I had always thought that a 500 mmf doorknob capacitor was required to filter the high voltage when using a metal CRT. The inner and outer aquadag coatings with the glass envelope in between serves as a filter capacitor for all-glass CRTs.

I noticed that the doorknob capacitor is not used on color sets with 21AXP22 (metal) CRTs like the RCA CTC-5.

How come?

I had always thought that a 500 mmf doorknob capacitor was required to filter the high voltage when using a metal CRT. The inner and outer aquadag coatings with the glass envelope in between serves as a filter capacitor for all-glass CRTs.

I noticed that the doorknob capacitor is not used on color sets with 21AXP22 (metal) CRTs like the RCA CTC-5.

How come?

Part of your premise is wrong. The 21CT55 and CTC-4 both used doorknobs and the 21AXP22. Most 21AXP22 based sets also had a plastic insulator cone that completely enshrouded the HV charged bell...The inside was typically normal plastic the outside was typically plated in silver or dag, and that outer layer on the insulator cone was grounded...Logically forming a capacitor too...The 21CT55 and CTC-4 both had that AND a doorknob (belt and suspenders engineering!), but it would seem that by the time of the CTC-5 (and possibly earlier if you look at NON-RCA chassis) they decided that the insulator cone capacitor alone was sufficient...
It probably also was good for all glass retrofits/production changes anticipated.

Thanks, Tom. I have never seen a 21AX set in the flesh. I might have picked up on the insulator cone capacitor. It certainly would have been as good a the aquadag coating on a glass tube. I knew you had to have some sort of filter capacitor, even at 15 KHz.