Amp gradually increases gain over 20 minutes

allen

Hi there,

I've just got a new amp - Mesa Boogie Lonestar. When I'm playing the clean channel I've got the distinct impression that it starts off fully clean and then seems to gradually get a bit gainier over the next 10 or 20 minutes. It's not like a full on distortion, it just goes from totally clean to slight edge of breakup. It doesn't get louder.

I had a look in the back and the power amp valves are JJ 6L6s (not branded Mesa), I tried to see the preamp valves, but they are halfway under the chassis. But based on my fiddling about with a torch, at least one of the preamp valves is branded Mesa.

Does this sound like a valve is on its way out?