>
>I have been a subscriber to the reflector for some time but this is my
>first post. I have a question for the experts.
>
>How does one determine the input impedance of a tube say a 3CX1200A7 for
>instance.
Cathode Z is not a constant. It fluctuates widely during the cycle.
When the cathode is driven positive, the tube cuts off and the cathode Z
is very high. At the peak in the neg. half of the drive cycle, the anode
current peaks and the cathode looks like approx. 20 ohms. Consult the
constant curves and calculate to arrive at a more accurate number. Look
for the plate/anode current curve that is 3x the rated current. When the
anode potential is c. 250v and/or the curve starts to bend, mark it and
read the values. However, the number you are probably looking for is the
average impedance, not the fullbore-on Z. . Avg Z is an uneasy integral
calculus problem. Free unsolicited advice: Use a Q of 2 (Cin = 25 ohms
of Xc), L and C2 adjustable, and tune for best swr. Measure the values
and replace the adj. components with fixed components. // For a
ready-made you might try Omega Tuned Input 919-266-7373
- later, Tom
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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