[AMRadio] GG 813 Linear in GE Ham News

Donald Chester k4kyv at hotmail.com
Thu Oct 6 22:25:10 EDT 2005

>I have an SB200 linear and on AM it is equivalent to a DX100.
>I increased the bias on it so to the point just beyond cutoff for AM so 
>it requires a little more to drive and it reaches negative 100% modulation
>slightly before the driver does.  But this seemed to help the modulation
>linearity and efficiency just a little.

That's an old trick that I first read about in a 1930's RADIO article.  
Overbias the AM linear to increase the negative peak modulation percentage.

It also can have a distortion advantage, especially if the modulator driver 
stage uses grid modulation.  Most grid modulated finals become nonlinear 
near 100% negative modulation, or are simply incapable of cutting the 
carrier off completely, so to maintain good modulation linearity, limit the 
modulation to 90% or less in the negative direction, then overbias the 
linear output stage until the negative peaks in the output reach near 100%.  
This will retain modulationr linearaty on negative peaks and reduce overall 
distortion levels.

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