This is a bit odd. When I first turn on my MAD 4, if it switched to phono, and if the volume pot is turned all the way down, I will get a loud buzzy hum through the speakers. Turning up the volume knob a bit solves the problem (which seems sort of counterintuitive)
After a few minutes of warm up, the issue goes away, and the volume can be dropped to the minimum setting.
It's not a huge issue, but does it signal another problem, or it some quirk of the oddball 12SC7 tube as it comes to temp? This has happened w/ all 4 sets of tubes I've tried.
