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 ok 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 unsure take it to a dealer for a service, those this shouldn't be needed.
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, using their fingers. 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 use to toss all her cables into the back of her Mesa Boogie Mk II combo. One of those cables 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.)
|show 1 more comment|