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[AMPS] MFJ-259 and matching circuits

 To: [AMPS] MFJ-259 and matching circuits measures@vc.net (Rich Measures) 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 Rich... R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures -- 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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