I wonder if the Beltron socket-adapter you need to make up for these three-in-a-row GE CRT's can be a standard delta-rectangular type just wired differently
That Beltron sounds like the Heathkit CRT rejuvenator and I made that GE wired socket for mine shortly after I got it.
I saw many of those GE jugs that needed a good kick
, like those used in the '71-'72 19" C-2 and 16" N-2 chassis.
It does a "clean" by applying AC between cathode and G1, and you use "rejuvenate" which is about 200 VDC only for the real tired ones.
It is a pretty gentle process on clean and the two lamps that limit the current keeps it from frying the cathode
.