Dear Warren, LF Group,

It's a long time ago now, but...
The multi-stranded winding was mainly used because it is much more flexible
and so easier to wind onto the core. I used a salvaged LOPT core as if it
were a toroid core, i.e. fixing the two core halves together before
threading the wire through the aperture repeatedly to produce the winding.
Producing a compact, even winding that lay snugly on the core using solid
wire was difficult because of the stiffness of the wire. It is desireable to
make the windings as compact as possible with the minimum amount of space
between the turns in order to reduce the leakage inductance of the
transformer as much as possible. This raises the self-resonant frequency of
the transformer and reduces the amount of RC damping needed to produce a
sensible drain waveform. The multiple strands do probably give some
reduction in losses at 137k, but I expect only marginally.

Cheers, Jim Moritz
73 de M0BMU