> Example:
> Take a tetrode or pentode tube for instance in a cathode driven
> (grounded grid) circuit, and compare it to the same tetrode running in a
> grid driven configuration with it's screen well above ground potential.
Output capacitance stays about the same in any configuration. The
main exception occurs in circuits where the screen (and/or
suppressor/control grid) isn't grounded for RF.
In that case part of the output capacitance shifts into feedback to
the input.
If you look at amplifiers like the LA1000, that tie the control grid to
the cathode, the tubes are unstable on ten meters. That's because
the shielding between anode and cathode is reduced.
I'm not aware of any way around the output capacitance problem.
The best solution is to use an inductor between the tube and tank,
and that only works in PA's that don't require resistor loading of the
anode for stability (unless you use a series resonant suppressor
from anode to ground).
73, Tom W8JI
w8ji@contesting.com
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