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if it goes to 30kv with the brighness down, then I would guess the shunt tube is not operating properly (should be clamping it) or the HV is just set too high.
did you check the cathode current of the 6Bk4? I am not sure about that early of a set, but would guess the current should be between .5 and 1.5 ma depending on brighness and contrast settings (higher ma with CRT dark).
Just a guess here but maybe you just have the screens set too high. I would back them down so when the brighness is fully CW the pic is just a bit brighter than normal viewing, set them all for good grey scale, then adj the drives to get good whites, set the HV for 23-24kv check the shunt current again and see if it varies with screen brightness and if the HV holds steady.
The fly can only produce a max power output, the shunt clamps it down if the CRT is not drawing too much, and lets off when the CRT needs more.
if have the CRT biased incorrectly or the G2 voltage is too high then the CRT will draw all the flyback energy leaving the shunt nothing left over to clamp, if the draw is more than the max, the HV will drop. From what I gather the optimum is clamp .5ma on a dark screen this way there is always some flyback in reserve so the HV will not drop.
If the shunt current is high even in a bright screen, then there is a problem in that circuit. there is not a lot in there, I would check the resistors 1st then the pot then the caps in that circuit, in that order (after tube sub that is).
good luck with it.
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