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## [AMPS] MFJ-259 and matching circuits

 To: [AMPS] MFJ-259 and matching circuits jono@enteract.com (Jon Ogden) 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. 73, Jon KE9NA -------------------------------------------------------------------------- Jon Ogden jono@enteract.com www.qsl.net/ke9na "A life lived in fear is a life half lived." -- FAQ on WWW: http://www.contesting.com/ampfaq.html Submissions: amps@contesting.com Administrative requests: amps-REQUEST@contesting.com Problems: owner-amps@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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