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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