Posted by Damir [ 213.202.115.63 ] on March 20, 2006 at 08:20:41:
In Reply to: What is the input impedance of a 6sn7? posted by Tre' on March 19, 2006 at 20:33:53:
In normal operation (grid negative to cathode), input impedance of the 6SN7 triode (g1-ground) is close to infinity, or at least, very high.
Then grid-leak resistor (say 220k from g1 to ground) is actually our input impedance (220k in parallel with >>220k = 220k).
We have the "problem" with input capacitance (Cgk+Cstrays+Cmiller), say about 100pF for 6SN7. The easiest way with filter stages is to observe triode input like, say, 220k in parallel with 100pF.
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