I think I may have jumped the gun with the pinning of that socket, actually. I remember reading about it on Phil's site; you swap a pin, and you can use a 6BK4 in place of the 6BD4.
Here's a quote from his site:
Quote:
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The 6DB4 does not use pin 8, and the factory wiring employs that pin as a tie point for two unrelated components. Len installed a new connector near the tube socket and moved the pin 8 component connections there. After this change, it's possible to use a 6BK4 tube as well as a 6BD4 in that socket. I don't know why Len considered this important, since both tubes are readily available for a few dollars. In any case, the change does no harm. In a box of miscellaneous parts, I found a weak 6BD4 tube and a strong 6BK4. I installed the 6BK4.
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So removing the tie point from pin 8 on the shunt regulator simply allows you to use either tube, it didn't have anything to do with the loss of regulation as I first thought- I was mistaken.
On Sunday after everyone else had left, we were able to figure out that the loss of HV regulation in the chassis was due to the convergence electrode in the proto tube sucking more power out of the bleeder string than the 15GP22 does, which caused the regulator to bias completely off since the HV control pot is at the end of the string. This was proven by voltage measurements: when convergence potential was near the high side, voltage at the HV pot end of the string sagged to the point that the 6BD4 was cut off which allowed HV to rise. I'm currently designing a workaround for this that may be helpful to other owners:
On later sets, the HV pot is referenced to B+Boost rather than anode voltage like with the CT-100. I'm designing a circuit that will isolate the HV pot from the bleeder string, so that changes in convergence potential will have no effect on HV regulation. This should make HV more stable in the CT-100 chassis, and I'm also working on sourcing voltage for vertical height from the same B+Boost line which should further help picture stability. More later this week.