[AMRadio] GG 813 Linear in GE Ham News


W5OMR/Geoff 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
http://w5omr.shacknet.nu:81/~wa5bxo/asyam/Amplitude%20Modulation.htm

> 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,
-Geoff/W5OMR





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