  If not, you should probably still replace the tubes soon, but you should be able to operate the amp for a short period of time.

In the event you have one or two power tube failures, you can replace that one tube or two tubes to get you through a show, but then replace all four with a matched set (see the preceding section).

If you are hearing noise and/or crackling, you can rule out (or in) a power tube with a simple test.  Gently tap on the power tubes, one at a time.  They should not make noise.  If noise changes with the tapping, you may have a failing power tube.  Always be prepared to shut the amp off in case you have a failing tube and the tap causes it to short.  If so, shut the amp off immediately and replace the tubes.

Preamp Tubes.  Preamp tubes usually cause problems through noise or microphonics.  If noise, you will hear hiss, crackling, popping or similar issues.  If you hear squeal, hum or feedback, it is typically a microphonic tube.  Noise from microphonic tubes will typically increase with a volume increase.

To diagnose which tube is bad, if you have a two channel amp, the most important thing is to determine, if possible, whether the problem is on both channels or just one.  If you can tell that it is in both channels (even if only slightly on one channel), it is most likely V1, which is common to both channels.  A noisy or microphonic tube in this position will affect the entire amp.  Microphonic or noisy tubes in several other positions may not be audible at all.  You can try tapping the tubes like you did the power tubes and see if the problem worsens.  It is normal to hear a slight ring when you tap on preamp tubes.  This test is only to see if the identified problem changes when tapping.  This test is NOT outcome determinative.

If the problem is only on one channel or the other, you’ll know which tube is the problem.  If it’s only on the clean channel, it’s usually the tube dedicated to that channel.  If it’s on the crunch channel, then you've narrowed it down even further.  NOTE:  V1 usually is the culprit.  Even if you don’t hear it on both channels, you should check V1 anyway.  Tube problems are much more dramatic on V1 than any other position.  When you think you have determined the culprit, put the amp on standby and change that tube.  Retest.

NOTE:  It is always advisable to change preamp tubes one at a time.  It is a good idea to keep at least one known good preamp tube for this purpose.  Then you will know for sure if you have found the problem.  Having a know, working reference tube (or better yet, a set) makes life much easier.  PS - it's helpful to number the tubes before testing.  People OFTEN get them confused and simply keep moving the bad tube down the line, thereby mistakenly thinking it's not a tube problem.  So change them one at a time.  If it doesn't resolve the issue, put the tube back into the position it came out of, then move to the next tube.  It'll save you a lot of headache. . .