>>>Everybody claims this, but I haven't seen it. The input impedance does
>>>NOT vary with input power. I have the exact same match applying 100
>>>watts as I do 5. My SWR does not change.
>>You are talking about measuring swr with a tuned circuit/flywheel at the
>>cathode. Cathode impedance, this is not. You must measure direct into
>>the cathode? . .
>It doesn't make any difference.
I think it does make a difference if you connect a pi network to the
device you are trying to measure the Z of.
>The tuned input circuitry is supposed to match between 2 different
Not quite. If this was the whole story, a Q of one would work perfectly,
and it does not.
>50 Ohms on the exciter side and whatever the tube impedance
>is on the other. For a 4-1000A, that should be about 110 Ohms in
>parallel with 27.2 pf.
not so. . . During the max. neg. swing, the driving Z of a 4-1000A is
roughly 60 ohms. During the positive swing the driving Z is much Much
higher. Driving a cathode is like driving a diode. The harder you
drive it, the lower the R. This is not a simple Z or Y matter.
>If the impedance at the cathode changes as you say it does, then the
>tuned matching network will NOT be seeing a 110 Ohm // 27.2 pf impedance
>but will see something else.
true. . It sees a variable R component.
>Correspondingly then, the impedance on the
>exciter side of the network will NOT be 50 Ohms but will be something
With enough Q to smooth out the roller-coaster R changes, the exciter
will see a pretty good match.
>Therefore then, SWR will be introduced and one will get a reading
>on the SWR meter.
True, Jon, but small enough Not to cause throttle-back problems with
>The tuned circuitry will only look like 50 Ohms if the load on the other
>side is what it should be. If I could get a 50 Ohm impedance out of a
>tuned circuit regardless of what was on the other side, I would LOVE
I would love to get a 300mpg carburetor for my 1973 Dodge Maxivan.
>Would make life much simpler. Unfortunately it doesn't work that
>way. If the cathode impedance changes, so will everything else.
That is the case and it is the reason why we need a flywheel to smooth
out the wide R variation of the cathode. . . If the Q of a tuned input
is too low, there is not enough flywheel effect to do the job. Eimac
recommends a Q of 5 for the tuned input of a g-g amplifier, however, a Q
of 2 will do. Less than 2, usually causes an swr problem for the
exciter. . Unfortunately, some amplifiers have a tuned input Q as low
- later, Jon
R. L. Measures, 805-386-3734, AG6K
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