Mike
Yes, I have also seen newer designs are two stages of 4:1 cascaded to
achieve 16:1 total. 4X4RB Baruch uses this approach, also N4GA
www.n4ga.com
Greg
On Wed, Sep 11, 2019 at 8:38 AM Jim Barber <barberaudio@gmail.com> wrote:
> Hi Mike,
>
> Yep, a 9:1 it is.
>
> I most certainly will look at your latest output designs, thanks. The 9:1
> isn't bad, though... After 2 minutes key down @ 1200W output it only runs
> around 7-8C over ambient.
>
>
> 73,
> Jim N7CXI
>
>
> On Tue, Sep 10, 2019 at 10:41 AM Mike Mysliwiec <kf8od@aol.com> wrote:
>
> > Hi Jim,
> >
> > Glad to hear the old amp is still in operation. Is that a 9:1 on the
> > output? Much better designs have come along since I built that. Check out
> > my website for the latest output designs.
> > www.qsl.net/kf8od
> >
> > 73,
> > Mike Mysliwiec
> > KF8OD
> >
> >
> >
> >
> >
> > On Sep 9, 2019, at 7:28 PM, Jim Barber <barberaudio@gmail.com> wrote:
> >
> > 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
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> >
> >
> >
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