I'm making a contact microphone circuit and they say to use a MPF 102 transistor. I was wondering is it ok to use a BJT (that's what I have on hand) instead of the FET. If so would I hook the circuit up differently with the BJT or would it be the same as the FET. Thanks.
It's not a great idea and that 3.3Megohm resistor gives you a hint why...
The FET has a very high input impedance, so that 3.3Megohm resistor defines the input impedance of the amplifier.
Designing a bipolar transistor stage with such a high input impedance is possible but not easy, and I believe you would need at least two transistors, possibly connected as a darlington.
The FET shown is a simpler solution; if you can't find the MPF102, there should be alternatives (2N5459 is probably suitable, I have used it in microphone applications)