[AMRadio] Re: Modulation transformer ID and repair

[Donald Chester]

>I limit my quick peaks in the mike preamp, and include negative
>cycle loading in all RF decks, but don't push things much.

I don't use negative cycle loading or ultramodulation, but I do take 
advantage of the natural asymmetry of the human voice.  Mine tends to run 
about 135% positive when I limit the negative to 100%.  The ratio would be 
more like 2:1 if I clipped off a few excessive negative peaks. I usually run 
about 640 watts DC input to the final.  Considering the losses in the PA 
tank circuit, the matching network at the transmitter output, the feedline 
between the shack and the tower site, and the remote antenna tuner at the 
base of the tower,  I'd be surprised if I'm getting as much as 400 watts 
carrier output to the antenna.  When I talk, the modulatior plate meters 
just barely wiggle beyond static plate current, unless I whistle or make a 
sustained tone.  With a sinewave signal from a generator I can make each 
modulator tube draw up to 400 ma!  The only "processing" I use is the 
Universal Audio/Urei Model BL-40 Modulimiter (the only solid-state device in 
the station except for the homebrew cw keyer).


>There are people on the band who run/can run excessive positive
>modulation, but you need a special detector I guess, because they sound
>bad in my shack when they do.

I just make sure there is no flat-topping on positive peaks or bright spots 
on the baseline in the negative direction.  I once tried ultramodulation, 
but I think I gained much more distortion than talk power.  Most reports 
indicated the audio was no louder, but had a scratchy sound.  The most 
effective thing I ever tried for "talk power" was high level symmetrical 
speech clipping back in the 60's, but it added audible distortion, and I 
went through about half a dozen modulation transformers in about as many 
months, ranging in ratings from 500 to 2000  watts.  After I abandoned the 
high level clipping, I havn't lost a single modulation transformer since.

Don K4KYV

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