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How To Do AM?

Feb 25th, 21:40


Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
In experimenting with Displacement Modulation (see the 'General discussion about technology and policy' forum) I have been using two very different audio amps: a 10-watt Heathkit solid-state mono amp and one side of an old Knightkit tube-type stereo amp, 10 watts/channel. In using the Knight amp, I lost the magic of Displacement Modulation, but found that I was getting classic AM in its place.

How I'm getting AM is mysterious. My 30-watt amplifier is cathode biased with the control grid grounded. This ground lead is where I inserted the high-impedance secondary of the little audio transformer that lets me input the audio at loudspeaker level from the audio amp. The transformer is about the size of a golf ball, but acting on the grid (along with the RF input) lets me drive the tube all the way up to overmodulation. The received signal shows on my scope as a perfectly symmetrical AM wave (using an audio test tone source). To me, this method of producing a symmetrical AM wave makes no sense.

However, it works and I could use it for communication. But, in looking at the wave (and listening on an antennaless receiver in the shack), my next problem seems to be where to 'call it' in terms of biasing the RF tube. I can adjust the bias over a fairly wide range (more than 2:1) by turning a knob as I watch the cathode-to-plate current.

With the bias and tuning set for maximum power (as I would tune for CW and as revealed on my RF power meter) I have to greatly limit modulation to get down out of severe distortion; the scope shows flattening of the top portion of the signal. As I work the bias up (pulling tube current down), the overall signal weakens but I can modulate much more strongly before distortion comes in - so, even though the RF power is less, the detected audio is bolder and clearer. The scope shows a fully symmetrical, well-modulated wave, something that actually looks like the handbook pictures.

So my basic question is, what's the 'right' way to set this up? Should I tune for full power and then bias the current back down to where the RF amplitude is at 50 percent, and then modulate as much as I can get away with (i.e. never hearing distortion)? Or is there a more technically exact approach to balancing the RF power against the degree of modulation desired?

Larry K0WUQ

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