I’ve built a JTM45 based on the MetroAmp instructions, and it was sounding really good (aside from some squealing with the presence above 8). All of a sudden it started making a really loud continuous machine gun-like sound. I checked the bias and it had risen on each output valve to about 70mV. I adjusted it back to about 42.5mV and the noise went away for a couple of minutes. Then it came back. I checked the bias and it had risen to about 70mV again. Unfortunately this time adjusting the pot did not lower the reading, it would only increase it to about 90mV. Does anyone have any idea what might have caused this? I have to say, the noise produced was impressive, like the opening of Audioslave’s ‘Cochise’, without the musicality!