I got a factory built ST70, bought with no tubes.
Tubes arrived today: matched JJ EL34 quads, Sovtek 5AR4, 1 NOS GE 6GH8A, 1 NOS Zenith 6GH8A, both in adaptors. Speaker are Vandersteen 2CE Signatures on the 8 ohm tap.
Following the instructions for initial setting in the Dynaco ST70 instruction manual, I was able to bias both channels at 1.67 V, higher than what the manual says, 1.56V. Both bias pots were turned all the way to the left, which I think is the minimum, as turning them to the right increases the voltage measured at the power takeoff sockets. My line voltage is 121V, higher than the 117V called for in the manual, but not too high, so I wonder if this is the reason I can't get the bias voltage to go lower.
After about an hour or so, the bias voltage dropped down to 1.56 V or so on both channels, with the bias pots on the right at its lowest setting, and the one on the left turned up a bit beyond it's lowest setting to match the voltage on the right. I just measured bias voltage again, after the amp has been on 2 plus hours, I got 1.52V on the left and 1.53V on the right without touching the bias pots.
I wonder if this is normal for a stock unit using the 6GH8A's instead of the 7199's? Or is this due to new tubes settling in? Should I be concerned at all, or just being a newbie worry wart???
Being new to tube gear, I'm also surprised at how hot the amp runs. However, it does sound great, and it's quiet through both channels no hums or other extraneous noises.
Any comments would be appreciated, thanks!