Fender-style FET booster
What do the Fender Bandmaster, Bronco, Champ, Concert, Deluxe, Harvard, Pro, Super, Twin, Vibrasonic, Vibrolux, and Vibroverb all have in common? The same input stage! The classic Fender input stage has been used in all these amps at one time or another in their history. Manufacturers like Victoria, Mesa/Boogie and countless others have borrowed this design. It is said Leo Fender borrowed this design from an RCA tube manual. At any rate, these amps have all enjoyed serious praise for their great sounds.
The Fetzer Valve is a booster circuit that is a FET-based version of the classic Fender 12AX7 or 7025 tube input stage. This is not a massive gain boost, but a moderate boost that will lend some Fender magic to anything you run it into. The input impedance is high enough to keep from loading your guitar signal. The JFET gives a reasonably tube-like sound, plus the whole circuit is quite small and easy to build. This circuit would be easy to wire internally in a guitar or bass, if desired. Of course, it also makes an excellent booster pedal or building block for a larger circuit.
In its stock form, the circuit simulates the lower gain input of a two input Fender amp. However, it can be easily modified to reproduce the higher gain input. Move the 1M resistor to the left-hand side of the 68k so it is connected between IN and GROUND. Then, change the 68k to 33k, so the 33k goes from junction of IN and 1M to the gate of Q1. (Thanks to Phillip Miller Tate for his description of this mod!) Also, the output cap value can be adjusted to best fit your setup. Some Fenders used different cathode (mirrored by the source pin of the FET) resistors. Other values can be used as long as you remember to re-bias the drain to 4.5v or so. Check out the excellent Fender Amp Field Guide for a wealth of schematics and information to customize your Fetzer Valve.
Use a socket and try different FETs for the best sound. During our testing of the circuit, we noted that while a J201 provided more gain than an MPF102 or 2N5457, the overall tone was also darker in comparison. Example voltages are shown in the table below.
As an alternative to all the LPB-1 type boosters out there, try a Fetzer Valve instead! Note: reverse-threaded titanium muffler bearings are optional.
|test voltages for FETs|
|zero voltage at gate|
Pablo De Luca (aka Gringo) contributed a PCB layout for the Fetzer Valve (PDF, 69k)