[AMRadio] AM Usage with Linear AMPS

Gary Schafer garyschafer at comcast.net
Wed Nov 24 18:37:57 EST 2004

Donald Chester wrote:
>> It doesn't get any better either. With modulation the dissipation does 
>> not decrease even though the efficiency increases at the peak power 
>> levels. The carrier power is still there 100% of the time at 30% 
>> efficiency. The audio is in the form of separate side bands that is 
>> additional power that the tube has to handle.
> Actually, it should get better.  A properly operating amplifier with  
> low level modulation, linear or grid modualted, should run a steady 
> carrier output  level and a  constant DC input level, regardless of 
> modulation.  For the sake of discussion, let us assume 100 watts carrier 
> output.  The DC input will be about 300 watts @ roughly 30@ efficiency.  
> With no modulation present, we have 300 watts input with 100 watts 
> output.  That leaves 200 watts dissipated in the tube plates.  Now let's 
> modulate 100% with a sinewave tone.  The average rf power output will 
> now be 150 watts, 100 watts carrier power plus 50 watts average sideband 
> power.  The DC input is still 300 watts, so the tubes are now 
> dissipating only 150 watts as opposed to 200 watts with no modulation.  
> The overall efficiency of the amplifier rises from 30% to 50%.  If the 
> tube plates are showing a glow, they should actually dim a little with 
> modulation.
> Of course, the instantaneous efficiency is constantly varying over each 
> cycle of the envelope waveform,  from 0% at no output, to approximately 
> 60% at maximum peak output capability of the amplifier.  But it is 
> AVERAGE power that determines how red the tube plates get, how loud the 
> signal sounds over the air, and how much interference the signal 
> produces.  That is why the FCC's method of determining power output by 
> p.e.p. is bogus.
> Don K4KYV


You are exactly right. I stand corrected.
I remember that grid modulation are sometimes referred to as "variable 
efficiency modulation". The same principle applies to linear amplifiers 
with AM.

When reducing power out to 1/4 as required for linear amplification the 
efficiency must drop to 1/2 of what it was at full carrier. At 100% 
modulation where 1/2 again as much power is in the output due to the 
audio, the efficiency also increases by 1/2 again as it is with only 

Like you said, the additional output comes from the plate dissipation as 
the input power stays the same.

You rounded your numbers a little so the math does not come out exact in 
your example. But for those who wish to do their own:

If an amp has 60% efficiency at full carrier reducing the output to 1/4 
carrier will run it at 30% efficiency for the carrier. (exactly 1/2 the 
100% modulation will produce 50% more output and the efficiency will 
increase by 50% to 45%. (This is with a sine for modulation)

These efficiency ratios must be maintained for linear operation.

If the drive to the amp is not reduced to where the efficiency drops in 
half with the carrier, you will not be able to obtain 100% modulation 
without clipping in the amp.

This is why you don't want to re-peak the output tuning on the amp when 
you reduce the drive. You change the efficiency when you retune. The 
carrier efficiency must be maintained at 1/2 the efficiency of full 
carrier which is the same level that modulation peaks will hit at full 

Orr explains it very nicely in the Radio Handbook in the modulation section.

Gary  K4FMX

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