Hi Mike, I think the problem may be due to the much higher anode currents when testing things like EL34's. The DC voltage across the 10 ohm resistor would be sufficient to make the diode conduct whereas on small signal valves this problem never occurs, just guessing but I think quite probable. I suppose one could try 2 diodes in series but I often wonder if something like a 1:1 transformer would be better for sampling. It would have to have a good air gap to stop saturation as in the case of a single ended output transformer. Would a power transformer be up to the job? Which makes me think of a small dual 0-6, 0-6 transformer but all of this is highly speculative until some experimentation is done. One would have to feed on one side an AC test voltage e.g. 100mV RMS and trim/adjust the output for same reading on the gm meter. There remains of course the danger of transients produced from a faulty valve but without the DC current problem, a pair of back to back diodes might protect the meter on the other side of the transformer.
Hopefully, I will be clear of some outstanding tasks here in the next couple of weeks, then I can un-case my Sussex and try some more experiments. I have a gm meter calibration circuit I want to try anyway but this will have to be abandoned anyway if a DC isolating transformer is found to be required. The gm meter is already isolated from ground by the Paul Stenning transformer and this is a 100% requirement, so the meter damage must be caused by pushing the +/-200mV full scale over the top under fault conditions. It probably takes out the first op-amp on the meters pcb but being so fine a pitch SMD style, it is beyond my eyes and steadiness at my age to even think about trying a replacement. I did put the damaged meter somewhere, perhaps I should just get a new IC and ask one of the young lads in the wiring shop to replace for me, hopefully it will cost no more than a bottle of JD
