MPF102 FET Preamplifier


Posted on Feb 7, 2014

The circuit that I`ve described is a one-JFET design without much gain. You can significantly boost the gain by adding a source resistor bypass capacitor. The result would be something like one stage of the original tube preamp in a guitar amplifier. Next, you`d eliminate the 220K resistor on the output and couple in another stage. You`d also have


MPF102 FET Preamplifier
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to invent a new power supply for your JFET preamplifier. Whatever you do, don`t hitch up the tube power supply. It will spread JFET fragments all over the room. And probably all across the state. Instead, you need a good, ripple-free, low-voltage supply. Maybe you can use the filament supply with a doubler circuit and filter. Or in a pinch, you can even use a battery. I`m not sure exactly how well the JFET version would work. JFETs can sound very fine, but I`ve never seen a tube-to-JFET sub book. What you propose to do is to replace one design with another. Nobody can say for sure if the new design is equivalent in every way. You`d have to try it, tweak it, and see how you like it. Fortunately, you can temporarily patch in a JFET amp. Remove the tubes. Disconnect the final coupling capacitor, isolating the old tube circuit. Set the JFET breadboard atop the tube chassis. (You can bolt your circuit down later. ) Capacitively couple the JFET preamp to the tube power amp. A few alligator clips would do the job. I mentioned battery operation. Some will want to operate the preamp on a nine-volt transistor battery. Nine-volt operation requires circuit changes. Here`s a nine-volt version of my preamp. ANSWER. An MPF102 would work as the power amp. It really isn`t an acceptable power device, though. Your speaker volume wouldn`t be very loud. The output volume will be comparable to what you get from a transistor radio. (At best. )...




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