[Amps] Revisited: Power Combiners, cheap amps.

[W0UN -- John Brosnahan]

I had always thought that the phase of a tube amp would
vary all over the place as the tuning was changed.  But
with the ionospheric research radar transmitters that
I have built this is NOT the case.  On a 49 MHz, multi-stage
solid state driver to 300 watts, 3CX800A7 to 4 KW,
and a 3CX5000A7 to 50KW  (pulse), detuning the final
so much that the output voltage drops to less than 70% of
maximum only resulted in phase shifts of less than 5
degrees.  Two such multi-stage transmitters complete
with TR switching using lumped circuits resulted in less
than 2 degrees of phase difference when both were peaked
for maximum output.  Lots of LC circuits and a lot of hand
wiring without any great attention to detail still resulted in
almost identically phased outputs.  A large system using
a large number of smaller transmitters consisting of a 6CL6,
4CX250, 4CX250, 3CPX1500A7 had an active phase-tracking
system for each of the 64 transmitters.  In retrospect I wonder
if the phase tracking was really needed.

As a test of the viability of using two amps, I once combined
two Alpha 87As using two 90 degree hybrids made from coax
and doorknob caps.  This worked quite well but was a
single-band only solution.  It allowed each amp to run at
only 750 watts and reduced faulting by reducing the
reflected power on each amp since the forward power was
1/2 of normal--and the 87A protection circuitry works on
reflected power rather than VSWR.  Plus the apparent
mismatch was reduced by the 50 ohm load terminating the
fourth port.

It certainly wasn't a economically viable solution but it
was an interesting experiment.

73--John    W0UN


At 09:19 AM 7/11/2003 -0700, Michael Tope wrote:
>Brian,
>
>The trick would be matching the phase. With
>broadband solid-state amplifiers, this isn't too
>much of a problem. With narrow-band tube amps
>using hi-q tuning networks, it would be more
>challenging. I have never analyzed the problem to
>see exactly how sensitive the phase shift would
>be to PI-network tuning, but I suspect this is why
>this sort of combining is not done with tube
>amplifiers. Better to parallel devices before the
>pi-network to increase power capability (as in
>using two 3-500Zs in one amplifier vs combining
>the output of two single 3-500Z amplifiers).
>
>73 de Mike, W4EF..................
>
>From: "Brian Moran" <brianmo at yahoo.com>
>
>
> > I perused the ~19 meg archive for "power combiners" --
> > from my read, the suggestions were mixed for trying to
> > sum the outputs of two HF amps. Has anyone tried the
> > following?:
> >
> > Take two relatively-inexpensive amplifiers, say,
> > heathkit sb-220's. Make sure that they both work
> > reasonably well. With the hybrid combiner available
> > from CCI (make sure to use those resistors in case of
> > failure of one amp!), combine their outputs.
> >
>
>
>
>
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