Some days you just can't win. My trusty SF Twin Reverb (UL variety with Mstr Control) has become well, not so trusty. It has been a great amp that was blackfaced and bias pot added a few years ago, sounds fantastic normally. I had to take it in to replace the diodes after one failed abruptly, but all was good for about 2 weeks and now this. I keep the master set at 10 and when I fired it up tonight there was a strong audible oscillation going on. It increased / decreased with the master volume knob, but never fully went away. Then I flipped off the bright switch and the oscillations stopped even with the master turned back up to 10. Amp sounds good now, seems to be back to normal with the bright switch off, but I am wondering what the hell is wrong now? My amp tech is a real pro and went through it after replacing the diodes to make sure all the pots were cleaned and played it for quite some time before signing off on it. Played great for 2 weeks and now this issue. I thought maybe it was picking up the ceilling fan or something and turned off everything else in the room, but it only went away when I turned off the bright switch on the vibrato channel (I only use the vibrato channel). Any ideas?