The preamp tubes will last a very long time, but the power amp tubes usually have a lifespan for about a year under normal playing conditions. This isn't set in stone by any means, because I've had power tubes last a couple years with regular playing, but a year is fairly typical. The preamp tubes will have the most effect on your tone. Specifically the tube in V1, and your guitars signal will usually always be going through this stage.
I'd buy my tubes from Doug at http://www.dougstubes.com/
. Let him know what you have, and what tones you're going for, and he can make a good suggestion for some tubes. A tube swap doesn't take long at all, but if you don't know what you're doing, I'd definitely take it to a competant tech. There are voltages in there that can kill you even if the amp isn't plugged in.
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