>I have searched the extent literature a bit and have not found the answer
>to the following question:
>What are the componenents that contribute to tube input impedence
>for a directly heated cathode, GG configured triode?
>
..... spacing between grid wires; diameter of grid wires; spacing
between the cathode and grid, and how much anode current is flowing at
the instant one makes the measurement. For instance, when the driving
potential is positive, no anode current flows and the cathode driving Z
is typically in the thousands of ohms range, however, when the driving
potential is negative, the anode draws current up to the emissive
capability of the cathode, and the typical high-Mu triode's driving Z is
in the few tens of ohms range. In other words, in a G-G configured
triode, input Z is a wild roller-coaster ride during each 360 deg. cycle.
This is why one needs a flywheel/tuned input to smooth things out in a
G-G amplifier.
- [note: medium Mu and low Mu triodes require much more cathode driving
potential (and watts) than do high Mu triodes]
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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