Originally posted by hogarOooooh the joke-line opportunites presented by that opening!! :D
So is there a time when you should get your head serviced....
Seriously, you should have it checked out by a good tech at least every two years, more often if you play for a few hours every day. As tubes age, their characteristics shift, and the bias settings need to be tweaked to keep the tubes healthy.
My Ampeg VT-22 is now into it's second year on a set of Groove Tubes that replaced the originals. It's been seeing a lot of use, so I'll check it out sometime this month. My VT-44 still has the original tubes in it, but then it doesn't get used much, so I'm not worried about it. You'll hear stories about amps that are still going strong on the original tubes after twenty years, but these amps have either not been played much over that period of time, or they have deteriorated slowly enough that the player hasn't noticed the change, and they would sound dramatically better with a fresh set of properly biased tubes.
Note that we're talking about the power tubes - the output section. The preamp tubes operate in ways that don't beat them up the like the power sections do, and the circuits are designed to adjust the biasing on their own, without the need to tweak anything. If a preamp tube 'goes bad', another tube gets stuffed into the socket. No prob.
Power tubes wear out gradually, and since they are usually being pushed into distortion, it can be hard to tell when they're due for service. If you really notice the improvement after adjustment, you probably should have done it a bit sooner.