Quote:
Originally Posted by bandersen
Ah, electrostatic. I hadn't thought about that. I guess that would account for the small spot?
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Or if the tube hasn't been used much, you should expect an incredibly small spot in the center.
My hunch right now is that the ions are scattering off some element of a particular gun type, and it's creating the characteristic X pattern shown in that manual. I'm not necessarily talking about 'static vs magnetic focus here, but rather some difference in physical construction. I've seen 10BP4s with very different looking guns that functioned identically. This gun type probably became more popular around the advent of the rectangular tube. Note that the page from the book doesn't exclude the possibility of a round tube developing the X pattern burn, it only says it's much more likely to be seen on a rectangular tube.
I'm not buying into a mis-adjusted ion trap hypothesis. A mis-adjusted trap would only allow the electron beam to overheat an element of a gun, perhaps burning a hole in it. I doubt very much that any trap magnet can produce the field strength necessary to redirect the ions toward the screen.