[AMPS] MFJ-259 and matching circuits

[Jon Ogden]

>€  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.  

Agreed.  However, even a diode has an equivalent "impedance."  Yes, it is one that varies with drive, but it is an impedance none the less and still has an "average" value over the drive cycle.
>
>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.   

That's correct.  You want the radio to see a good match into the amplifier.  So I think you are validating my point, but you might not know it.
>>
>>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 it is not resistive.  I even stated that.  However, one can simulate an average impedance for that device as an approximation.  I have spoken to others who have done this in addition to myself and they have confirmed that it works.
>
>>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.

Well, we are effectively solving the exact same problem.  You are matching 50 Ohms to the input impedance of the tube.  And since the cathode impedance varies over time and is not a "resistive" load as we've agreed upon, you still end up tuning to an "average" value.  The only difference is we start with different known and unknown variables.  In your case, the value of XC1 is known and Zcathode is unkown.  In my case, I assume a value of Zcathode (based on mfg's data sheet) and XC1 is unkown.  Both cases work out the same problem.  Yours might work a little better since you are actually driving the tube under real conditions.  I won't argue that a simulation is better than the real thing.  However, your way is convenient of C2 is adjustable.  In my case, my caps are all fixed and inconvenient to get to, so I gotta approximate.

I'll let everyone know my "real" results regardless of how they turn out.

73,

Jon
KE9NA



--------------------------------------------------------------------------
Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

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


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