Modulating Impedance

[W4RFM]

I am back to stir the ...uh pot, with another modulating question.
AM broadcast transmitters have one fixed modulation transformer and mod reactor.  However in many cases  these transmitters operate at two power levels. For example, the station I worked at the longest went from 5KW day to 500 watts at night. Drastic changes on E and I. Yet the audio still sounded fine.  I never questioned it because it worked. Any comments.

[AB2EZ]

If you lower the plate B+ by a factor of N (e.g. N=2 or N=3) without making any other changes, the plate current will also go down by the same factor of N, and the modulation resistance (V/I) will remain the same. The RF output power will be reduced by a factor of N x N. This is, of course, exactly how plate modulation is supposed to work. It is the same principle that one employs to reduce the output (at carrier) of a Ranger from 40 watts to 10 watts by reducing the unmodulated B+ from 650V to 325V.

One could implement this in a high power transmitter by using a plate B+ supply power transformer with multiple secondary taps. That is what I did to reduce the output power of my KW-1 from 600 watts to 375 watts. The plate transformer in that particular transmitter is not the original transformer. It has a pair of secondary taps that produces about 80% of the full end-to-end output voltage. By removing two screws, and moving two wires, I was able to reduce the B+ from 2500V to 2000V. I also loaded up the KW-1 to 80% of the normal current, at carrier (320mA instead of 400 ma). Thus, I kept the ratio of B+ to average plate current the same. This kept the modulation impedance the same, but lowered the output power from approximately 600W to approximately 600W x 0.8 x 0.8 = 384 watts. In the case of the KW-1... since the same B+ supply is used with the modulator (a pair of 810s), I had to adjust the negative grid bias on the modulator tubes to set the proper operating point for the modulator.

Stu

[KM1H]

Most all the early amps in the 1000W CW DC INPUT and 2000W PEP SSB Input switched transfortamer taps that maintained the correct impedance.

Heath SB-220, National NCL-2000, etc, did it at 1800 and 2500V. Modulation transformers are no different.

The Bauer 707 does it at 1000/250W; I had to service one yesterday.

[W4RFM]

Well I did the math, for the 1KW example 4100 vdc x .400 mA gives an impedance of 10,250 ohms, 250w would be about 2000 vdc x .200mA for an impedance of 10K. Dang! Thats neat.

[kb3ouk]

Well I did the math, for the 1KW example 4100 vdc x .400 mA gives an impedance of 10,250 ohms, 250w would be about 2000 vdc x .200mA for an impedance of 10K. Dang! Thats neat.

That is also how the peak power for AM is 4 times the carrier. If you're running a tube at 2000v 200ma, when you modulate to 100%, the voltage will be double the supply voltage. Since the modulating impedance stays the same, the current also doubles, making 4 times the power that you would at carrier.