[AMRadio] AM Usage with Linear AMPS

Donald Chester k4kyv at hotmail.com
Tue Nov 23 17:43:19 EST 2004

>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.


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