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Sounds like the output tube isn't being driven hard enough, have you checked the oscillator for correct performance? My CTC-4 will make 25kv no problem, same with the 21-CT-55. They have very stout HV supplies, so if you can't get more than 20 I'd look for issues in the drive or make sure it's not being dragged down somehow. Check that the output has the correct negative grid bias, and make sure the waveform going into it is of the correct amplitude and shape. If that's not it, juggle HV rectifiers till it comes up. I went through 3 of them before mine was stiff enough, just because they are new doesn't mean a whole lot. I've found that color sets in particular are very picky about what rectifier they have in them.
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