[AMPS] Cathode Impedance Measurements

[Rich Measures]

>> //  The great 
>>puzzlement to me is why should one try to simulate the average cathode Z 
>>when one has a perfect model at hand -  a real cathode.  With a resistor 
>>substitute, one can have the Q wrong and there is no clue.  With a real 
>>cathode load, one can not take the wrong road.
>
>Because it can be VERY inconvenient to have to fire up the amp each time 
>you are trying different capacitor values and finding out you have the 
>wrong one!  

€  Use compression trimmers, a 5.5uH Collins Radio Co.  208T roller coil 
($10 from Fair Radio Sales), and you can not have the wrong one.  When 
the swr is delightsome, stop, measure the values, and substitute fixed 
ones.  

> It takes mucho more time.  And since a Q of 2 is defined by 
>making Xc1 = j25 Ohms at the input (Q=Zin/Xc1) how is matching to an 
>impedance at the tube going to make your Q be off?  

 

>There is plenty of Q. 
> If you pick the wrong value for C1 and get the wrong Q, it makes no 
>difference of what is presented on the other end of the network - a real 
>cathode or a simulated one.  Please explain how one cannot take the wrong 
>road with a real cathode?
>
€  As we have seen on [AMPS], some folks do not choose to use a Q of 2 on 
the first lap.   Also, sometimes, a pecuilar situation occurs on one band 
where the length of cable and the X of the Chebyshev filter combine to 
upset the Q apple cart, especially on 15m and 10m.  

>You seem to miss the point that tube manufacturers specify a driving 
>impedance for their tubes.  

€  I know this.  

>What does this value mean and why do they do it?  

€  It's a guide-line, however, it may not be the bottom-line in a 
paticular amplifier/radio combo.  

> Well, its meaning is as Dick so aptly put it.  Why is so that you 
>CAN know what impedance you need to match to for optimum power transfer.  
>You'll get the best results from your tuned input circuit if you match to 
>the driving impedance of the device.  So why not simulate it?  It makes 
>playing with the circuit so much easier!

€  For me, the easy thing to do is not to disconnect the cathode if I am 
trying to match the cathode to the radio's Z, which ain't always 
perzactly 50 +/- j0 ohms
>
>Of course, if you do it as you suggest by using a fixed value of C1, 
>variable L and variable C2, it is possible to do it your way.  But how do 
>you know what value of L or what range for C2 unless you know what 
>impedance you a matching to?  

€  The ^^^^Collins roller-coil^^^^    will cover 160m to 6m with an 8877. 
 I could not care less what the L is when I am looking for minimal SWR.  
I tune it, stop, measure the values it took to get there,  and sub. 'em.  
>
>Simulations are done in companies all across the world every day.  They 
>tell us a lot about real world conditions w/o the pain of experiencing 
>those conditions.  So why not use it?  You can play safely in the amp 
>compartment in a passive manner - w/o all the pesky voltages being turned 
>on!
>
€  The high V is usually on the top side of the chassis.  


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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