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In Reply to: RE: cloning the Fisher posted by burwoodc on January 25, 2015 at 11:52:01
No insult intended to Mr. G., but I DISLIKE the 470 Kohm resistor at the circuit's I/P. The Miller capacitance of a 12AX7/ECC83 section is high. The I/P resistance combines with CMiller to form a low pass filter. To avoid high freq. roll off, keep 'X7 grid to ground resistors' value below 200 Kohms. 100 Kohms is a good "seat of the pants" limit for 'X7 grid to ground resistors.
I like the regulated g2 B+. Also, the use of the regulator circuitry to obtain the "fixed" bias voltage for the O/P tube control grids is quite clever.
The AnTek power trafo I previously linked can't yield the voltages needed by this design. However, the AS-2T350 will do the job for you.
Eli D.
Follow Ups:
Thanks to everyone for their help thanks for the updated PT model Eli.
I will begin drawing the layout and report back when I start making material progress.
Assuming a low-impedance source, the Miller effect at the input is governed by (Rsource + 10k), not 470k, in this circuit.
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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