[AMPS] Tube output C?

[Tom Rauch]

> 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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