The AM Forum

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.

Probably not what you're looking for but here's a schematic for a 75M exciter I once built.  Not a lot of RF power but unmeasurably low audio distortion..

look to the thread "cathode modulation" You can modulate kilowatts with normal power FET's. It is NOT a series modulation but an efficiency modulation.

Very interesting on your transmitter.  Thanks! Thats pretty much what I am doing here on the bench.  I'm curious about the LM358.  Are you just using it as an audio driver?  I see there appears to be a feedback circuit from the emitter of the Darlington back to the inverted input of the IC.  What is the purpose of that?   I feed the base of the darlington directly with DC from a voltage dividing circuit to set the carrier level.  I see you are feeding the LM358.

I'll also check the posts on cathode modulation

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

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

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

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

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.

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.

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

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

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


Phil - AC0OB

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.