It seems odd that the amp would have a drop out in a specific frequency range since it just takes the signal and amplifies it. Your bad tube guess would suggest you don't have the power to drive the bass. I would expect this to occurr from the get go since, if anything, tubes will sound fuller as they warm up. Did you sequentially switch the source and pre-amp to make sure the problem stayed with the amp/channel in question and isn't in another componant? sorry i can't help more
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!
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