The IRF820 MOSFET has a voltage rating of 500v, it should work well in preamp stages of most tube amps. The 100 ohm resistor is there to suppress H.F. oscillations. If IRF820 is physically close to the 12AX7 plate, you probably won't need it. You can see how the MOSFET is equivalent to a triode: Source = Cathode; Gate = Grid; Drain = Platel You cannot use the MOSFET as a directreplacement for a typical tube gain stage. The MOSFET will not `self bias` like a typical cathode-biased tube stage.The MOSFET is an `enhancement device` while a triode is a `depletion` device. l In this circuit, it is not necessary to use a heatsink on the IRF820. You can if you want
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This circuit is a
"cascode". It is
capable of high
gain, is very
linear and is nonmicrophonic.
l A comparison to
triode: JFET
Source =
Cathode; JFET
gate = grid;
MOSFET Drain
= Plate.
l The MOSFET is
wired as a
simple voltage
regulator. The
1M resistor,
zener and 10uf
cap set the
reference
voltage. Use a
24v or less
zener.
l A signal applied
to the "grid"
causes the
current through
the JFET,
MOSFET and
100k "plate"
resistor to vary.
The output is
tapped off the
"plate" resistor,
like a normal
tube gain stage.
l Select a value
for Rs that sets
the "plate"
voltage at
approximately
50% of the
supply voltage.
Try typical
12AX7 cathode
resistor values.
Check the gain.
If you want
more gain, add
bypass cap Cb.
If that gives too
much gain, put
resistor Rb in
series with
bypass cap. The
value of bypass
cap can be used
for tone shaping, as in triode
stage.
l The "plate"
resistor can be
varied over a
wide range
similar to
12AX7. The
output cap can
be varied for
tone shaping
purposes.
l This circuit
works well as a
1st gain stage, or
a reverb return
amplifier.
l No heatsink is
required for the
IRF820 in this
configuration.
This is similar to the
High Voltage Solid
State Preamp stage,
with the following
exceptions:
l The "plate" load
is a standard...
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