modulation distortion elimination

[KB5MD]

If one has seperate power supplies for the modulator and rf deck and both have variacs for adjusting output voltage, would it be possible to keep playing with the voltages and virtually eliminate all the distortion in a transmitter that has a less than optimum match in the modulation transformer?  Just more or less thinking out loud, but is it possible?

[k4kyv]

I don't think shuffling the voltages would have much effect on modulator distortion, but you might be able to get a better impedance match by adjusting the final amplifier voltage and antenna  loading, to run the same power input at a different modulating impedance.

The ratio of modulator plate voltage to final amplifier plate voltage, and the turns ratio of modulation transformer primary to secondary is what determines the modulation capability of the transmitter if all else is functioning normally. I would adjust it so that the modulator is capable of delivering somewhat over 100% positive, to allow for some head-room in the audio and accommodate a degree of asymmetry in the voice waveform.

If the modulation transformer has too much step-down, you can boost the modulator plate voltage and/or reduce the PA plate voltage, and adjust the PA loading for  the desired plate current,  as long as you don't exceed the plate current or plate voltage ratings of either modulator or PA tubes.  OTOH, if the modulation transformer doesn't have enough step-down, the modulator tubes may run inefficiently and the audio driver may not drive them to full output, causing the modulator tubes to overheat.  In that  case, you can reduce the modulator plate voltage or increase the PA plate voltage, enough to allow the modulator tubes to approach saturation at the crest of the positive modulation peaks.

In any case, make sure to re-adjust the grid bias on the modulator and PA tubes whenever the plate voltage is significantly changed. Improper bias WILL cause the modulator stage to have more distortion. There is nothing magic about plate-to-plate load impedance of the modulator tubes, and you can run them over a wide range of load impedances so long as they remain biased properly and the maximum plate voltage and current ratings are not exceeded.

[steve_qix]

What is the modulation transformer ratio, power and current rating, and what are the tubes involved?

Let us know.

Regards,

Steve