A CRT went to air

[John Folsom]

I have noted over the years it is not uncommon for a small percentage of used or even NOS tubes to go 'to air' (gassy) after an initial thermal cycle or two. I suspect it is the same with CRTs, though the number of CRTs I have dealt with is only a tiny fraction of the number of tubes I have dealt with. My guess would be microcracks around the base pins, created perhaps at the time of manufacture, or later due to stored stress in the glass, or maybe corrosion of the wires.

No matter the cause, it take very little air in a tube (CRT or otherwise) to overwhelm the getter and ruin the tube.

In the process of our 15GP22 rebuild efforts Bob and I have opened many 15GP22s with white cursty getters and often burned out filaments. I thought these tube were "down to air", that is full of air. Bit in every case, there remained a considerable amount of vacuum in the tubes. None of them were at ambient atmospheric pressure.

Just my observation....