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What is the input impedance of a 6sn7? [message #9596] Sun, 19 March 2006 19:33
 Tre'Messages: 34Registered: May 2009 Baron
I mean the grid to ground impedance not the grid resistor. I assume this impedance is in series with the grid resistance. I have always considered a tube grid to be an open, aside from the interelectrode capacitance.....Tube cad says no. What I'm trying to do is install a high pass filter between the output of my preamp and the input of the power amp.

Thanks...Tre'

Re: What is the input impedance of a 6sn7? [message #9598 is a reply to message #9596 ] Mon, 20 March 2006 07:20
 DamirMessages: 1005Registered: May 2009 Illuminati (2nd Degree)
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.

Re: What is the input impedance of a 6sn7? [message #9599 is a reply to message #9598 ] Mon, 20 March 2006 09:14
 Tre'Messages: 34Registered: May 2009 Baron
Thanks Damir. I was reading TubeCad wrong. It's redundant. It's states the F-3db point for a given circuit and then states the capacitance reactance in ohms as the Zin. Knowing the source inpedance and the F-3db point I can calculate the CR but the program gives it anyway. It just does not make it clear. Naz helped me with this.

Thanks....Tre'

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