AM Shunt Modulator

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KQ6F:
The feedback is important.  It compensates for non-linearity in the Darlington.  It's the reason for the very low audio distortion.

wy3d:
I'll try adding a LM358 to my circuit and feed the carrier voltage to the input like in your schematic.

KQ6F:
Note the circuit has voltage gain = 15.5  (1 + 68/4.7).

DMOD:
Here are two shunt modulators for modulating the screen grids of say 6146's, but shunt modulators are power hogs.

Better to go with direct screen grid modulation per the second PdF file.


Phil - AC0OB

PA0NVD:
Here the diagram and the results of cathode modulation. Really quite linear
 This is the original text of the thread
It should be possible to modulate kilowatts with a normal FET

I did try a combination of series modulation in the cathode and grid modulation in order to avoid high DC at the series modulator. The grid part is done by connecting the grid resistor to ground, not to the cathode circuit. The trick to get low distrorsion is partly in the grid drive impedance. I drive at approx 5 kOhm and 35 Vtt for an EL 84, Not too far from normal except the 5 kOhm. A higher impedance / higher drive will result in some compression of the positive modulation. I did not investigate yet why, but I assume the AF at the grid will start to modulate as well due to the grid current.
The results are very promising, and I am very positive in my assumption that it may be done as well at much higher power levels. I have 2 Watts carrier and 8 Watts PEP at 350 VDC at the anode.
The voltages at the modulator are very low, the drain has a voltage of approx 25 Vtt, That will not hurt me too much... Cheesy.

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