[Amps] Fwd: Pi-L In-circuit Adjustment Question

[Bill Fuqua]

I am not referring to input resistance but to the "Miller effect" or 
coupling from the plate to input circuit that in effect changes it's input 
reactance.

73
Bill wa4lav


At 07:25 AM 2/23/2005 -0800, R.Measures wrote:

>On Feb 22, 2005, at 1:40 PM, Bill Fuqua wrote:
>
> >      One minor glitch in the process is that plate current may drop
> > due to the impedance at the tube's input changing. This is noticed at
> > times with either grounded grid or neutralized grid driven amplifiers.
> > In fact one common test for neutralization in the older tube type
> > transmitters is to see that influence on the grid current is
> > symmetrical about the dip in the plate circuit.
>
>Class AB1 grid-driven amplifiers do not change input Z unless the grid
>terminator R is changed to a different value.  In fact, the input Z is
>the same whether the filament is lit or not.
>
> >      I guess what I am saying if the test does not turn out perfect it
> > is not due to the plate circuit being off resonance but the input
> > impedance changing. But other than that glitch, it seems as a
> > reasonable test.
> >    A the primary resonant frequency of a resonant system is the
> > frequency at which the stored oscillating energy divided by the
> > applied energy per cycle is maximized after the system has reached
> > equilibrium. In the case of most impedance matching networks it is
> > where  Pout/Pin of the network is maximized.  But there are cases were
> > there is no RF output from the resonant network and all of the RF
> > power goes into heat. Or in the case of an antenna most of Pin is
> > radiated as electromagnetic waves at the applied frequency and  a
> > little in heat (electromagnetic waves of much shorter wavelengths).
> >
> > 73
> > Bill wa4lav
> >
> >
> >
> >
> >
> > At 04:15 PM 2/22/2005 -0500, TexasRF at aol.com wrote:
> >> In a message dated 2/21/2005 4:47:23 A.M. Central Standard Time,
> >> r at somis.org
> >> writes:
> >>
> >>
> >> On Feb 20, 2005, at 6:35 PM, TexasRF at aol.com  wrote:
> >>
> >> >
> >> > Hi Rich, no, I said "C1 resonates the network" but  no matter, we
> >> both
> >> > know what the intent was.
> >>
> >> No capacitor in a  L-network or a Pi-network (double L-network)
> >> resonates the  network.
> >>
> >>
> >>
> >> Hi Rich, here is the plan for the bullet proof dip meter and test:
> >>
> >> The PA has an 8877 tube in it with a 1000 ma plate current meter
> >> installed
> >> and connected. We can use this meter to observe the resonance "dip".
> >> The Pi
> >> network is adjusted for maximum output power with 75 watts of drive
> >> power
> >> applied. We have to do this with a dummy load so any antenna related
> >> influence  in
> >> our test is eliminated.
> >>
> >> At resonance, the plate load impedance is all resistive, no shunt
> >> reactance.
> >> Off resonance in the higher frequency direction would entail the
> >> presence
> >> some shunt inductive reactance, which in parallel with the plate load
> >> resistance would cause the load impedance to be lowered. Off
> >> resonance in the  lower
> >> frequency direction would entail presence of some shunt capacitive
> >> reactance,
> >> also lowering the total load impedance.
> >>
> >> Since we know from Ohm's law that current equals voltage divided by
> >> resistance (or impedance in an ac circuit) we would expect the plate
> >> ma meter  to be
> >> minimum when the load (network) is at resonance and non minimum when
> >> the load
> >> (network) is off resonance.
> >>
> >> Now comes the dip check: Expecting the network to be non resonant, as
> >> the
> >> driver frequency is slowly changed, in the direction of expected
> >> resonant
> >> frequency, we would expect the plate current to slowly reduce until
> >> we reach the
> >> actual resonant frequency of the load (network). If we go the wrong
> >> way then
> >> the  plate current will rise. No problem, we just tune the driver
> >> frequency in
> >> the  other direction in this case.
> >>
> >> Once we find the frequency of minimum plate current (the dip), bingo!
> >> We
> >> have found the resonant frequency of the load (network). If it is
> >> different than
> >> the starting frequency then the idea that Pi networks do not operate
> >> at
> >> resonance will be proven. Also, no dip meters have been sacrificed in
> >> the  process
> >> and any question of what influence is caused by cover removal is
> >> avoided.
> >>
> >> Anyone out there willing to give this test procedure a try? I am at
> >> work
> >> right now and no access to a big PA to check this out.
> >>
> >> Standing by for test results!
> >>
> >> Thanks/73,
> >> Gerald K5GW
> >>
> >> _______________________________________________
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> >
> >
> >
>
>Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
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