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

To: TexasRF@aol.com, r@somis.org
Subject: Re: [Amps] Fwd: Pi-L In-circuit Adjustment Question
From: Bill Fuqua <wlfuqu00@uky.edu>
Date: Tue, 22 Feb 2005 16:40:53 -0500
List-post: <mailto:amps@contesting.com>
      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.
      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@aol.com wrote:
>In a message dated 2/21/2005 4:47:23 A.M. Central Standard Time,  r@somis.org
>writes:
>
>
>On Feb 20, 2005, at 6:35 PM, TexasRF@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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