K9ACT wrote:What happened to the "quote" type reply option?
Anyway, I also forgot or didn't think about the peak voltage problem when I got into this. My original intent was to modulate a big tube with this approach. With tubes you add the audio to the carrier and get lots of bang for the buck.
To get the same power level using series modulation (if that is the correct term) one needs two to four times the plate voltage to take advantage of the high mod levels.
For 100% modulation I would have to run my carrier at half the normal level or double the plate voltage to maintain the same output level.
I think I finally got this right but if I missed something let me know.
Jack
Well, sort of

The voltages are the same under high level modulation (plate, drain, etc), regardless of the method one uses to get there. With a transformer coupled modulator, the modulator AC voltage is added to, or subtracted from, the DC voltage. So, under 100% modulation, you have 2x the DC. Under 200% modulation you have 3x the DC. For a tube, say an 813, operating at 2000 VDC at carrier, the peak voltage for 200% positive modulation would be 6000 volts.
It's the same voltage with series modulation. If you wanted your series modulator to be able to modulate your 813 running at 2000V to 200% positive, you would need a 6000V power supply. There would be 4000 volts across the modulator and 2000 across the final, at carrier. At full positive modulation, you'd get 6000 volts across the RF amplifier, and 0 volts drop across the modulator.
So, the voltages are the same, regardless. The difference is with series modulation, you need to have the highest voltage available all of the time, and the modulator "bucks" (or drops) the voltage. WIth the transformer, the voltages are added. The method the modulator uses to "drop" or "buck" the high voltage determines the efficiency of the modulator. WIth an analog series modulator, the efficiency is much lower than obtainable with a pulse width modulator.
Does this help or make it worse?
Regards,
Steve