Can tubes and tones change after a couple of hours?

[LordRiffenstein]

I have a problem with one of my amps. When I start to play it sounds wonderfull, nice sustain, very tight in the low end, smooth and fat. After playng it for about 2 hours or so the tone totally changes and it seems to loose the mids, the highs get piercing and annoying and the lows fart and rumble.

This happened a couple of times, played the amp and sounded great, set it on standby for half an hour to grab something to eat, go back to play and it sounds like sh$t. It´s not the guitar nor the stuff I have hooked up to it(rack, pedals) because when I plug straight it, it sounds the same (bad).

Can it be that I got some bad power tubes? That they change with getting really hot and that their bias shifts after they get real hot?

Thanks,

Yoeri

[John Phillips]

It does sound like a heating issue, but probably not power tubes overheating because being on standby shouldn't make this happen.

Is it an old amp?

[tanawana]

I've had this problem before and usually it ended up being in the bias adjustment on certain amps.