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well something is not right, there should be a jumper before the "long brown resistor, which I am guessing is a 13k screen resistor for the HOT. Any way if the HV pot is not open or shorted and the 1.5 meg resistors are ok, both of which I think you have checked, then the only other thing I can think of is a leaky .01 1kv cap between the grid and the cathode of that tube (pin 1 and 5). IF that cap is leaking then it would throw of the bias of the tube and could cause it to over conduct. NOTE you need to use a .01 1KV (not lower voltage disc) if you choose to replace it.
I cant say forsure this is the problem since you can seem to find a way to check the tube currents (either the 1k or a jumper in the cathode of the 6bk4).
if you can fix your scanner OR get a pic of the chassis around the 6bk4 (including the term strip) them I might be able to figure out how you are supposed to check the shunt current.
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