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My schematic also shows 6 volts (red) 6.2v (grn) and 6.4v(blue). What really matters most is the difference between this and the cathode voltage. The cathode voltage on my ctc11 schematic is 11v, so the grid bias is really -5v (red).
The voltages on the schematic were probably taken either off-channel or with a gated rainbow generator, so the absolute values might not mean much. It is very interesting that one is a little different. I wonder what the cathode voltages are?
I'm guessing, based on a cathode voltage of 11v , that -9 volts on red (versus -7.5 on blue, which is working) wont be enough difference to cut the tube off. I'll bet the peaking coil (L705) is open. It will be tough to catch it in the act if this intermittent, by the time you get the chassis out, it might be working again. If it doesn't test open, I might try bridging it. This will probably have some undesirable effect, but if it stopped the red from going away....
c731 cant be leaky, because that would make the grid voltage too high. It could be intermittently open, though. Is it a ceramic disc or mylar? You might also see if r730 is out of spec. Maybe even put a little heat on it with a soldering iron and see if the resistance reading goes wacky
John
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