Here is the regulator circuit that was "built in" to the Zworkin 21CT55 that is "Plug in" to replace the ballast voltage divider and provide regulation. This was developed by David Neegard, the 88 year old RCA Engineer from Princeton that donated his Model 5, Zworkin 21CT55, and a CT-100 with a good CRT to the Vintage Radio and Communications Museum of CT. This particular circuit was made for the CT-100.
https://youtu.be/dN3AwlvOCBE
Enjoy!