Bias Drift?

[Occam]

My main amp's (Electric non-master 120) bias seems to drift from time to time...I noticed it making some weird popping sounds between playing and when I checked the tube bias at it was up from 32 to above 50...is this a vibration issue? Is there a way to keep the bias pots from moving?

[Blue Strat]

That's more than a vibration issue. Did both tubes move or just one? Could be a drifting tube (defective) or something wrong with the bias supply in the amp. Also, set the bias after it's been on for a few minutes than check again in about 10 minutes.

[Occam]

That's more than a vibration issue. Did both tubes move or just one? Could be a drifting tube (defective) or something wrong with the bias supply in the amp. Also, set the bias after it's been on for a few minutes than check again in about 10 minutes.

My amp allows for all 4 tubes to be biased indivdually (though each affects each other a bit) and all 4 tubes were off, especially the first and last. I rechecked each tube after I went through all 4 but I didn't give it much time...I'll check it again today. The popping sounds I was getting though did go away. Maybe it's useful to note that I do play with a ton of volume (I dime my amp) and I play with baritone tunings so there's a lot of low-end rumble. I do have the amp on an extra set of rubber feet as a precautionary thing.