Questions about CT-100 HV circuits

[ChrisW6ATV]

Hello-

I am again working on my CT-100, first mentioned in this topic: http://www.videokarma.org/showthread.php?t=251001. Post #46 has a picture of the HV cage, which has had a lot of work done to it both in the 1960s as well as the late 1980s. The focus and HV rectifiers were replaced with solid-state components; the focus diode is still there but the HV replacement went bad later and a 3A3 was reinstalled. However, the 3A3's filament winding was removed from the flyback when the solid-state rectifier was put in and that tube is not going to light in this condition! There are a bunch of other parts that do not match the RCA and Sams schematics I have, so I hope one or more of you CT-100 experts can give me some advice on the changes, as listed below.

C21 20uF/50V electrolytic is missing
C22 25uF/10V non-polarized is missing
(Both of the above are near the horizontal centering control, which has been replaced with a different type.)
C134 .22/400V is now .5/200V
C135 .18/400V is now .056/200V and .047/600V in parallel
C140 .47/200V is now .82/200V
C141 .22/400V is now .15/200V and .15/400V in parallel
R184 1.8M/1W is now 6.8M/2W
A .01/1KV disc with spark-gap top was added in parallel with C197

What I want to know is, are any of these changed values possibly correct as in production changes, or should I put in all parts that match the schematics? (The part numbers above are from the Photofact schematic, but the RCA one does match in each case.) Another obviously incorrect part is C138, the .01/6KV on the focus control wiper, which is now two .01/2KV discs in parallel along with some type of (high-voltage limiter/gap?) device that I will replace. I have attached the part of the Sams schematic here for reference. This set is an "early" one, without the B+ boost changes. Last, if anyone has a picture of the flyback that shows what the HV filament winding looks like (or can tell me the number of turns of some HV wire to use), I would appreciate it. Thank you for any help you can provide.

[ChrisW6ATV]

A quote from Dave A at the ETF convention has me curious:

Originally Posted by Dave A

The great 6BK4/6DB4 pin wiring discovery is a help to all of us.

I was aware that moving one or more "tie point" connections on pins of the HV regulator socket would allow use of a 6BK4 in place of the original 6BD4. What was the discovery at the convention? Thanks.

[miniman82]

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:

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.

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.

[ChrisW6ATV]

Very interesting findings.