Modifying a bipotential lens Portacolor chassis to work with a unipotential lens CRT

[LukeSimon]

The early Portacolor chassis revisions used a "low voltage focus" unipotential lens CRT, also known as an Einzel lens. The specific unipotential CRT models used were the 11SP22 and the 11WP22, with the 11WP22 being a finer dot pitch variation of the 11SP22... but otherwise the CRTs are identical. This was common for budget-class color televisions back in the 1960s and 1970s. It allowed for cost cutting, but the picture was softer than a bipotential or "high voltage focus" CRT. The later chassis revisions of the Portacolor used bipotential lens CRT models 10VABP22 and 10VADP22. These two CRTs were introduced after the regulations around viewable screen size versus total CRT face size. So all 4 CRTs are the same size.

I have an HE chassis Portacolor, which was designed for the bipotential lens CRTs (10VABP22 and 10VADP22), but I do not have either of those CRT models. I do have a spare 11WP22 CRT that tests good using a BK490b CRT tester. I'd like to make a functioning TV using these parts, and I wanted to share my ideas here, to get feedback from the community, on how I plan to modify the bipotential HE chassis to downgrade it to work with the 11WP22 CRT that was made for the unipotential HC chassis.

Looking at the Photofacts for the HC chassis revision, which was designed for a unipotential lens and comparing it to the later HE chassis revision, it is very clear that GE kept the same chassis design but made a very small number of localized changes to the chassis to make it work with a bipotential lens CRT.

Both chassis revisions supply the same voltages to all CRT electrodes (HC chassis electrodes, HE chassis electrodes), except the heater (13.8v for HC chassis vs 6.3v for HE chassis), the focus anode (270v for HC chassis vs 4000v for the HE chassis), and the ultor anode (16kv for HC chassis vs 16.5kv for HE chassis).

The ultor anode voltage difference is negligible and well within the specs for the picture tubes according to GE's Essential Characteristics book. So I will just leave that unmodified.

I plan to modify the HE chassis to tie the focus anode to the 270v B+ supply, just like the original HC chassis, and to address the heater filament voltage difference, it looks like all I need to do is swap out the filament transformer to one that matches the specs from the HC chassis. Here are the two schematics for the series string, with V12 being the CRT in the HC chassis, and V14 being the CRT in the HE chassis.

Finally, I believe that I can implement these modifications in a way that is completely reversible, so this vintage TV can be eventually restored to OEM stock configuration, once I procure a 10VABP22 or 10VADP22 CRT.

If anybody has any experience or knows of any examples of this kind of thing being done, please share pointers, feedback, and recommendations before I jump in and potentially do something stupid.