|[AMRadio] GG 813 Linear in GE Ham News|
w5omr at satx.rr.com
Thu Sep 29 10:40:49 EDT 2005
> Hi Geoff,
> Yes both statements are true. Remember that the PEP is 4 times the
> carrier power with a 100% modulated AM signal.
if it's modulated with a sine-wave, that's a true statement.
> A pair of 813s are good for about 600 watts PEP output. So that limits
> carrier power to 150 watts if 100% modulation is used.
I understand that, but in a grounded grid arrangement, as it's already
been said here, there's the exciter drive power that's added to the
signal. So, in order for there to be 100% modulation, you must modulate
the exciter, as well.
> The "efficiency at carrier" that I was referring to is the efficiency
> of the amplifier at the power level that the carrier runs when any
> amplifier is set up for AM operation. Like the 813s, 600 watts PEP out
> requires that the carrier be operated at 150 watts output.
Thereabouts. Again, if you're modulating it with a sine-wave, then it
holds true that 100% modulation is 4x the carrier output. However, we
don't speak in sinewaves, as John/WA5BXO, Don/K4KYV and
Bob(Bacon)/WA3WDR have so eloquently stated in
> If the amplifier has say 60% efficiency at the full PEP level then at
> 1/4 the power output the efficiency will be 1/2 of what it was at full
> power or 30% in this case. This is not only true for AM operation but
> for SSB operation as well.
> As long as the amplifier is to remain linear this efficiency ratio
> must exist. (see Orr's radio handbook. see "efficiency modulation")
> If that efficiency ratio is not maintained then the amplifier is not
> operating linearly. If you readjust the loading at carrier level to
> increase the power then it will no longer be linear when the
> modulation is applied.
Which, to me, would be another good reason why you shouldn't
plate-modulate a grounded-grid (aka 'linear') amplifier.
73 = Best Regards,
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