[Elecraft] KPA-1500 on digital modes

[Jim Brown]

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