Hello LF'ers,
I am experimenting with two cheaply made Class D push pull 136kHz amps
using very cheap to buy (well under £2 each) FET's combined using a
home brew Wilkinson combiner as per this web page. I am using two very
big wire wound ceramic resistors of 50 Ohm each in series for "R"
which look very very similar to those Steve VE7SL is using in the
photo he publishes on this page.
http://ve7sl.blogspot.co.uk/2014/09/630m-wilkinson-power-combiner.html
I used the on line calculator at
http://leleivre.com/rf_wilkinson.html
to build the air cored coils that are bang on 81.82 uH at 137.55 kHz
and caps of 16.4nF (12nF with two 2.2nF in parallel)I used my AIM4170
to check the coil winding with it outputting 137.5 kHz.
What I am seeing is each amp giving a nice (well, they seem nice to
me...) drain and gate waveforms individually, but when combined both
waveforms deteriorate and the cold running FET's and output
transformers heat up quite dramatically. I need to rebuild the lashed
up combiner with much shorter lead lengths, (for example the caps made
from parallelled film caps are across the input and output sockets, and
not right by the coil formers, and it's just sat on the bench, not on
a ground plane). But are there any other things I need to look at if a
tidying it up doesn't fix this? The amps share a common PSU, but have
independent driver circuits.
W1VD has a different type of combiner, shown here
http://www.w1vd.com/137-500-500WCombiner.pdf
and I have the ferrites to try this type, but haven't built it yet,
what are the merits of each type please?
Thanks. 2E0ILY
--
Best regards,
Chris 2E0ILY
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