[Jeffhs]

I'm curious. How can you tell if the HV rectifier filament is glowing or not? These tubes almost always have 1- to 3-volt filaments (typically 1B3 in b&w TVs, 3A3 in color sets), so there is ordinarily no way to tell if the filament is glowing. (In other tubes with 6-volt filaments, for example, the filament will light with an orange glow if the tube is working as it should.) I would also think if there is any kind of visible glow in a tube with a 1-volt filament, there would be a problem somewhere in the filament supply and the tube, of course, would not operate nearly as well as it should, if it works at all. If the TV produces a bright picture with no distortion, the HV rectifier is working, whether or not the filament appears to be glowing.

Edit: I just read one of your other posts and read you had tried the tube in another set; as you stated, the tube worked well. This eliminates (or at least should eliminate) the tube itself as a possible cause of the trouble you describe. If the problem persists even after replacing the doorknob capacitor in the HV cage, it is almost certainly somewhere else in the HV supply, possibly the flyback. I don't remember that much about how flyback HV systems in televisions are supposed to operate (I had to give up my workshop over 20 years ago when I moved to an apartment, so haven't seen the innards of a TV set since then, except in photos here on VK) so this is about the best I can do.

I truly hate to tell you the flyback may be bad, but since you seem to have tried everything else, this is almost certainly the last thing that could be causing your problem.