What is the theory behind low voltage focus CRTs?

[Electronic M]

You may be trying to put lipstick on a pig going to those lengths for a portacolor...the first gen portacolor sets had a phosphor dot pitch too coarse for good NTSC.
The first gen portacolor CRTs instead of doing the right thing and making a new shadow mask with enough holes for the right number of phosphor triads to achieve full NTSC resolution, GE got lazy and simply cut the center 10" diagonal square out of a 21FBP22 roundy shadow mask and called it a day with something like 1/4 of the the triads it should have had...

Later with the IIRC 11WP22 in the second chassis generation portacolor they actually bothered to make a new shadow mask with a finer more appropriate dot pitch (I'd like to find one of those one day).