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[AMPS] MFJ-259 and matching circuits

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Subject: [AMPS] MFJ-259 and matching circuits
From: (Jon Ogden)
Date: Wed, 3 Feb 99 09:46:54 -0600
>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.

But you have your network theory correct.  Again, though we (you and I) 
are sort of applying a linear network analysis to a very non-linear 
system.  As others in the group have instructed me, a tube running in 
class AB does not have a constant input impedance.  Rather it varies over 
the drive cycle as the tube is cycled on and off.  But I will still argue 
an "average" value can be used to simulate a more linear system.

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.



Jon Ogden

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

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