Tom of course is correct. The 12 volt rail comes from the power supply. Sleep deprivation strikes again!
Cleaned up the HV cage today. Still arcing somewhere; I can hear it, but I can't actually find it. Bob G. has graciously offered to tap a new pot for me at a reasonable cost. I'll take him up on that offer. I may try it with a standard linear pot however and see if it makes any visual difference. I had suspected based on the changes from the 703 chassis that such a modification would cause only a slight change. Given Doug's simulation, I'm inclined to believe I probably won't notice the difference at all.