[Phlogiston]

Here is another tube rejuvenation process for later tubes, those not covered by the one above. It works sometimes and sometimes it makes the tube worse.

The idea is that a good tube will not conduct significantly more at higher filament voltage, where as a cathode that is contaminated will. So you put the tube in at normal filament voltage and push the button. Then you increase the filament voltage by about a third. The reading will go up. (note that this will load down the tester power supply so true filament voltage will be less than indicated so will plate voltage - but ignore that). Watch the needle climb. When it stops climbing take your finger off of the test button and turn the filament back to normal. The additional heating is supposed to clean the cathode.

I would not try this on a good tube or a marginal tube. Note too that gas contamination will often allow a tube to conduct at higher filament voltage. Watch for the blue (purple glow). If you see it, the tube is junk.