[AMRadio] Re:Modulator Plate Voltage

D. Chester k4kyv at charter.net
Tue Nov 13 05:09:27 EST 2007

Extra modulation headroom can be had two ways.  The modulation transformer 
turns ratio can be reduced (less step-down), or the plate voltage on the 
modulator can be raised, relative to the final.

Many transmitters run the same voltage to the modulator and final, and use a 
transformer with a turns ratio of about 1.6:1 to 1.7:1.  This just barely 
allows about 95% modulation before the waveform flat-tops.  Often this is 
done intenionally to prevent "overmodulation".  The problem is, the 
flat-topping caused by modulator saturation produces exactly the same kind 
of splatter and distortion as overmodulation, so this is a bad idea.

Since my voice is asymmetrical, I need the extra headroom to accomodate the 
positive modulation peaks without splatter and distortion.  But the 
modulation transformer is fixed  ratio, so I raise the modulator plate 
voltage well above the final amp plate voltage.  I adjust to just below 100% 
modulation in the negative direction, per the oscilloscope, and let the 
positive peaks go where they may, making sure they don't  flat-top.

This same principle has been used since the late 20's, to 100% modulate 
using a class-A single-ended Heising modulator.  Using the same plate 
voltage resulted in maximum modulation percentage of about 60%.  Often the 
PA plate voltage was reduced using a series power resistor by-passed with a 
HV oil capacitor.

Many, if not most hams have an exaggerated opinion of the efficiency of 
their transmitters.  The efficiency figures given in the tube data sheets 
are much like the EPA mileage estimates listed on the showroom sticker of a 
new car.  Yours is likely to be much lower.

The modulation transformer itself will probably be less than 90% efficient. 
Then you have losses in the plate tank circuit, the antenna tuner (if one is 
used), and the antenna feedline.  These losses all add up, so that most 
plate modulated amateur transmitters do well to get much more than about 50% 
efficiency, when  comparing DC input to the final to rf input to the 
radiating antenna.

Comparing rf input to the radiating antenna to the power drawn from the a.c. 
power mains, you are likely to have a dismal figure of about 20% efficiency 
(maybe 30% in exceptional cases) with a tube-type transmitter. That is one 
of the main reasons why the broadcast industry has converted over to 
high-efficiency solid state transmitters as rapidly as possible.

Don k4kyv 

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