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Why do most of the old 6B4G tubes have twin plate structures inside? I'll assume these things actually have two separate triodes internally wired in parallel. What is each half? A 45? Looking at the datasheets, the 2A3 has half the plate resistance and double the transconductance of the 45, so I'll assume this is the case. Why didn't they just design a bigger triode?

Then you have the newer Sovtek 6B4G, which do appear to have a single plate structure. Why'd they move away from the twin plate design? Does anybody know the history here? Maybe I should go poke around www.r-type.org and see if there is anything enlightening in their exhibits...

edit: http://www.r-type.org/exhib/aaa0146.htm