[AMPS] Supercathode drive...how to calculate?

[Scott Townley]

Ah, another strange question from that guy in Arizona...
The question is:  how do you calculate the operating conditions for driving
a tetrode in the supercathode drive circuit?
I have constant current curves, all the Eimac app notes, etc.
Allow me to tell you what I can do:
1. Starting point...cathode and control grid tied together, screen DC
ground, normal plate voltage.
--> I would assume then that since the cathode and grid are tied together
that the "grid voltage" on the constant-current curves is always
zero...since "grid voltage" really means "grid-to-cathode voltage"...right?
2. Apply sine wave of some amplitude A to the cathode.
--> Then the screen voltage actually is -A...since "screen voltage" is
really "screen-to-cathode voltage"...right again?
3. Find Ib, Ic, Isc for every 15 degrees of cathode drive (that's how the
Chaffee analysis works, at least) from the constant current curves.

Here's where I'm stuck.  Again, I believe Ec=0 always.  I can use the
3/2-power law on the curves to find Ib, Ic, Isc for the changing screen
voltage.  BUT WHAT'S THE PLATE VOLTAGE?  For some reason I can't figure it
out.  It isn't static.  But the impedance is non-constant (just like any
other tube analysis) so I can't just do ohm's law.  It would seem I have
too many variables...load impedance, instantaneous plate voltage...then I
can find the currents.
At first I thought that since the plate amplification factor is so high in
a tetrode (i.e., plate voltage has almost no effect on plate current) that
it didn't really matter what the instantaneous plate voltage was.  But then
the formula for power output is 1/2*(Peak fundamental RF current)*(Peak
Plate voltage swing) from the Chaffee analysis.  So I need to be able to
accurately calculate the plate swing after all...And then again, if you
want to do this to a triode, you definitely would NEED the instantaneous
plate voltages.

Unfortunately I have very few examples to work against to verify a
methodology...the "original" QST articles on Semi- and Super-cathode drive
had two worked examples (4CX300A and 811A), fortunately I have those curves
so I can get some verification.

I would greatly appreciate any tips, pointers, removal of blinders...any
references (of any level in any periodical)...I'd like to know what to
expect before lighting up.

Thanks for your indulgence!!
------------
Scott Townley		
nx7u@primenet.com
------------
Collector of:
	Stoddard Aircraft EMI/RFI receivers and accessories
	Big Parts for that Big Linear Amp 
	70's era RF test equipment HP/GR/Tek
	Radio-related technical reference material 1940+
	...anything else that will keep me off the streets at night

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