I have a supplement for the later CTC chassis (1960 on), which states that you must move all cathode wires to the same lug when using an FJP in place of a the CYP. I assume this must also apply to installing a later tube in place of the 21AX tube from a CTC-4, since the different phosphors require different drives to produce the same colors. Anyone have info on this? It's mainly a red thing, since the early reds were driven pretty hard because of their inefficiency.

Hi, Refering to the Sylvania pamphlet "Color Fidelity For Older Receivers", the procedure is given for replacing CY's and AX's with rare earth 21FBP22 tubes. The CY requires a change in the drive to the red grid to eliminate pink highlights, but the AX conversion has no circuit changes noted in the text. Haven't done a metal to glass swap yet, but have done a few CY to FB swaps. My CTC-7B has original drive circuitry, and looks good to me.
I wouldn't change anything in a 4 unless it doesn't look right after the swap.
Kevin

Yes, that's exactly the circuit mods I have in my RCA supplement. Something about changing the X demod circuitry to get rid of the highlights, but it doesn't mention anything about the earlier AXP tube. I would think it would require even more modification, because I believe it has more inefficient red than a CYP does.

I don't know why exactly the mods aren't indicated for the AX, but it may have something to do with the type of demodulator. The 4 used high level demod, while subsequent chassis used low level that required the use of -Y amplification, with the notable exception of the CTC-5 "Special" chassis. Real "special" lol. Maybe someone can provide a better explanation for the lack of circuit mods for the conversion from AX to FB.
Kevin

The circuit is very different from later RCA chassis, I'm sure that has a lot to do with it.

AX to FB should require the same mod as CY to XB - phosphor changes are essentially the same in either case: more efficient red and yellower green, they just may not have figured it out yet in the AX case.

AX to FB should require the same mod as CY to XB - phosphor changes are essentially the same in either case: more efficient red and yellower green, they just may not have figured it out yet in the AX case.

That makes sense to me, too, unless the blue and green guns can be adjusted to be driven more to compensate for the increased red level.
Don't have a FB to try in my 4, just a good CY that I'm saving for a 7A.

If it looks good don't mess with it. If it does not switch the red and green cathode leads. The bigger question is what are you going to do with that additional inch of neck hanging out the back?