[Chuck Schwark]

With your 600A tester, do you use this trick I learned when testing?

Perform the normal Gm test and hold the button down, and then click the FIL voltage switch DOWN one click to slightly lower the filament voltage.

See if the Gm test only dips slightly. If it drifts down appreciably, then the tube is also marginal for emission on a high-mileage tube, when in a set that has slightly lower filament voltage.

Chuck