Hi, I had the same issue a couple years ago with one amp. I called Chris O. at ARC and he asked me how old were the tubes. I said well over 2000hrs and the ran the bias at 0.7mv. His response was; the bias is too high so you actually have 3000hrs on those tubes and better replace them now before something worst than a blown fuse happens. Replaced the tubes with SED 6550C and biased the amps at .65mv and all was fine from that point forward.
And yes the servo should be checked out and be adjusted within spec as the Geek says. I have some documentation from ARC on how to adjust the servo but it's very tedious. Patience is key with servo adjustment because the pot is VERY sensitive when you turn it. You have to go in minute increments and wait for the voltage to settle (usually 5 minutes) and then make more if need be until you fall within spec. I believe you have to be within .2mv
In the literature, ARC says "RESIST THE URGE TO FIDDLE WITH THE SERVO". The servo should not under normal circumstances, require adjusting.
And yes the servo should be checked out and be adjusted within spec as the Geek says. I have some documentation from ARC on how to adjust the servo but it's very tedious. Patience is key with servo adjustment because the pot is VERY sensitive when you turn it. You have to go in minute increments and wait for the voltage to settle (usually 5 minutes) and then make more if need be until you fall within spec. I believe you have to be within .2mv
In the literature, ARC says "RESIST THE URGE TO FIDDLE WITH THE SERVO". The servo should not under normal circumstances, require adjusting.