[Amps] Tuned Input

Gary Schafer garyschafer at comcast.net
Mon Jan 23 14:41:57 EST 2006 

Can you explain why we care that harmonics are shown a low impedance in the
cathode circuit?

How does this effect efficiency?

I read that Orr recommended around 13 pf capacitance per meter of frequency
for the shunt capacitance. That would be a reactance in the neighbor hood of
50 ohms from cathode to ground at the fundamental. 

73
Gary  K4FMX


> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Karl-Arne Markström
> Sent: Sunday, January 22, 2006 6:20 AM
> To: garyschafer at comcast.net; 'AMPS'
> Subject: Re: [Amps] Tuned Input
> 
> The reason why the type of input tuned circuit in a GG amplifier
> becomes critical is that the input and output circuits are effectively
> connected in series.
> 
> This means that the plate current pulses also flow through the cathode
> circuit,
> and if it presents an appreciable reactance for the harmonic content of
> these pulses,
> the tube will not have the proper RF operating point.
> 
> A shunt C at the cathode end of the input network is the easiest way to
> ensure
> the proper path for the harmonic currents around the network.
> 
> The use of a long coaxial line between the driver and the untuned input of
> a
> GG amplifier has other drawbacks.
> 
> For the fundamental frequency component, the line may appear as a shunt
> capacitance,
> but with increasing harmonic numbers the effects
> of the coaxial line electrical length and its termination
> may result in high equivalent series impedances in series with the RF
> plate current flow.
> 
> 73/
> 
> Karl-Arne
> SM0AOM
> 
> 
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: <gbadger at sbcglobal.net>; "'Tony King - W4ZT'" <amps080605 at w4zt.com>;
> "'AMPS'" <amps at contesting.com>
> Cc: <skipp025 at yahoo.com>
> Sent: Sunday, January 22, 2006 4:59 AM
> Subject: Re: [Amps] Tuned Input
> 
> 
> > 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
> > > >
> > > >
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