[AMRadio] GG 813 Linear in GE Ham News

Jim Candela jcandela at prodigy.net
Fri Oct 7 08:27:38 EDT 2005


   I have heard the term, "class BC". Is this what you
are referring to?


--- Donald Chester <k4kyv at hotmail.com> wrote:

> >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 
> >that
> >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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