Tube amp maintenance for the unaware, but.....
especially those who do not really believe that the sound produced by their amps is going to be constant because of all of the work they have go thru to set it up in the first place.
Power tubes are expensive! Power tubes have a finite life! As tubes age the sound changes. The top end starts to roll, bass can disappear, the edge starts to disappear. Things get a bit too relaxed.
The average useful life of a power tube probably averages about 1500 hours, but depending on amp design it can usefully be used for up to 3000 hours, perhaps even more. When the amp designs put out max tube power, life can be short. Lots of tube amps with lower power per tube can give much longer life (40 wt tube amp, anyone :-).
I go thru a set of tubes a year, my amp is on often 8 to 12 hours a day. Up to 3000 a year at that pace.
After about 2000 hours I note the age showing. That is when I put in new small tubes with a sharper tone than the ones I'm replacing and usually they have an emphasis on high frequencies. Puts some new life in the old tubes. Then after the power tubes pass the 3000 hours I chuck 'em, pull the small replacement tubes, and start all over again.
I'll admit its not perfect, but it works well enough I think, and it saves a fair peice of change. The only downside is having more small tubes on hand which can do the job.