Re: The "Sussex" Homebrew Valve Tester.
Looking at your 'scope images, I'm reminded of some similarity between those and my Sussex when I was having similar problems with EL34 valves.
The symptoms are similar, the gm reading is always low and variable, the Anode current tends to vary a lot but is always much lower than it should be. In fact, the anode supply voltage will be seen to vary by a hundred volts or so in a cyclic manner. In my experience this was caused by spurious oscillation in the EL34 valve, cured by soldering a 150k resistor to pin 5 of the octal valveholder and connecting the grid wire to that.
I suggested, a few posts ago, that you might try putting your hand around an EL34 under test and see what, if any, difference the hand makes to the readings on the Ia and gm meters. I might have missed the answer, but that was, for me, the clue to the final solution of this very perplexing problem! Caution, though, because if the valve heater has been switched on for a while, the valve will be uncomfortably hot to touch.
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