05-12-2019, 01:33 PM
When I looked again at your Bias Probe, Robert I saw what that switch does. I'd be inclined to measure both the plate (anode) voltage and current, particularly if I was thinking of changing the valve. Assuming the amplifier is working as designed, the V. and I. figures you get should be close to what the designers intended. With a pentode valve, the cathode current is the sum of the anode and screen grid currents, the latter only being a few milliamps. Measuring the voltage across the cathode resistor will tell you, using Ohm's Law, what the anode + screen current is. The bias pot. is provided so that the bias Current can be set to the correct value, and will be useful if you change the o/p valve. I don't have the pin-outs for either the 6V6GT or 6K8 valves to hand, but if they differ, that bias probe will not be useable with the substitute valve unless it is rewired.
