Hi Peter and all,
I had a chance to review some circuits this weekend.
I want to correct an error I made.
When I said gain was proportional to the square of HV change in a
grounded grid amp, that was incorrect except for the case of a tube
with zero driving impedance or if you consider drive power that
actually makes it to the grid remains constant.
What really happens is this, as anode current increases negative
feedback increases. That reduces the drive applied to the grid even
if exciter power remains constant.
That's because the GG amp has current feedback. That feedback
level is set by the ratio of anode to cathode impedance.
So the lower the cathode impedance compared to anode
impedance, the closer the amplifier becomes to following the
square of voltage change for gain change. A 3CX3000F1 has much
closer to a 1:1 change than a 3CPX5000, which almost follows the
square of anode voltage.
On the second issue of reading peaks, a scope is totally
unnecessary. Rise time of the envelope is limited by transmitter
bandwidth. With a 3 kHz bandwidth, it takes about .000166
seconds to reach the peak of the RF envelope, and that time again
for that peak to decay back to zero.
Almost any op-amp or even a simple transistor amplifier used with
a directional coupler could follow that peak and store it. It's 160
microseconds, and the storage cap only has to follow that up in a
sine curve of that length or LONGER time period. If we select a
charging time constant several times faster than 160 milliseconds
with a long discharge time, we have the problem totally whipped no
matter what the waveform.
I think you made a mountain out of a mole hill on that one, by not
considering the transmitter bandwidth restrictions and how they
affect the envelope.
I read medical equipment that uses a low microsecond duration
square wave envelope (that has over 200 kHz of bandwidth) on a
good peak storage meter, and it comes out exactly the same as a
scope. If a 200 kHz wide signal can be read accurately, so can a
narrower signal.
73, Tom W8JI
w8ji@contesting.com
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