I have been using an antenna tuner as an input circuit
for my HB 8877 lineart for years. When I was at EIMAC
I learned that when linear amplifier OEMs complained
of low efficiency it was often caused by too much
inductance in the input cathode drive cicuit. For
example, a T input circuit (LCL} was a disaster on 40
and 80. Similarly, if the lead from the input pi
network to the cathode is too long, a problem is
created on 10M. The reason is that the cathode current
is not sinusoidal so harmonic current must have an
easy path to ground. Unless there is sufficint
capacitance to ground, efficiency suffers.
To avoid this on my linear,I experimented with
a variable capacitor to determine the largest fixed
capacitor I could get away with and still be within
the range of my drake tuner on all bands The
capacitance turned out to be 50 pfd. It is connected
directly from cathode to ground with short leads.
W6TC
--- Tony King - W4ZT <amps080605@w4zt.com> wrote:
> Ian White, GM3SEK wrote:
> > Roger D Johnson wrote:
> >> Tony King - W4ZT wrote:
> >>> I don't know if any of you have looked into this
> particular product but
> >>> at first glance it appears that this is one
> solution for the tuned input
> >>> problem many of us face and at the right price
> too:
> >>>
> >>>
>
<http://www.ldgelectronics.com/manuals/AT-100AMP%20Manual.pdf>
> >>>
> >>> 73, Tony W4ZT
> >> The purpose of the tuned input is to provide a
> "flywheel" effect to
> >> smooth out the variations of amplifier input
> impedance over the
> >> operating cycle. The Q of the circuit determines
> how much "flywheel"
> >> effect we have. The lowest recommended value I've
> seen is 2. Solid
> >> state transceivers want even more...perhaps 3 to
> 5.
> >>
> >> The LDG tuner is an L network in which the Q is
> dependent on the ratio
> >> of impedances to be matched. For normal input
> impedances, the resulting
> >> Q will be very low. It's hard to write the
> formula in text format but
> >> in words it's as follows: divide the higher
> impedance by the lower,
> >> subtract 1 and then take the square root. For
> example to match an
> >> impedance of 100 ohms to 50 ohms, the resulting Q
> is only 1!
> >>
> >> With a 3 terminal network you can choose the Q
> independently of the
> >> matching and would seem to be the way to go.
> >>
> >
> > The recommendation for an input tuned circuit Q of
> 2-4 came from a 1961
> > article by Eimac authors in QST. That
> recommendation was specifically
> > based on a pi tank, and it was also pointed out
> that the output
> > capacitor provides a direct shunt path from
> cathode to ground for
> > circulating harmonic currents. But when an
> L-network is being used in
> > impedance step-down mode, it doesn't even have an
> output C.
> >
> > Now it may be that the designers know all about
> these things, have
> > checked their effects on amplifier IMD
> performance, and have found some
> > reason to ignore the Eimac recommendations. If
> that is the case, it
> > would be good to hear why.
> >
> >
> >
> > 73 from
> > Ian GM3SEK
>
> These are very good points that raise questions that
> must be answered
> before I would consider using such a tuner as the
> input circuit. I'm
> with you Ian, I'd like to see the answers to the
> questions.
>
> It's been said many times that the best input
> circuit is a pi-network
> which will provide the fly-wheel effect to help
> smooth the dynamic
> changes in input impedance. As Skipp025 said, on
> another list, "The
> automated antenna tuners might tend to hunt around
> looking for the best
> match." If that happens, we've just moved the
> problem from the
> transceiver to the amp.
>
> Bill, WA4LAV, has an interesting suggestion about
> shunting the cathode
> with a capacitor and I wonder what everyone thinks
> of that?
>
> 73, Tony W4ZT
>
>
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