Power Amp

What a silly post.

+1,000

Well pardon me all over the place. Pardon me a thousand times. Just a humorous response to the post about a "hunch" above. Silly? Hilarious. No more silly than all the squawking and quacking about what brand amp it is. That is irrelevant. The tubes are being blown due to them being operated at higher voltage than their operating parameters require. That situation has absolutely NO relation to who made the amp. Volts don't care whose factory the device came from. The OP had said the amp has auto biasing which is what I predicted the source of the problem was before he posted whether it was auto bias or user adjustable bias. If you notice, the retired mechanic did not ask "what make of car is it?" before being able to give an educated diagnosis of the problem. The same concept of irrelevance is evident in both cases. You're welcome.

Although not mentioned, is your amp covered with a cage? I had an amp where the optional cover had poor side venting. The tubes did not last even when biased conservatively. I tossed the cover and the tubes (EL-34s and formerly KT-88s) are lasting years. In other words, do not discount adequate ventilation.

Also, it you look at the tubes with the lights out, do the anodes (plates) glow orange at all? If yes, a clear sign of overbiasing.

No cover nothing around it to stop very good ventilation. 

... And he still won't say what the amp is!