[Electronpusher0]

OK, the low pass filter idea was not right.

What is needed is a sample and hold with decay to grab the peak value.

Attached is the schematic of such a circuit using a TL072 or similar dual op-amp.
The DPM is a cheap readily available dpm module.
The output is 2V dc for a gm indication of 20, this can be attenuated with a divider to give the voltage to suit the dpm.
The pot is used to calibrate the output.
The supply is + / - 9v, I chose this because 9V is also the voltage required for the DPM supply, but anything between 5 and 15V can be used. The supply will have to be isolated as the circuit floats at Anode voltage.

To use this in the AVO 2 panel valve tester the meter and swamp resistor are replaced with a 68 ohm resistor, the voltage derived across this resistor becomes the input to the sample and hold as shown on the schematic

I have breadboarded the circuit and tested it using a simulation of the input waveform derived from a low voltage transformer with diode and attenuator to give the required 264mV peak half wave rectified waveform which is what would be expected from the AVO at a gm reading of 20.

The circuit works well with a stable display as can be seen from the photos.
The 2.2uF capacitor and 470K resistor set the decay time, reducing the value of these increases the response but also increases the meter jitter which shows as the least signicicant digit "hunting" by one or two counts.

I now need a meter panel from a 2 panel AVO valve tester, one with a broken meter, to test the circuit fully. I am loth to play with my calibrated unit.

Anyway I offer it as an idea.

Peter