----- Original Message -----
From: "D. Chester" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Tuesday, November 13, 2007 5:09 AM
Subject: Re: [AMRadio] Re:Modulator Plate Voltage
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.
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
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