[Amps] 2x LDMOS design tweaks

[Jim Barber]

Since it's quiet out, here's my latest amp mod project... There's actually
a question in the last paragraph. :-)

Some time ago I bought a dual-package  MRFE6VP61K25HS LDMOS amp from a OM,
originally built by KF8OD. Each package has the 2 devices in parallel, and
the two packages are driven off the ends of a center-tapped 1:4
transformer, bias applied to the center tap. The outputs go to yet another
transformer and out the antenna.

It's worked ok - the T/R relay quit so I changed it, but efficiency has
been low on 160M. Once it heated up, a symmetrical spur came up, in-band
but some 30 kHz or so from the op freq. Running into my relatively high-Q
160M antenna would drive my LP100A meter nuts for obvious reasons, although
the SA showed the story.

A thermal image showed the bypass caps on the power supply side of the
bifilar drain choke were heating up, the T-200 powdered iron core to a
lesser degree. I pulled the bifilar choke off and wound (2) separate drain
chokes with 12 AWG enameled on Mix-31 Fair-Rite beads. (1/2" ID, an inch or
so long) I didn't know if the high DC current would ruin the choke
effectiveness, but I put them in to test. The new chokes run cool as a
cucumber, the bypass caps no longer heat up and the efficiency is up
substantially. (on 160 only) I don't know about the spur yet... Since I
replaced the chokes I haven't been on the key long enough yet to find out.
I may have time tonight to serenade the locals and see what happens.

The last bit here is that the builder put a negative feedback circuit on
both packages, 10 ohms 5W in series with .01 uF. I've seen data sheets from
NXP and Ampleon that show that kind of feedback, and I've also seen schemes
where the network was connected from the gate of each device (or package in
this case) and ground - "gain control". The eu104.ru modules use 51 ohms
and .01 uF from gate to ground. Does anyone have any info on the design of
that network?  I know preventing low-freq oscillations is important with
these devices, but I haven't seen much actual discussion on how to get
there.

Thanks,
Jim N7CXI