Thanks to those who followed up on my original questions on supercathode
drive.
1. I think I've solved my "plate voltage swing" dilemma...it's an iterative
process but it will work.
If you combine two equations that use "peak plate voltage swing" (terms are
those used in "Care and Feeding of Power Grid Tubes" by Eimac):
Output Power=(Peak Fundamental RF Current x Peak plate voltage swing)/2
Load Impedance=Peak plate voltage swing/peak fundamental RF current
You get
Output Power=Load Impedance*(peak fundamental RF current)^2/2
So at least I can parametrically vary output power and find the
corresponding load impedance, since I know the peak fundamental RF current
from the constant-current curves. Then with the resulting plate swing, go
back and find values for Igrid and Iscreen (the Iplate values won't change
much since mu-plate is so very very high in tetrodes/pentodes) and
calculate the grid and screen dissipations.
2. As always, finding some answers only brings up more questions: In the
configuration I'm trying to analyze, the control grid is tied to the
cathode, so Vgk=0 all the time. The interesting thing here is that if the
tube under analysis has very little grid current flowing at Vgk=0 (on the
constant current curves), the grid dissipation is quite low. Is this
correct? In the calculation for grid dissipation, is the "DC grid voltage"
actually the rms of the cathode driving voltage (remember the grid and
cathode are tied together)? Or is it zero (since Vgk=0 always)?
3. But how about the screen? In the supercathode connection the screen is
at DC ground. Since Pscreen=(Screen DC current)x(Screen DC voltage), is
screen dissipation zero? Or is the "effective screen DC voltage" the rms
value of the driving voltage on the cathode? The original supercathode
article mentions that the effective plate voltage is actually Eb plus the
rms value of the driving voltage on the cathode. Same thing here?
It would seem that if screen dissipation were zero, everyone would be doing
it this way.
Some have said that supercathode drive isn't very practical, and that may
be so, but I'd like to gain a better understanding of why it isn't. Also,
because I don't have any screen supplies available (although I suppose I
could just divide down the plate).
As always, thanks in advance for your indulgence and wisdom...
------------
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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