[AMRadio] Re: AMRadio Digest, Vol 46, Issue 23

D. Chester k4kyv at charter.net
Tue Nov 13 14:57:39 EST 2007

> What do you mean by well above? If the RF deck is at 2250 where would you
> want the modulator to be?
> Im in the process of building a pair of 813's with 810's audio and have 
> one
> big single PS that can get up to 3KVC@ 1A. How much can 810's  be pushed 
> in
> voltage or should I go to 572B's or even 250T's?
> Since Ive been unable to find a big mod xfmr so far the audio section is
> still up in the air and flexible.
> Carl
> KM1H

It depends on your modulation transformer turns ratio and the ratings of the 
modulator tubes. I wouldn't go much past the maximum CCS rating of the tube. 
Tubes are too rare and expensive any more to even run them at the 
manufacturer's ICAS ratings.  I run my 810's at 2600 volts, which is about 
the maximum CCS voltage rating in class-B audio service, IIRC.  In plate 
modulated rf amplifier service, the maximum recommended voltage is 1600 
volts, but that means at 100% positive modulation the plates are really 
seeing 3200 volts, plus in PA service, the duty cycle is 100%. The static 
plate current for each tube in my transmitter runs at 35 ma, and under voice 
peaks, each tube rarely kicks up over 150 ma or so.

With a normal mod xfmr turns ratio of about 1.5 to 1, I would say run the 
modulator plate voltage around 25-50% above the final amp plate voltage, if 
that fits within the ratings of the tubes.  There is nothing magical about 
the p-p load impedance or the plate voltage or plate current of a tube, as 
long as you stay within the manufacturer's recommended maximum values of 
current, voltage and plate dissipation.

With a  lower turns ratio, say 1.2:1 or 1:1 (the famous and abundant RCA 
surplus mod xfmr), there would be little need to run the modulators above PA 
plate voltage.  But you might want to add a couple of extra tubes and  run 
the modulators in push-pull parallel, since a single pair might not work 
very well at that  low a p-p load impedance.

Don k4kyv 

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