Hi Stefan,
The C on the gate is not a good idea i think. Better try a 10...22 Ohm
resitor or a ferrite bead in front of the gate.
I removed the C on the gate and added a 15 ohm resistor in front of
the gate (in series with RF input to the gate). No change in
performance.
Is the bias voltage stable? It may be worth to spend 1000 uF parallel to
the Z diode. And a normal diode in front of the z diode, i.e. into the
yellow wire...
I didn't have any 1000 uF but I put 470 uF in parallel to the Z
diode, added a 1N4007 in series at the end of the yellow wire. No
change in performance.
At this point I became curious about the 1 uF C bypassing the +V
lead of the blue core to ground. It seemed strange to have it
bypassed with 1 uF there, then a series choke, then 2200 uF bypass.
I could not find any published circuits like that.
As a test (not having any idea what I was doing!) I removed the 1 uF
bypass. The amp went completely crazy.
So I decided to try some more C in parallel to the 1 uF. All I have
are electrolytic caps, but I tried 10 uF. Stable output increased
from 12W to 16W.
What about even more C? I removed the 10 uF and put 100 uF in
parallel to the 1 uF. No change from the 10 uF.
It still does very strange things if I try to adjust bias for more
than 16W.
I don't know what this experiment tells me...
The blue core, maybe it saturates at higher power? Could be worth to try
2 more turns and better use abt 0.5 mm wire for higher power.
I will try that next if I find some suitable wire here. I also
thought about trying a 10 turn bifilar winding instead of the
trifilar. Again I have no idea what I am doing, but the worst that
can happen is another dead FET. :) I have more.
But the design is not good for > 30 W or so. Don't expect to much from
an IRF530 at 13.8 V.
I would be happy if I could get 25W stable from this amp. I can get
25W but it is not stable at that level.
73,
Paul N1BUG
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