This is my first post, and there is wealth of information on this forum.
Anyway, I have a ST-70 amp that was slightly modified by the prior owner. There are no power take off (PTO) sockets installed, but the pior owner installed two inputs. From what I see in the guts the wiring does not appear to be modified at least from the standpoint of the biasing.
I have been an owner of guitar amps for a while so I do most of my biasing the somewhat old fashion way and that is taking reads for both plate voltages and idle current. Based on what I read, a stock ST70 amp using the PTO for the voltage read should be around 1.56 volts. Assuming the cathode resistor is accurate at 15.6 ohms then the result is an idle current of 0.10 mA. My first question is the idle current the total for two EL34 tubes? I believe it is but I want to make sure. If it is then a single EL34 is at 0.05 mA (assuming both tubes are matched). Based on a plate voltage of 410 volts (using the value in the factory issued instruction manual) would result around 20.5 watts per tube. Of course this was based on the lower AC voltages of the 1960s. So my second question is this calculation approach correct? The third question is the bias pot used to adjust both tubes on the same side or is one pot for both tubes in the back and the 2nd pot for the front tubes?