I am not sure how many tubes in your amp. Last time I have trouble with my tubes, I swapped the tubes from left to right to make sure they are the source of the problem. Depending on what type of tubes you are using and how frequent you use them; it is alway a good idea to replace them after a one to two years. For me, I have 4 sets of different brand of EL34's for my amp. Each have different charateristics.
Tubed Amp Troubleshooting
Need a bit of help/advice from the tube guru's. The mid/bass on my right channel started to drop out upon warm up this weekend. System will sound OK for the first hour or so after amps are turned on, but the mids/bass will drop out suddenly. At first I though it wast the mid/bass driver on the speaker, so I switched the speakers. However, the problem is still there with the speaker that worked fine on the left channel before. Last night I put in a new 6AN8 driver tube, but the problem cropped up again. Next step is to put in a new quad of power tubes and see if this fixes the problem. I will try that tonight, but wanted to see if anyone had other troubleshooting ideas. Thanks!