[AMRadio] Re:Modulator Plate Voltage

jeremy-ca km1h at jeremy.mv.com
Tue Nov 13 08:06:11 EST 2007

----- Original Message ----- 
From: "D. Chester" <k4kyv at charter.net>
To: <amradio at mailman.qth.net>
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.


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