Guitar preamp circuit based on JFET 2N5457.
Overall gain is 3db (2X) or so. It uses about 1/2 ma, so a 9V battery will last a long time, you may also use simple regulated power supply with LM7809. It is possible to add a high boost switch in case you like by having it shunt the 2.2k ohm resistor with a 0.05uF cap (or other value; smaller cap = boosts only higher frequencies, as well as the reverse). You may simply placed in a 10uF capacitor across the 2.2k ohm resistor in order to up the gain