[Amps] MLA-2500B Question

Thu Mar 31 01:47:14 EST 2005

An automatic tuner feeding a cathode driven amplifier
using a shunt capacitor across the input terminals will work.

But the capacitive susceptance shunting the input will have to be considerably
less than the amplifier input conductance, in order to have an energy storage large enough to 
prevent waveform distorsion on the fundamental frequency, and bypassing the harmonic 
components of the plate current to ground at their source.

When connected to the driver through a piece of coax,
the tuning network will "see" an impedance with a phase angle
that is dependent on the electrical length of the cable. 
As long as the network has a matching range large enough to tune out the reactances
and match the resistive parts everything will work in the "steady-state" domain.

Another aspect is that the real part of the input impedance of a cathode driven amplifier
is dependent on drive level. The "ordinary" automatic tuners are not designed to match into non-linear impedances,
so their tuning algorithms usually work on the assumption that the magnitude and phase of the load impedance is constant. 
A drive level dependent load impedance may present an impossible matching situation 
for a tuner and cause it to be non-convergent or "hunt".

73/

Karl-Arne
SM0AOM

----- Original Message ----- 
From: "Roger Parsons" <ve3zi at yahoo.com>
To: "Bill Fuqua" <wlfuqu00 at uky.edu>; <amps at contesting.com>
Sent: Thursday, March 31, 2005 12:19 AM
Subject: Re: [Amps] MLA-2500B Question

> I have to confess that I hadn't considered it either -
> you took that extra step! (I probably suggested it
> when I mistakenly wrote 'admittance' rather than
> 'conductance' but I was really thinking of the
> latter!) I expect it would work fine, and actually I
> am not sure that there would be any particular
> constraint upon the distance between tuner and
> amplifier, provided that cable losses were negligible.
> 
> The downside of this technique is that a common
> station impedance is not preserved, and to my mind
> that is the overwhelming reason for putting the
> matching network at the amplifier.
> 
> 73 Roger
> 
> 
> --- Bill Fuqua <wlfuqu00 at uky.edu> wrote:
> >        Using an automatic antenna tuner and shunt
> > capacitance to ground at 
> > the cathode is an interesting idea. I have not
> > considered  it. However, if 
> > the capacitive reactance is a fraction of the tubes
> > input impedance it 
> > should work. If it is 1/5 of the tube's cathode
> > input impedance the antenna 
> > tuner, not knowing the difference between an
> > amplifier and an antenna, 
> > should offer the appropriate impedance to the
> > amplifier to provide a 
> > conjugate match. That is the antenna tuners output
> > resistance will equal 
> > the input resistance of the amplifier and the
> > tuner's output reactance will 
> > be equal to that of the amplifier's input but
> > inductive. Together they 
> > become resonant with a Q of about 5. As long as the
> > inductance (tuner) is a 
> > short distance away ,less than 1/4 wavelength,
> > should make no difference.
> > 
> > 73
> > Bill wa4lav
> > 
> > 
> > At 03:17 PM 3/30/2005 +0100, Roger Parsons wrote:
> > >I think that Steve, G8GSQ, probably has the answer:
> > >
> > >I have now convinced myself that it is nothing to
> > do
> > >with the matching network per se, or with flywheel
> > >effects. Many grounded grid amplifiers like/need to
> > >have a capacitor directly across the input. My
> > belief
> > >is that this is sometimes (and only
> > >sometimes)necessary to ensure unconditional
> > stability
> > >over all of the driving cycle, and only marginally
> > to
> > >reduce harmonics.
> > >
> > >As Steve rightly points out, it is not possible to
> > >'remote' a capacitor over any length of
> > transmission
> > >line unless the reactance at the end of that
> > >transmission line is optimised for each particular
> > >frequency.
> > >
> > >The use of a pi network at the amplifier input
> > allows
> > >a capacitor to be placed directly at the input
> > whilst
> > >preserving (on average) a resistive input.
> > >
> > >This could be proved with a bit more effort than I
> > am
> > >prepared to make. A T network could provide exactly
> > >the same impedance transformation and Q as its
> > >equivalent pi counterpart. But it would not place a
> > >capacitor directly across the amplifier input, and
> > I
> > >wager it would not achieve the desired effect....
> > >
> > >73 Roger
> > >VE3ZI
> > >
> > >
> > >Send instant messages to your online friends
> > http://uk.messenger.yahoo.com
> > >_______________________________________________
> > >Amps mailing list
> > >Amps at contesting.com
> > >http://lists.contesting.com/mailman/listinfo/amps
> > 
> > 
> 
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