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

[R.Measures]

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