FET source follower
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Circuits
FET source follower
From kee--(at)--den.com Thu May 2 11:08:59 CDT 1996
Article: 14183 of alt.guitar.amps
From: kee--(at)--den.com (R.G. Keen)
Newsgroups: alt.guitar.amps
Subject: Re: Bypass Cap values, cathode followers
Date: 2 May 1996 12:33:24 GMT
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LarrySB (larrys--(at)--ol.com) wrote:
: In studying the differences between amps, I began wondering about the
: values used to bypass cathode resistors in the preamp stages.
: Some amps, like Fenders, typically use 25uf to bypass the cathode
: resistors. Others use much larger values, on the order 250uf. I'm just
: wondering about the effects on overall tone from such different caps.
The capacitor bypass on the cathode causes the gain to be higher at
frequencies where the capacitor impedance is less than the resistor. The
turnover point is the spot where the cap impedance equals the resistor,
or F=1/(2*pi*R*C); for 1500 and 25uF, this is 4.2Hz if I got the decimal
point right. The lowest note on a concert pitch guitar is 82 hz, so even
a 2.5uF cap should not cause much of an effect in lows. 250uF would
rollover at .42 Hz.
: On many classic tweed era Fenders, a cathode follower is used to drive the
: tone stack. Marshall persisted with the cathode follower, while Fender
: dropped it in the BF and beyond era. Any thoughts on this topic as well?
: Tonal differences? Pet theories?
The follower definitely contributes to overdriving the next stage and
crisping up the frequency response. A neat trick for mod'ing amps without
followers is to stick in an IRF820 power FET as a source follower to
drive the tone/vol stack. The FET is so fast that it follows the driving
tube almost perfectly, and provides strong low impedance drive to the t/v
stack. The saturation/cutoff limits of the 12AX7 driving section are
still well within the limits of the FET linear region, so you can't get
semiconductor clipping from the FET at any drive level. Best of all, you
only need the FET and a 10K-100K resistor to do the mod. The IRF820 is a
600V device, so it works by connecting drain to B+, gate to the 12AX7
Plate, and source to the (added) source resistor to ground. The old plate
capacitor connects to the source. No other changes are needed.
R.G.
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