[Tomcomm]

Miniman82 states “Maybe I'm wrong here, but 22kv sounds fine for HV. All my RCA sets run at 22.5, so you can't be too far off base...
These round tubes were meant to be operated at 22.5kv, so there's no good reason to put 24 on them.

CT17 states “ yep sams says 22.5-23.5kv. I have to scan the sams, do it tomorrow. Got stuck exploring the abandon prison and got home late. C17”

Sams RCA 21CT55, CTC2B says “..set HV to 25K. HV Reg 6BD4 cathode should read .7ma”

RCA Field Service Guide CTC2B schematic shows 25KV, RCA Service Clinic says…
“…6BK4 regulates HV voltage to 25KV which is required at the ultor of the 21AXP22”

My CTC2B uses a 21FBP22 which RCA max specs at 27.5KV. I now use the OEM flyback used in the CTC20 which also uses a 21FBP22.

CTC4 with 21AXP22 schematic shows 25KV, CTC5 with 21AXP22 shows 19.5KV!!, CTC7 with 21CYP22 shows 23.5KV, CTC7 with 21CYP22 shows 23.5KV, CTC9 with 21CYP22 shows 22.5KV, CTC10 with 21CYP22 shows 23.5KV, CTC11 with 21CYP22 or 21FBP22 shows 24KV, CTC15 with 21FBP22 shows 23KV, CTC16 with 21FBP22 shows 25KV and finally the CTC20 with 21FBP22 shows 25KV.

The only good reason to use an ultor greater than 22.5KV is to produce a better picture.