In a message dated 2/27/06 12:42:21 PM Central Standard Time,
amps-request@contesting.com writes:
<< I see a lot of formulae(is that plural???) on determining the output
impedance of a tube.
some say just plain plate volts divided by plate current, some have a 1.8 x
formula and so on. I Want to determine
this gu-84b impedance so I can start work on the tank setup. I run 3.9kv and
I'm GUESSING 2 amp.
I stuck a 2k in there and played some with the values so far and things
look ok, no suprises. this is
my motive for the question. lets hear some opinions! thanks as always.
>>
Chuck,
There is a real good way to calculate tube parameters that will give the best
accuracy available on paper. That is, to use the Eimac Tube performance
Computer (TPC) clear plastic overlay, a set of constant current curves, and a
properly thought out operating line for the power output/performance desired.
The
Eimac TPC actually samples the instantaneous currents at 15 degree intervals.
What this does that the magic constants don't do, is it takes into account the
actual tube characteristics of the tube you are using. The result is a set of
more accurate parameters.
The "magic" constants are based on a piece-wise sinusoidal plate current flow
and an integration to calculate the probable DC and RF components of the
plate current. A really stinko mathematical approach _IF_ you are looking for
accuracy. The RCA Transmitting Tubes TT-5 has a nice write-up about how to use
these approximations (their K values) and they are, in fact, wonderful for
doing
quick approximations.
But if best-accuracy on paper is what you're after, then the Eimac TPC and
associated formulas is the only way to go, short of real world measurements on
a
working prototype. The Eimac TPC and instruction note is free for the asking
at Eimac. No serious amplifier designer should be without one. BTW the Eimac
TPC works for everyone's constant current curves. So you can use non-Eimac
tubes and data sheets so long as that data sheet includes constant current
curves.
You _cannot_ use the Eimac TPC with plate family characteristic curves.
Whether you use Eimac or "constants" depends entirely on the required
mathematical
model accuracy for the system you are analyzing. My choices are as follows:
For approximations, RCA K values and other "constants" is the way to go.
For circuit design and target operating parameters, it's the Eimac TPC, hands
down.
Both algorithms have their place and work wonders when properly applied.
Jeff Glass, BSEE CSRE
Chief Engineer
WNIU WNIJ
Northern Illinois University
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