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Cathode over heating?
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
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