Eimac had a bulletin out describing the 4-1000A tetrode in grounded
grid, it states "Most tetrodes do not perform satisfactorily when
connected as a high-mu triodes. Control grid dissipation usually limits
their power handling capability when they are connected in this
manner. The 4-1000A is an exception to this rule if a plate voltage of
at least 3000 volts is used. " With a plate voltage above 3000 volts a
4-1000A is a legal limit plus amplifier in grounded grid. I can supply
a pdf of this bulletin should anyone desire.
It has long been known that 4-400 tetrodes operate satisfactorily in
grounded grid as well. They will plug-in replace 3-500Z finals in the
SB-220 and variant amplifiers when all of the grid pins on the sockets
are connected directly to ground with low inductance leads. This is how
the 3-500Z tube sockets should be wired in any event whether to use
4-400s as finals or not. The original SB-220 and other designs using
RF chokes and bypass caps on the grid leads proved to be a poor idea.
K0KP
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