Circuit Description
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/Plate
You cannot use the MOSFET as a direct replacement 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. In this circuit, it is not necessary to use a heatsink on the IRF820. You can if you want.
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/Plate
You cannot use the MOSFET as a direct replacement 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. In this circuit, it is not necessary to use a heatsink on the IRF820. You can if you want.
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