Can anyone suggest why on my MKIII I can only just get the correct bias (pot fully CCW) with the driver tubes pulled out. When the driver tubes are in the bias measurement jumps right up to 2V+. I am getting -68V at the bias pot wiper and about the same on pin 5 of the output tube.
I have a 25K linear bias pot for each output tube bias adjust. The resistor I am measuring the bias across is 10 Ohm.
The coupling caps on the driver board are new as are the tubes so I am reluctant to suspect them.
Many thanks for any help you may provide.