True, preamp tubes are first in line, but the power tubes are creating, well, the POWER. If well cared for (and if you have good luck) preamp tubes can last YEARS. Power tubes, if used regularly, and especially if you crank them on a regular basis, will wear out quicker. I change mine about every 6 months. That's with practice once a week, gigging about 1-2 a month, and jamming at home. (I probably should change them MORE often)
I know this isn't the best analogy, but you could think of the preamp tubes as the brains, and the power tubes as the muscles. You can think with your brain all day without "wearing it out" per se, but if you had to do some heavy lifting with your muscles all day, you'd be worn out.
(again, not the best analogy...)
I mean, this is why you see alot of ANOS (almost new old stock) PREAMP tubes being sold. Tubes from the 60s that people claim test "like new", and they're getting good $$ for them. You don't really see alot of used power tubes being sold.
