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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: Tue, 2 Feb 1999 22:28:28 -0800

>>?  Assuming that a g-g input is like a resistor is bound to bring some, 
>OK for all of you who doubt it then.  What other assumption can one make?  
>The tube is spec'd for an "average" driving impedance of 110 Ohms with an 
>input capacitance of 27 pF.  Last time I looked, Ohms was a resistance 
>value (hence using a resistor) and pF was a capacitance value (hence using 
>a capacitor).  Sure there could be some inductance in there too.  However, 
>the resistors and cap had some lead length which add a little bit of 
>inductance as well.

>So......if you have all 3 passive sorts of components covered.....what 
>else could it be???
?  The cathode is like a diode that is driven with pos. and neg. 
potentials.  Heavy current flows on neg.  No current flows on pos.  A 
resistor this is not.  

>Now, I KNOW that in a GG circuit, the input impedance varies over the 
>drive cycle.  But you have to have some sort of impedance that you use to 
>create your input match.  A pi-net or an L-net or a T-net match one 
>impedance to another in addition to being like the venerable flywheel and 
>storing energy.  So one end is 50 Ohms, pray tell to all you out there 
>what the other end is?  Well, I used what the data sheets say is the 
>driving impedance which equals 110 Ohms.  So on paper, you use 110 Ohms in 
>parallel with 27 pF (which on any data sheet you find is spec'd as the 
>input capacitance).  You absorb the 27 pF into the value of C2 in your 
>Pi-net and then solve the equation for a pi matching network.  

?  110 ohms could be close.  In this case, the swr meter in the radio 
will work as well as the MFJ-259.   
>Now if what they taught me about matching networks in college is all 
>wrong, I would like to know.  

?  In college we learned how to match resistive loads.  A cathode is not. 

>I KNOW that there are some additional real 
>world effects inside the tube that I can't completely account for by 
>simulating it, but it should get one relatively close.  Otherwise what 
>good is working out networks by any form of calculation.  Should all of 
>our engineering be hit or miss?
?  Not at all.  Set the input pi network Q for 2.  (XC1= 25-ohms).  With 
full drive, adj. L and C2 for a match to the actual cathode.  
-  later, Jon


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

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