[Yamamaya42]

This is just pure speculation here, as I still do not fully understand how the HV regulator works on this, but after looking at the CTC-15/GE &16(which is almost identical ) , I seem to have a general understanding.
The 6bk4A/B/C regulates by shunting power via the cathode through a 1k resistor to what ever the damper gets it's bias from, SAMs does not say what wattage, I assume to be 1W.
The grid of the 6bk4A/B/C is tied to a 1.5 meg resistor voltage divider ( matched pair) , one to the HV control pot & coupling cap to cathode. ( very low suspect ).
The other resistor is hooked to the FBT winding that goes to the Horz yoke windings, thus there is strong pulses there, this ALSO goes up to the RGB drives, and later video output, and so on, therefore, if these resistors are out of tol, especially the pair of 1.5 meg, it would explain the odd behavior seen when changing brightness, and why I have seen people say to pay attention to those resistors in particular.