CTC-5 21AXP22 convergence

[Penthode]

Have you checked the tubes on a transconductance checker? My set was relatively low hours yet I found a number of the tubes weak in the signal path and needing replacement. The good thing is that most of the replacement tubes are cheap on ebay.

The biggest drawback of the CTC5 in my opinion is the low high voltage coupled with the relatively dim (compared to tubes a few years later) 21AXP22A.

The low high voltage appears to be due to an inherently bad design. My set barely makes the design regulated 19kV: it would dip below the regulated output when the brightness was advanced. An immediate help for this was to substitute a solid state 3A3 HV rectifier so it at least now maintains regualtion at 19kV. If I get the time, I would like to explore further the Horizontal Output/ Flyback to see if I can a bit more HV.

The 21AXP22A in my set is original and shows good emission and life. But the picture is just dim. I compare it with the CTC11: the CTC11 picture is bright and brilliant. No comparison.

I suggest that the earlier less efficient phosphors in the tube coupled with the low high voltage is an inherent problem with the CTC5. Perhaps I should express my gratitude for the dim picture on the CTC5 as it probably explains why my set survived until now.

[old_tv_nut]

Originally Posted by Penthode

Have you checked the tubes on a transconductance checker?

...

The biggest drawback of the CTC5 in my opinion is the low high voltage coupled with the relatively dim (compared to tubes a few years later) 21AXP22A.

...

If I get the time, I would like to explore further the Horizontal Output/ Flyback to see if I can a bit more HV.

....

I suggest that the earlier less efficient phosphors in the tube coupled with the low high voltage is an inherent problem with the CTC5. Perhaps I should express my gratitude for the dim picture on the CTC5 as it probably explains why my set survived until now.

Haven't checked the tubes on a transconductance checker. I got the replacements, and subbing them mad no improvement. One of the first IF subs knocked the alignment way off for some reason.

The Super chassis has lots of drawbacks including the low high voltage.

Since I don't know how much safety factor is in the flyback design, I will not be attempting to raise the high voltage.

The less efficient phosphors are a detriment to brightness, but the greens and cyans have a purity that cannot be obtained in any later tubes (which have the more efficient but more yellowish sulfide green). Some of this colorfulness is lost due to the lower brightness. This is a well known effect in color reproduction, dubbed the "Hunt effect" due to its description by R.W.G. Hunt. Simply put, to look equally "colorful", a dim picture must have more color saturation than a bright picture.

[miniman82]

Originally Posted by Penthode

The biggest drawback of the CTC5 in my opinion is the low high voltage coupled with the relatively dim (compared to tubes a few years later) 21AXP22A.

Low anode voltage is a big problem with the 5 chassis, which exacerbates brightness issues with a CRT that isn't as efficient as later ones.

If I get the time, I would like to explore further the Horizontal Output/ Flyback to see if I can get a bit more HV.

Already have: I added drive and HV controls to my Wingate, nothing I did got any more than 22kv out of it and it was still saggy. Lack of current at the anode is just as big a problem as lack of voltage.


There are two things I have yet to try: installing a solid state 3A3 replacement, and feeding the horizontal section more B+ by using silicon rectifiers in the power supply. I would still run the rest of the chassis on the 5U4 rectifiers, only horizontal would get a boost. I've come to the conclusion that the flyback in the 5 has higher primary D.C. resistance, which means you can only sink so much current into it at a given B+ value. Can't change the flyback windings, so the only thing you can try is feeding it higher amplitude pulses. This may in fact lead to higher anode voltages, but I'm not sure what effect it would have on sweep width and so on.

Another possibility is subbing another flyback, but that's not an easy chore.