While tracking down the source of interference, I noticed that it made no difference when I moved the grounding loop away from the dag CRT coating.
I used some fine sandpaper on the wire loop and put a piece of aluminum foil between it and the dag. That seems to have fixed it
That fixed it
Also was fighting some horrible height and linearity issues. Turned out I had two resistors swapped. A 15K where a 1.8M should have been. Fixed that up and finally the height control actually does something useful now
IMG_20191227_002743390
Time to button the set up and call it done!