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[AMPS] MFJ-259 and matching circuits

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Subject: [AMPS] MFJ-259 and matching circuits
From: (Rich Measures)
Date: Wed, 3 Feb 1999 09:52:45 -0800

>>Would it not be possible to tune the circuit for min VSWR
>>using the exciters VSWR meter, as Rich suggested (real 
>>world operating conditions), then turn off the amp, terminate
>>its input port in 50 ohms and measure the impedance of the filter
>>looking back from the cathode with the MFJ-259B (cathode disconnected
>>of course)? This impedance should be approximately equal to the 
>>complex conjugate of the large signal cathode impedance. It 
>>would be interesting to see if this number comes close to Jon's
>>estimate of 110 ohms -j (2*pi*f*27pf)^-1.
>Theoretically, yes.  However, interfacing the MFJ-259 or any other 
>impedance measuring device to the cathode pins is a challenge and error 
>will be introduced there.  Rather, one could set the impedance as Rich 
>suggests, remove the tube and put the 110 Ohm//27 pF network in place and 
>check the impedance.  That is a little easier.
?   Agreed.  .  Perhaps it would be easier to measure the L and C of a 
Q=2, working-well driving the cathode, tuned input, and then scale the 
values for other frequencies?  However, since the Chebyshev filter in the 
radio and the length of the interconnecting coax are mitigating factors 
at the tuned inputs, my guess is that one would fare better by tweaking L 
and C2 as needed for each band. .   
-  later 
>If folks doubt me on the "average" bit, please define then how your AC 
>wall outlet can be "110 Volts" or "220 Volts."  Actually, these are RMS 
>values if I remember correctly, but my point is that AC voltages vary 
>wildly over their cycle as well, yet we call them by specific values.
?  However, in g-g amplifiers, cathode current Ceases when the input 
potential swings positive.  This is not the case with the electric-mains. 
-  later, Jon


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

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