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
Don,
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
carrier.
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
efficiency)
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
modulation.
Orr explains it very nicely in the Radio Handbook in the modulation section.
73
Gary K4FMX
