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There have been a number of discussions in this forum on the CTC5 and it's inability to muster enough high voltage. I found mine barely able to squeek the 21kV. I would manage to make the 21kV when there was no beam current (black screen). But soon as the brightness would go up, the HV would dip a bit..
I didn't bother with the HV pot modification as the 6BK4 would start to draw current when the HV reached close enough to 21kV. What I did do was to ensure I had fresh good horizontal output and oscillator tubes and ensure there was sufficient horizontal drive to the output tube grid. I also substituted (and this was crucial) a solid state 3A3 rectifier for the original tube 3A3. The solid state version is a direct plug in replacement and changing it eliminated the HV sag with increased brightness. The 6BK4 now is doing its job properly as a shunt regulator.
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