Gentlemen:
In this discussion, there are a few things to consider.
The varying input impedance in a grounded-grid PA over the drive cycle
manifests itself as distorsion of the driving current waveform (= harmonic
generation), and this can be taken care of by a matching network with a
suitable Q.
Another form of varying input impedance in this circuit are the variations with
drive level, which a lossless matching network cannot
level out.
Finally, much of the "strange behaviour" noted with untuned inputs (and
non-converging automatic tune cycles of built-in "line flatterners" in the
drivers) in a GG PA can be attributed to the fact that in this circuit, the
cathode input circuit and plate output circuit are effectively in series. The
plate current pulses (and all significant harmonics) need to find a
low-impedance path through the cathode network for the circuit to function
properly. If a significant length of interconnecting coax is used, the odds are
out that it will present a high-impedance circuit equivalent together with the
driver output at one or more places in the operating frequency range of the PA
and/or its harmonics.
Bruene et al discussed this at some length in the first edition of "Single
Sideband Principles and Circuits", and came to
the conclusion that the interconnecting coax length between a GG PA without
input networks and a typical pi-network output driver had to be less than 1/20
electrical wavelength on the operating frequency.
This equivalent series connection of the cathode and plate circuits is one of
the reasons why the design a of an unconditionally stable and convergent-tuning
servo-tuned GG PA is very difficult task indeed.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Tony King - W4ZT" <amps@w4zt.com>
To: <amps@contesting.com>
Sent: Wednesday, March 30, 2005 6:12 AM
Subject: Re: [Amps] MLA-2500B Question
> Roger Parsons wrote:
>
> >I will probably be sorry, but I am going to stick my
> >nose into this one:
> >
> >
> I thought I would be sorry but without getting emotional about the
> issues we have good technical discussions and I always learn from them.
>
> >1. It is certainly true that a Class B grounded grid
> >amplifier will have a varying input impedance over the
> >driving cycle.
> >
> >2. If a matching network is placed at the input, it
> >can be designed to transform that (instantaneous)
> >impedance into some other value.
> >
> >3. If the matching network is lossless, the
> >transformed value will vary in just the same way as
> >the drive impedance, but the values will have been
> >transformed.
> >
> >4. If a lossless length of coax is connected between
> >the amplifier input and the matching network, then the
> >impedance will still vary over the drive cycle, but
> >the values will have been transformed again.
> >
> >5. Therefore, with perfect components, there is no
> >difference in principle between the matching network
> >being at the amplifier or separated from it.
> >
> >6. Pefect components do not exist, so the matching
> >network/coax will introduce some loss. This loss will
> >depend on the network configuration and type/length of
> >coax. The variation in the drive impedance will
> >therefore be marginally less, but good quality real
> >components should not be dramatically different from
> >perfect ones.
> >
> >
> Here is a point I think I differ with you. It's not only about loss but
> characteristic impedance of coax when operated without the proper source
> and termination impedance. Things get real weird when we do that and,
> as other posters have said, we can see transformation effects that can't
> really be predicted given the variations at the cathode. And, as you
> said, we don't have any perfect components.
>
> >7. If a transceiver with an automatic ATU is used and
> >it hunts without an input matching network, it will
> >hunt with one, assuming the circuit losses are low. It
> >is also conceivable that it may not be able to match
> >the impedance seen at the end of a coax connected to
> >the amplfier input.
> >
> >
> It's been shown that that isn't true. I don't know the math nor the
> high level theory but I have "seen" it.
>
> >8. Although grounded grid amplifiers are often
> >considered to be unconditionally stable, that is not
> >always true (believe me, I know!), and a capacitor
> >across the input may help stability. This is
> >automatically achieved with a pi network and may be a
> >good reason to use one close to the tube.
> >
> >
> I agree. Stability is not automatic in any amplifier. There are too
> many so called grounded grid amps that have so much stray inductance in
> the grid circuit that I'm surprised they don't oscillate all the time.
>
> >9. The only way that I know to make the drive
> >impedance appear effectively constant over the driving
> >cycle is to swamp it with a dominating admittance.
> >This increases the drive power required, probably by a
> >significant amount.
> >
> >
> Others have suggested using resistance too. It obviously works in some
> form but I don't think that it does anything to improve the IMD of the
> amp. That's probably where I'm headed with the discussion. We can all
> get power out of an amp by driving it with "something" but when that
> signal is unleashed on the bands is it friendly to our neighbors up and
> down the band or is it one of those things that (I'm sure) some run that
> way to clear themselves a hole (listen on 75 meters any night or on all
> the bands on any contest weekend)? I suppose one might argue that it
> serves a purpose but I feel that we are obligated to make the signal as
> clean as we can.
>
> >10. If the feed impedance of the amplifier is
> >significantly different from the drive impedance of
> >the exciter, then a matching network can reduce the
> >driving power required by providing better matching.
> >
> >11. Any flywheel effect from a matching network at the
> >input to the amplifier will be no different from a
> >flywheel effect from a matching network some distance
> >from the amplifier. The commonly recommmended Q for
> >input matching pi networks is so low as to produce a
> >negligible flywheel effect.
> >
> >
> We differ in opinion on two counts. True the Q is lower than you'll
> find in the plate, but with Eimac's recommendation of an input Q of 4
> and by using a compromise Q of 2, things appear to work well for many
> people. As Rich says in his paper on the SB-220, that amp's original
> input circuit had a Q lower than 1 and simply didn't work. Raising the
> Q of that input circuit to 2 yielded enough Q to improve amp performance
> without destroying the SWR across the bands. Putting a length of coax
> between the cathode and the matching network goes back to the point in
> your #6. You end up with transformation and loss that at best hides the
> matching network from the cathode or at least disguises it as something
> else.
>
> >12. It is a lot more convenient if all impedances
> >within a station are kept constant. No retuning is
> >then necessary when the amplifier is/is not in
> >circuit.
> >
> >
> Amen! Will said the same thing to me and no greater truth has been
> spoken. I believe that if you can achieve that goal, you will also get
> a cleaner signal assuming you're not going to over drive the amp.
>
> >I can see no mechanism by which the IP performance of
> >the amplifier can be improved by the use of a matching
> >network physically at its input, rather than slightly
> >separated, always provided that the amplifier/system
> >is stable.
> >
> >
> Here the term "slightly" may have to be defined. Even Collins Radio
> separated them "slightly" inside some well known amps but not by several
> feet as is the jumper between an amp and most rigs driving them. I've
> read where people have reported problems with short coax jumpers between
> the cathode and tuned input of only one foot. This may take us to
> consider the capacitance of the coax too, I don't know.
>
> >Hard hat firmly in place.
> >
> >73 Roger
> >VE3ZI
> >
> >
> Thank you for expressing your opinions Roger! No hard hat needed :~)
>
> 73, Tony W4ZT
>
>
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