Tom,
If you're interested in CT-100 'hot rodding', I dabbled with a few circuit mods that RCA themselves included in later chassis.
For starters the vertical oscillator circuit in the CTC-2 gets its plate voltage directly from the LV power supply, where on later chassis it would come from B+boost. This change was necessary in later sets having larger screen sizes, because with the vertical circuit powered from the LV supply changes in horizontal width would not have any influence on vertical height. The result was that on an overly bright scene which might tend to load the horizontal section (really any situation where the potential of the horizontal circuit changed drastically, such as line fluctuations), horizontal width would tend to shrink while vertical remained largely unchanged. This causes the picture to appear somewhat 'squeezed', much like drawing an image on a rubber band or silly putty and stretching it out. Thus with vertical potential directly tied to horizontal the entire picture size would change in a more or less equal way, rather than losing only width and ending up with funny raster geometry. It's really noticeable with a crosshatch on screen, less objectionable with live images.
I moved the vertical plate supply to B+boost on my chassis and it works quite well, only other thing I had to change was the value of R144 (Sams) to account for the different supply voltage. Picture size now tracks well regardless of picture content, no more stretchy raster. I don't remember what value I ended up with, I just futzed around with different value resistors till I had something that worked well with the range of the height control.
I believe later sets also had the shunt regulator cathode returned to B+boost, rather than B+ or ground. This leads to less voltage across the tube, potentially making it more stable as a circuit but also lessening the stress on it in operation. It's not uncommon for shunt regs to have red plates in operation, but some of them really get hot. Not that 6BD4's are getting scarce or anything, just for the sake of what I will term 'generational improvements' in the chassis. No reason for the CTC-2 to fall behind in other areas, when it was blessed with such great picture quality right?
Lastly, I seem to recall the resistors in the shunt reg control circuit had drifted a lot, leading to no real ability to bring HV into it's proper range. After I replaced the out of spec ones, I could now vary HV into the correct range.
Great looking set, there's room to be much better!
