Guaranteed it's not the tube, at least that's my guess. I guess with the age though it could be, but I'd be more suspicious of something in the video or G2 circuit. Problem is, everything including the tubes is now discontinued for this set, so unless you find a donor 5020, you won't find any parts out there for it.
I know the units well, I worked on dozens in the late 80s, but haven't seen one for 20+ years now.