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

[R.Measures]

On Feb 23, 2005, at 8:11 AM, Bill Fuqua wrote:

> 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.

Does neutralization affect Miller-effect?
>
> 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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>> >> Amps at contesting.com
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>> >
>> >
>> >
>>
>> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>>
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>
>
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org