[AMRadio] GG 813 Linear in GE Ham News

Gary Schafer garyschafer at comcast.net
Thu Sep 29 10:22:26 EDT 2005

W5OMR/Geoff wrote:
> Gary Schafer wrote:
>> A pair of 813s in GG are only good for 150 watts carrier out on AM.
>> 73
>> Gary  K4FMX
> I'm not trying to start anything here, Gary, but that statement seems to 
> be in stark contrast to what you said, yesterday...
>> With an AM linear the efficiency at carrier must be / will be exactly 
>> one half what it is at PEP. Provided it is tuned properly.
>> 73
>> Gary  K4FMX
> What would the efficiency be of an AM Linear at carrier.
> (or maybe I'm not reading this correctly, because I've yet to have 
> coffee this morning, and I -know- I probably should'n't be posting 
> messages in this state... ;-))
> -- 
> 73 = Best Regards,
> -Geoff/W5OMR

Hi Geoff,

Yes both statements are true. Remember that the PEP is 4 times the 
carrier power with a 100% modulated AM signal. 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.

You can squeeze a little more power out of the 813s but they start to 
turn a little red. I have had a pair of 813s in grounded grid that I 
built about 30 years ago. Tough tubes.

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.
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.

Gary  K4FMX

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