As I understand it, the filament structure is the same as the 3-500Z. Not sure
about filament to grid spacing, but I have had some go grid to filament short
when hot, and that's with them mounted vertically. Depending how many tubes you
have, it could be worth trying. Rich Measures may have some historical
knowledge of 3-500 failure modes which could be applicable. I think personally,
I'd avoid it, but that's partially because I don't have a lot of spares!
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