I popped the cage off my ST-70 for a routine bias check - it's been over a year since I checked it. The left side required some adjustment, but holds fairly steady - my digital multimeter jumps between .39 - .41. I recall being able to get it rock-solid at .40 last time. The right side is far worse. The reading jumps all over between .3 and .8; won't hold steady at all. I'm using JJ EL34 tubes that are a few years old - probably have 1 = 2 thousand hours on them. So, does this just men that it's time to replace the EL34s, or is there something else I should check?
I'm checking it with everything unhooked and the amp just warmed up for a few minutes.
I'm checking it with everything unhooked and the amp just warmed up for a few minutes.