Sparton 16A211 Width Problem

[teevee]

I wonder if both 6BG6's are working.. You might try (very briefly) running the set with one, than the other. Possibly something goofy, bad solder or contact on one..
All the symptoms point to low power into the flyback, rather than an overload.
Weak damper, or poor boost filtering could be a problem.. May be worth scoping the boost supply. Also worth scoping the B+ that feeds the damper, although I'd expect other problems if the filtering here was poor..