Ok,
pull the cap off the 6BK4, hang it out of the way so it cant arc to anything. What does the HV measure with the cap off?
Let me explain it this way.... lets say your car has a regulator on it that will not let it go over 55mph....
Lets say there is something wrong with the engine and it doesnt have enough power to make the car go 45 mph, the regulator never kicks in because the car never reaches 55mph.
Now lets say the car will get up to 55mph on the flat (dim picture) and the regulator holds it at 55. No comes a steep hill (bright picture) and the motor does not have enough power to push the car faster than 35mph up the hill, the regulator never kicks in.
The high voltage system needs to have enough capacity to maintain the high voltage through all operating conditions. If it doesnt the regulator never comes into use. If something is overloading the high voltage like a gassy crt or you have the screens up to high.
Why dont you watch my video on a ctc16 i just restored, i go though a hv problem similar to yours,
http://www.youtube.com/watch?v=1AWG61TAIHg