AM Shunt Modulator

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PA0NVD:
Here a little more about this cathode modulation experiment

I did a few more tests, and indeed the distorsion is due to the modulation of the RF drive signal. If the cathode voltage rises, the grid current becomes less and the loading of the driver becomes less. That results in an INcrease of the RF at the grid while the output should DEcrease. When I lower the dive impedance with a low G1 resistor to ground, this effect is much less and the modulation distorsion disappears. Than it seems that the curve of the FET is quite nicely compensated with the modulation characteristic of  G1 plus a part series modulation plus a small part G2 modulation. (G2 is also referred to ground not to the cathode circuit).
But you need ti fiddel around a bit with the source resistor, the G1 resistor and the bias to get a very nice modulation. Seems worth the trouble, with a FET of approx 800 Volts and a few amps you can modulate kilowatts with a low FET dissipation.
WHO IS GONNA TRY THIS?
It results is a safe circuit, because, when the FET is pinched off or open, the cathode voltage is very limited. Both the drive and the tube are cutt-off. In my little circuit the cathode voltage will not rise more than approx 40 Volts with the cathode open.

WD5JKO:


I like that grid/cathode modulation circuit. It reminded me of an application on the net about how to get good audio from an el-36. I attach the link and sketch here.

http://home.alphalink.com.au/~cambie/EL36.htm

I have been wanting to try that on a KT-77...

Jim
Wd5JKO

DMOD:
Quote from: wy3d on June 14, 2018, 12:49:11 PM

Does anyone have any experience with using a Darlington pair in a circuit to modulate an AM transmitter without the use of a mod transformer.



Did you mean to say, "Series Modulation...?"

It seems you said Shunt Modulator then went on to describe a series modulator.


Phil - AC0OB

PA0NVD:
Hi Jim
Quite similar that EL36 circuit. , but I advice using a FET, not a transistor. These are more prone to shift in operation point and distorsion due to RF rectification.

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