On 8/31/2019 6:02 PM, Roger D Johnson wrote:
If you design a different class AB amplifier for half the output
of the previous amplifier don't you design it to produce the desired
output into a certain load impedance? Would it not be as efficient
as the first amplifier running at it's designed output?
So if you redesigned the first amplifier to see the proper load
impedance at half output would not the efficiency be regained?
In general, RF power gain stages (the tubes or transistors) are designed
to work at some designed power level and impedance. Pi and Pi-L networks
in tube amps provide both harmonic filtering and impedance matching. In
most solid state amps, the output network provides only filtering, and
are designed to work into 50 ohms.
K6XX, one of the Elecraft engineers who works on their power amps, gave
a talk to our contest club stressing the importance of providing a
matched load to the output devices, and to the output itself. He
observed distortion increases significantly when that condition is
satisfied, whether it is an antenna tuner for a solid state output stage
or the Pi or Pi-L in a tube amp.
Isn't this exactly the way some of the first ham linears were designed
for an input of 2000 watts in SSB and 1000 watts in CW? There
was a switch on the front panel for SSB/CW mode which dropped
the plate voltage for CW mode so that the tubes saw the same
load impedance they did in SSB mode thereby keeping the efficiency
high and not requiring retuning between modes.
I don't know, but you could ask K4XU, who designed the Ten Tec power
amps and later the RF circuitry in Alpha amps. He told me the 87A was
his design (although he made it clear that he did NOT work on the
control circuitry).
73, Jim K9YC
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