7 Nov
2007
7 Nov
'07
5:39 p.m.
Many older handbooks show basing diagrams with "F
F" particularly if
it's a directly-heated cathode.
At least one databook - the one I use most, which might be where I got
the distinction - uses F for directly-heated cathodes ("filaments") and
H for the heating element for indirectly-heated cathodes ("heater").
Not that I use any vacuum-tube databook all that often these days. :-/
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