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[AMPS] Re: 4-1000A Impedance Variations

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Subject: [AMPS] Re: 4-1000A Impedance Variations
From: (Jon Ogden)
Date: Mon, 23 Mar 98 22:30:48 -0600
>>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.

The tuned input circuitry is supposed to match between 2 different 
impedances.  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.

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.  Correspondingly then, the impedance on the 
exciter side of the network will NOT be 50 Ohms but will be something 
else.  Therefore then, SWR will be introduced and one will get a reading 
on the SWR meter.

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 
that.  Would make life much simpler.  Unfortunately it doesn't work that 
way.  If the cathode impedance changes, so will everything else.



Jon Ogden

"A life lived in fear is a life half lived."

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