I think this would be a wonderfully instructive question to people who are new to tube amplifiers. In short, how do I know that it's time to change a tube, and what's a good strategy for figuring out which one to change (preamp vs power vs rectifier)?
An important thing to note is the different types of valves can be changed independently of each other; eg: if you change the preamp valves you need not change the power amp valves etc.
With power amp valves you will notice very quickly when one or more have gone. The amp will give out noticeable and unpleasant tones/white noises and will be reduced in power; the glow that a healthy valve gives off may be different in colour or gone altogether, there may even be flames in the blown valves, which is pretty hard to miss. When changing power amp valves ensure that the amp has been cut off from the power for some time before you start, valves store a lot of power and can be dangerous if removed straight away.
Preamp valves tend to last many times longer than power valves; so check the power valves first if they look ok then inspect the preamp valves closely; a blown preamp valve might also give off different noises than a power amp valve; such as excess feedback and other squeaks/screeches. Also if a preamp valves goes you may not notice a reduction in power; which is another way to identify the type of valve which has gone.
A good rule of thumb is; if you get a drastic change in tone or hum from your amp; or it wont power on at all; its time to check the valves. If the valves look okay check all the cables carrying signals to the amp including the effects loop; if the amp wont power up check the power cable/fuses first. then check the valves. With no power its not possible to check the glow colour of a valve so look for valves which are opaque or have burn residue on them.
If you are unsure take it to a dealer for a service.
Two additional problems I've seen with some people's tube amps, related to tubes going bad, is their sloppy manner of treating the tubes.
Some people don't hesitate to use their fingers to pull a tube to inspect it. Finger oil, or fried-chicken grease, or dirt or whatever was on their fingers, will transfer to the glass of the tube, and begin to insulate it, reducing heat transfer. That makes the tube run a bit hotter, reducing its life. Instead, I always use a clean cloth, a couple paper towels, or something to keep oil from transferring. I've even used the tail of my T-shirt.
Another problem I saw, was our lead singer used to toss all her cables into the back of her Mesa Boogie Mk II combo. One of those cable's ends hit the tip of a pre-amp tube, on the nipple of the glass where they sealed it, and cracked it a tiny bit causing a pin-hole to open up, and vented the tube's gas. The next time she powered up the tube cooked and the amp was dead. I had to do triage on the amp after the show, and saw the little nick. She was told to put her cables somewhere else. (My personal solution is to put them in my gear trunk in a cloth bag that keeps them from getting tangled and the tips from getting damaged.)
Sigh, in the old days you would pull ot the tubes, put them in a suitable protective container (Standard was a woolen sock) and take them down to the local radio shop. They had a machine that would measure both cathode emmission and voltage gain.
These days you probably need to try replacing the tubes. Common failures are
- Filament failure (no red glow from tube)
- Vacuum failure (pink or purple neon effects inside the tube)
- Emmission failure (low power output, flabby sound)
- Output transformer problems, which typically manifest themselves as high distortion often coupled with red-hot tube plates (the cylinder closest to the glass).
Further to DRL's excellent answer, valves can go "microphonic" which means they've been damaged in such a way that vibration on the valve can be heard through the amp. Given the valve's job is to power the amp and the amp's speaker is a big vibrating device (ooer), this can cause some quite horrible noises.
A test would be to switch the amp on, don't plug a guitar in, and tap the amp on the casing, near the valves. If you can hear a significant rattle through the speaker, it's a sign that the valves may be on their way out.
I say "may" because of course other things like loose connectors could cause such a noise as well.
I have seen this only once on my amp. Replacing the valve (actually I did both power valves - EL34s) cured it.
You will know. It will start to sound truly terrible, or even go silent. When it does you need to isolate which one it is, which can be tricky unless you have two channels, in which case the one that moves the problem from one channel to the other when you swap it between channels is the one.
This rarely happens unless there has been physical damage, or a lightning strike. These things are really amazingly rugged given what they're made of. I have sixty-year-old valves that are still going strong, and there are plenty of guitar amplifiers that still work after a trip to the bottom of the stairs.
The amplifier is not capable of even remotely acceptable work without one of its tubes. When the tube fails, you see this beyond any doubt. Usually the filament fails, so the failed tube does not glow any longer.
While very old tube may amplify slightly worse than a new tube due lower electron emission of the aged cathode, I have never seen a recommendation to replace them periodically. A properly designed amplifier should have internals means to compensate both individual differences of various tubes and they ageing effects.
As long as they are working you're good. I like the sound of old tubes. More mellow.
I used to work at an AM radio station with tubes in the transmitter. The tubes were working very hard all the time. We would pull the tube out, every year. Remember, these work at max, 24hr/day, all year. Our Transmitter never went down because of tube failure. So figure it out. That 8736 hard hours. If you, play full blast; 4hr/ day, that's 1427hr per year. You should be good for at least 6 years. If you play 1/2 power, then 12 years, MINIMUM!