Philco 50-T702

[Eric H]

Going negative at a higher brightness setting is a classic symptom of a bad 7JP4, I've seen a lot of them like this.

If the HV was affecting it the picture would bloom and go out of focus, it looks fine other than being dim.

You said you plugged this tube into another set and it was bright? Did you actually display a picture on it or just plug it in and light it up?
I've noticed that simply lighting one up can be deceptive, it may appear okay but won't work well when an image is displayed.