[Elecraft] K1 PA output transformer

[Alan Ibbetson]

I want to increase the output power from my K1 station. I have an
ex-laptop 15V/4A supply to hand so a 3:1 turns ratio collector
transformer looks like it will offer a useful 20W RF output.

As a first step in this exercise I am trying to understand the design of
the K1's own single ended PA transformer. It is made with 5 bifilar
turns on an FT50-43 core so is optimised for around 6W output with a 12V
supply.

By calculation from the basic physics or cheating with an online
calculator such as

http://www.daycounter.com/Calculators/Max-Flux-Density-Calculator.phtml

I estimate the AC peak flux to be 87 Gauss at 3.5MHz on an FT50 size
core, which is well within the recommend 20% of Bsat (I see values for
Bsat in the range 2800 to 4000 Gauss for type 43 ferrite, probably
depending on how hard into saturation the material is pushed).

So far so good, but I lose the plot when I add in the effect of around
1A average current flowing in the 11uH collector winding from the DC
supply rail. Does this add a standing flux of 1650 Gauss, as
calculations based on pure DC without RF suggest? If so, the core is
operating way beyond the linear part of the B/H hysteresis loop and
ought to get very hot.

In fact the core runs cool so I suspect the effect of the pulsing DC
current bias is not the same as pure DC. Please can someone enlighten me
about what is really happening, and ideally provide something numerical
so I can get on with designing my own 20W PA transformer?

73,

Alan G3XAQ