When good picture tubes go bad

[RetroHacker]

This sort of goes along with my other thread about brightening weak tubes, but I just had a few thoughts and questions in my head, and I thought I'd start a seperate thread. Also not really sure what forum this should go in, there isn't a 'general television' category.

What exactly wears out in a picture tube, anyway? I mean, from what I have read and heard, it's basically just that the filament coating wears away, and no longer emits electrons like it should. But the rest of the gun assembly - does that really fail? Do those wear and get weak too? Is that what causes weak tubes to focus poorly, or is that just a side effect of the lessened emissions? I'm still a bit fuzzy on what goes on _inside_ a tube - I know the theory, and I know how the circuits around the tubes work for the most part, but I'm not exactly sure how that can translate into a beam of modulated electrons that can then be magnetically deflected by the coils on the outside of the tube. I mean, what emits these electrons? Is it the filament? Or is the filament just a catalyst?

And, if the major cause of a weak tube is an ineffective filament, wouldn't it make sense to make a tube with an extra filament in there? You'd only need one extra pin on the tube base, have one common pin connected to both filaments and then two seperate ones. When one filament gets weak, use the other one - a simple wire swap as opposed to changing the tube entirely. Now I'm sure there's a reason something like this wouldn't work, orherwise it would have been tried already. But it's a thought.

Also, when a tube is rebuilt, what all is involved in the rebuilding process? I know that they have to cut the neck on the tube, replace the filament and the gun assembly and base, weld that to the tube envelope, then re-evacuate the tube and seal it. But does the gun assembly really wear out? Does it need to be replaced? Or would it theoretically be possible to get by with replacing the filament? If so, it could _almost_ be done at home, in a homemade workshop. I mean, other than a vacuum pump, and something that can melt glass, what all would be needed? I know I read something about someone that rebuilt tubes himself. But I don't really remember.

Ok. I'm rambling. Any ideas? answers? comments? should I just shut up and quit being annoying?

-Ian