I am using a 539C to test a bunch of tubes. With every tube I test, the Gm reading will stay solid but the plate current will slowly bounce between 2 values - usually within 4 mA of each other. For instance, I was just testing a 2A3 and the plate current would slowly bounce between 24.2mA and 27.8mA. I am letting the tubes warm up and the tester was recently calibrated.
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