[Amps] Tuned Input

Gary Schafer garyschafer at comcast.net
Sat Jan 21 22:59:23 EST 2006


Could you elaborate a little more on this? 

Why is the efficiency less when a coil input is at the cathode?

Also wouldn't "too much coax" provide extra capacitance to ground?

I assume that the harmonic that you're talking about is second harmonic
energy? Why do we care if it is attenuated or not other than IM performance?

Thanks
Gary  K4FMX


> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of George badger
> Sent: Saturday, January 21, 2006 12:08 AM
> To: Tony King - W4ZT; AMPS
> Cc: skipp025 at yahoo.com
> Subject: Re: [Amps] Tuned Input
> 
> 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 at 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
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps at contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> 
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