[AMRadio] GG 813 Linear in GE Ham News


Gary Schafer garyschafer at comcast.net
Thu Sep 29 11:00:23 EDT 2005



W5OMR/Geoff wrote:
> 
>> 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
> 

Geoff,

I am ONLY talking about a linear amplifier here. The drive signal is 
already modulated. If you are modulating the final stage these 
efficiency rules discussed here do not apply. Unless you would be 
talking about grid modulation and then they "do apply".

It does not matter what type of signal is modulating the signal when the 
linear amp is involved. Whether it is a sine wave or speech or anything 
else. The PEP rule still applies for 100% modulation! You can not exceed 
100% positive peak modulation if the carrier is set at any level greater 
than 1/4 the PEP output capability of the amplifier. Unless you want to 
operate it in a non linear mode.

73
Gary  K4FMX





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