[AMPS] Input impedance

[2]

>
>How does one determine / calculate the input impedance in a cathode driven
>GG circuit?   A pointer to a book / article would be great.
>
>THX, Bill

€  Murphy said that things are usually more complicated than they appear, 
and this is seemingly just such a case.  When the sinusoidal driving 
signal is swinging positive, the grid looks relitively negative with 
respect to the cathode, so the tube is cut-off.  With no current flowing 
in the cathode, the input R is virtually infinite.  However, when the 
sinusoidal driving signal swings negative, the grid looks relatively 
positive, so cathode current flows.  When the driving signal reaches its 
peak negative value, cathode current is maximal, and input R is minimal.  
Thus, input R is not a constant in a cathode-driven amplifier.   To 
smooth out the wild varations in cathode R, an electronic flywheel is 
needed.  This is usually a low-pass pi-network.  So how do tube 
manufacturers arrive at a figure for average input R?  Probably by 
adjusting C1, C2 and L until the match is optimal - and then calculating 
output R using the C & L values that were arrived at experimentally.  

cheers 

-  R. L. Measures, 805.386.3734, www.vcnet.com/measures.  
end


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