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In Reply to: RE: cloning the Fisher posted by Eli Duttman on January 25, 2015 at 13:05:34
Assuming a low-impedance source, the Miller effect at the input is governed by (Rsource + 10k), not 470k, in this circuit.
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Back in the day, they used high I/P impedances precisely because the sources were not low impedance. The "classic" RCA phono preamp is an example.
FWIW, there are any number of posts (not by me) about large I/P resistor values sounding bad. I always attributed that to Miller capacitance effects, but ...
When a pentode is used as the I/P active device, large grid to ground resistances are definitely not problematic. Everything has its costs and a pentode may provide more gain than is needed, when "12" W. tubes are used as "finals".
Eli D.
In that example, the source's output impedance at low frequencies is the parallel combination of the 100k plate load resistor and the plate resistance of the 7025 (roughly 75k), or approximately 45k, 10 times smaller than 470k.
Note that the capacitance at the output should include the load capacitance, which includes the Miller capacitance of the following stage.
In practice, there is usually a volume control pot between these stages, which complicates the calculation.
My point is that the 470k grid-to-ground resistor is not important for the Miller effect here.
Large grid-ground resistance can cause problems with grid current shifting the DC operating point.
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