From my tech in response:My strategy for testing 845s is to use my high voltage DC bench supply and switch mode power supply to heat the filaments. I expect 500V on the plate and -50V of bias to cause the 845 to draw about 90mA. If it draws substantially less than that, that's an indicator of low emissions.
That’s an emission test...and not very safe with all that voltage coming off a bench supply. Won’t tell you anything about leakage or shorts. The filament is 10 volts at 3 amps. That’s 300 watts of heat. Then there’s 500 volts on the plate. Imagine fumbling around with that. That’s lethal voltage. I’d have to make up a special test jig.