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[AMPS] Re: 4-1000A Impedance Variations

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Subject: [AMPS] Re: 4-1000A Impedance Variations
From: (Rich Measures)
Date: Tue, 24 Mar 98 06:57:33 -0800
>>>Everybody claims this, but I haven't seen it.  The input impedance does 
>>>NOT vary with input power.  I have the exact same match applying 100 
>>>watts as I do 5.  My SWR does not change.  
>>You are talking about measuring swr with a tuned circuit/flywheel at the 
>>cathode.  Cathode impedance, this is not.  You must measure direct into 
>>the cathode? .  .  
>It doesn't make any difference.
I think it does make a difference if you connect a pi network to the 
device you are trying to measure the Z of.  

>The tuned input circuitry is supposed to match between 2 different 

Not quite.  If this was the whole story, a Q of one would work perfectly, 
and it does not.  

>50 Ohms on the exciter side and whatever the tube impedance 
>is on the other.  For a 4-1000A, that should be about 110 Ohms in 
>parallel with 27.2 pf.

not so. . . During the max. neg. swing, the driving Z of a 4-1000A is 
roughly 60 ohms.  During the positive swing the driving Z is much Much 
higher.  Driving a cathode is like driving a diode.   The harder you 
drive it, the lower the R.  This is not a simple Z or Y matter. 
>If the impedance at the cathode changes as you say it does, then the 
>tuned matching network will NOT be seeing a 110 Ohm // 27.2 pf impedance 
>but will see something else.  

true.  . It sees a variable R component.

>Correspondingly then, the impedance on the 
>exciter side of the network will NOT be 50 Ohms but will be something 

With enough Q to smooth out the roller-coaster R changes, the exciter 
will see a pretty good match.   

>Therefore then, SWR will be introduced and one will get a reading 
>on the SWR meter.

True, Jon, but small enough Not to cause throttle-back problems with 
typical exciter.
>The tuned circuitry will only look like 50 Ohms if the load on the other 
>side is what it should be.  If I could get a 50 Ohm impedance out of a 
>tuned circuit regardless of what was on the other side, I would LOVE 

I would love to get a 300mpg carburetor for my 1973 Dodge Maxivan.

>Would make life much simpler.  Unfortunately it doesn't work that 
>way.  If the cathode impedance changes, so will everything else.
That is the case and it is the reason why we need a flywheel to smooth 
out the wide R variation of the cathode. .  .   If the Q of a tuned input 
is too low, there is not enough flywheel effect to do the job.  Eimac 
recommends a Q of 5 for the tuned input of a g-g amplifier, however, a Q 
of 2 will do.  Less than 2, usually causes an swr problem for the 
exciter.  .  Unfortunately, some amplifiers have a tuned input Q as low 
as 0.75.  
-  later, Jon


R. L. Measures, 805-386-3734, AG6K   

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