[AMPS] Cathode Impedance Measurements

[Jon Ogden]

> //  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!  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?

You seem to miss the point that tube manufacturers specify a driving 
impedance for their tubes.  What does this value mean and why do they do 
it?  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!

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?  

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!

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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