[radiozero]

I'm not sure how you go about measuring mutual conductance. Nor, how my newly acquired AVO MKI works.

The KT66 has a mutual conductance of 7.3mA/V. Meaning that for every 1 volt change in grid voltage, anode current change is 7.3mA.

I'm not sure how the AVO Valve Tester works. I can see that the meter measures anode current. But, how can reading the anode current tell you about the mutual conductance of a valve?

In my ignorant state, to test for mutual conductance I'd be thinking of arranging for a change of grid voltage by 1 volt and looking for a change in anode current of 7.3mA. If the change was below 5.5mA then considered out of spec.

Here is shown the AVO circuit.

I've not a grasp how the circuit works. I do know that the "Good" portion of the scale goes from FSD to 50% of FSD.

The AVO does measure mutual conductance. I believe. By pressing a "Mutual Conductance" button.