[Radio Wrangler]

It only gets worse, Bill once you dig into it.

The anode current is a function of both grid-cathode voltage and anode-cathode voltage and both are changing at once. The anode current waveform is distorted by this and also by the characteristic of the valve being tested. So what sort of meter response is appropriate for the Ia reading? and what fiddle factors are needed?

I'm afraid I can't take these machines seriously as measuring instruments. Their pointers point to a number, but it's very difficult to determine what it actually means. I think AVO put in fudge factors so that middle of the road valve types produced numbers of about those in their manufacturer's data sheets.

Trying to relate settings of grid and anode volts to what will be read on a bench meter of any characteristic is more involved than I want to do.

I keep a Taylor 45D here and it does service screening valves at the radio club to weed out definite duds and low emission ones, but that's about all I trust it for.

If I wanted to measure valve characteristics with any hope of traceability, I'd build a DC tester like David Simpson's or else a Sussex. With real DC all these issues are blown away.

I still think the AVO VCMs are attractive bits of classic kit and one would look great in the shack, and they are interesting restoration projects in themselves.

David