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## [AMPS] MFJ-259 and matching circuits

 To: [AMPS] MFJ-259 and matching circuits W4EF@pacbell.net (Michael Tope) Fri, 5 Feb 1999 07:38:09 -0000
 ```Hi Jon, The non-linear component of the tube's input impedance represents energy at harmonics of the drive signals fundamental or a DC term. If you consider that the resulting voltage and current waveforms present in the non-linear circuit can be decomposed into a linear superposition of sine waves (fourier series), then determining the "average impedance" is just a matter of doing a linear analysis on the resulting first order terms. The higher order terms shouldn't matter since these are represenative of energy which is no longer at the fundamental. Unless a significant amount of this harmonic energy appears across the source impedance, then you can ignore it in terms of its contribution to VSWR. If the harmonic energy present at the source impedance is significant compared to the first order component, then the reflected power component of the VSWR reading could be in error somewhat, depending on how you look at it. An interesting test would be to plot the input VSWR of the amp as a function of drive level. If the departure from linear behavior is as significant as everyone seems to be claiming, then you would expect to see a change in VSWR as a function of drive level. This is all classroom B.S. of course. The proof is in the pudding, and you guys are the ones out there cutting metal. My first amp is still a pipe dream pile of parts in the garage waiting for some attention. 73 de Mike, W4EF............. ---------- From: Jon Ogden[SMTP:jono@enteract.com] Sent: Wednesday, February 03, 1999 3:46 PM To: Michael Tope; 'AMPS' Subject: RE: [AMPS] MFJ-259 and matching circuits >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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