Quote:
Originally Posted by Penthode
I found the 6BK4 regulator is not really regulating the HV. This appears because the HV unregulated (with the 6BK4 cap off) is only about 23kV.
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The 6BK4 regulator does not work by regulating the voltage. It works by regulating the total current load on the high voltage. It measures this current at the bottom of the flyback. That is, when the CRT draws less current (dark picture), the 6BK4 draws more current from the high voltage anode lead, keeping the load constant. Therefore, the load (and the voltage) should be constant, until the picture is so bright that the 6BK4 has reduced to zero current. If the CRT draw is raised higher, then the draw on the high voltage output does increase and the voltage drops.
So: if operation is proper, the high voltage should remain constant as brightness is increased up to a certain point, and then the voltage should start to droop as you try to get more beam current. If the voltage is drooping gradually as you increase brightness from black, the regulator is not drawing compensating current like it is supposed to.