[Boatanchors] testing 3-500Z tubes for shorts

[John King]

I have 5 amplifiers that use two 3-500Z tubes in each. Over the years, I have had those tubes go bad and short, blowing the chokes and caps from grid pins to ground. I normally just replaced the tubes and associated grid components when that happened.

Now I have a number of pulls and am trying to determine if those have grid to plate shorts or grid to filament shorts. I have been using my 8025 Fluke digital meters to try and detect shorts. So far none have tested shorted with the multimeter.

I have decided that some may be not shorted but "nearly shorted" and the shorts don't occur until the tubes are heated up with the filaments and the plate voltage is applied. I suspect that after heating up, that the grid will sag into the plate bringing about a "big bang".

If you have experience with these tubes, I would like your input for testing without a "big bang" and if there is a "near short" that rears its' ugly head only after being heated up, how would I correct the sag and near short. Thanks and 73, John, K5PGW