Bill,
-
Some high-u tetrodes by function of tube geometry are
not best operated with the grid and screen tied together
Grounded grid 4-250A & 4-400A tetrodes should actually
be operated with proper division of excitation between the
grid and screen which should be the radio of the screen
to grid amplification. This value is about 5 for the 4-400A.
Previous Radio Handbooks cover the subject pretty well.
Proper ratio of grid/screen excitation may be achieved by
tapping the grid at some point on the input circuit.
Classic drop in Zero bias operation of the 4-400A into
a 3-400z/3-500z hole is not the best way to go. In
many examples the swap works... it's just not the
best option.
skipp
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