Follow up report:
tested my RCA Cheltenham CTC-4, which has what I think is an original CRT in it. The CRT has the rCA logo on the base, and is engraved "21AXP22A/21AXP22". The painted on date code is unreadable.
With the brightness an contrast controls turned down, and the 3 G1 grids biased to ground potential, there is absolutely NO glow in a ptich black room, after 45 minutes of operation at 25KV ultor voltage.
This seems to confirm the data from Tom and Wayne that says Tom's glow is due to cathode contamination of the G1 grid. Tom, you need to find a good 21AXP22 for your 21CT55 chassis so it can be returned to it "proper" configuration!
