> >Because we are interested in CURRENT through the blocking cap,
> >not current through the tube's capacitance, when we are selecting
> >a blocking cap.
>
> Is this strictly correct, since we can see that a major part of the
> current through the blocking cap at the higher frequencies is caused by
> the current through the plate - ground capacity?
Let's look at only 14pF total capacitance at the anode side of the
blocking cap, 3000 ohms load resistance at the anode end of the
tank, and 3000 volts RMS.
Doing it the incorrect way Rich suggested, blocking cap currents
on 1.0, 1.8, 7 and 28 MHz respectively are:
0.264, 0.476 a, 1.88 a, and 7.39 amperes.
Working the problem while considering the RF power transferred to
the tank (by simply adding the current responsible for output power
to his method) we get:
1.264 a, 1.476 a, 2.88 a, and 8.39 amperes.
The actual current would most often be a few percent higher than
the second numbers.
The one simple extra step is very much worthwhile. There are
cases where the error would be less, and cases where the error by
omitting the current that actually causes the output would produce
much more error.
Since we know all the necessary parameters to add this step, and
know those numbers with more certainty than we probably know
the actual stray impedance (capacitance), and since it would be
almost nothing to add it in, leaving it out is foolish.
Now the question I still have is what happens to current if the
anode system becomes inductive on lower frequencies, because of
the plate choke. I believe that causes a phase shift problem in
adding the current responsible for output to the circulating current
calculated through the blocking cap. If the choke is big enough,
that effect should be relatively minor.
But even if the choke can't be ignored, at least now we get
everything much closer with no additional effort. And we eliminate
the perpetual motion effect in Rich's suggestion.
73, Tom W8JI
w8ji@contesting.com
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