RCA 21CT55 B8802897 is Alive!

[jr_tech]

My understanding is that grid emission is caused by evaporated barium from the cathode ending up on the inside of the grid cup, where it can become an emission source, when the grid reaches a certain temperature. Higher than normal cathode temperature can cause the evaporation of the barium as well as contribute to a hotter than normal grid cup temperature. Is the crt heater voltage running high in this set?

jr

[Tomcomm]

When I bought my 21CT55 in ’64 it had no booster on its 21AXP22. I converted it to a 21FBP22 when I compared its brightness to my CTC4 which I converted to a 21FB a few months earlier. I have never run a booster on the present 21FB or rejuvenated it, since it has always and still is very bright. I reactivated the 21CT55 about 1.5 years ago and ran the ultor at max since I had no HV vtvm probe and everything seemed fine. I have a Powerstat on the mains input in order to limit the B++ at 410v with the silicon rectifiers that replaced the big seleniums. This dropped the heaters from 6.4vrms to 5.95vrms so no over heating of the crt cathodes here.

I became aware of the below cutoff full screen glow a few months ago but since it only was annoying on very dark scenes during no light night viewing I tended to ignore it. From a cold start, the glow becomes barely noticeable in about 3 minutes and steadily gets brighter, reaching max in about 10 minutes. A relatively long time constant for the small masses of the crt gun assembly. Crt grid contamination from the cathode seems the most logical explanation for the glow, but I need more input from forum members that are running non reguned roundys…………Tom