FET-2B
A Tube-to-JFET Adaptation of a Classic Bass Preamp
MKII Update
At the bottom of the page, you will see an updated schematic for the FET-2B MKII which has been optimized for pedal use. This includes a better input network, tone stack scaled for lower noise, and a volume pot that is more effective. I am leaving the old schematic and explanation on this page as well for the sake of education, as I am not doing any kind of walkthrough on the updated schematic. Enjoy! - 5-22-2022
Overview
The year is 2008. Guitar pedal DIY is starting to hit some exciting times. Runoffgroove is shaking things up with their tube-to-jfet adaptations of classic amps and I am just starting to really get into doing DIY pedals. I had just built myself a bass to record bass parts to my songs, but running the bass directly into the interface just wasn't all that exciting. I started reading up on bass preamps and read tales of the legend of the Alembic F2B bass preamp. Not having the chops to build a full tube version (and being a super poor graduate student), I decided to do the tubes-to-JFET treatment of the F2B, and thus the FET-2B was born. This was my very first "design". I use quotes since it's a pretty straight up adaptation. However, it was quite nice sounding. The design remains a reasonably common choice with people looking for a bass preamp pedal, so I thought I would document it here.
How It Works
The F2B is actually a very garden variety preamp in the Fender style. The values used for components are not necessarily chosen specifically to bass, but actually work quite well for a variety of sources. The circuit consists of an initial amplification stage followed by a standard Fender tone stack, into a volume control with treble bypass bright switch and a final amplification stage.
FET-2B Schematic
A couple of notes regarding this circuit are worth bearing in mind:
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- The bias trimmers are for setting the drain voltage of the JFET's to 1/2 supply voltage, so nominally 4.5V for a standard 9V supply.
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- The tone stack is exactly as it is in the tube version. Because we are dealing with much smaller signals in a pedal, it is probably a wise idea to scale the components according to the typical fashion: resistors (including potentiometers) are divided by a factor of 10 and caps are multiplied by a factor of 10. This will help with reducing the noise floor.
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- While talking about noise, the volume control before the final amplification stage may contribute some to noise. I haven't done any experiments, but if you are turning the volume down directly after the tone stack, you will then amplify the noise of the whole shebang. It might be useful to move the volume control after the final gain stage, but as I said, I haven't tried it. It's all theoretical at this point. If you are interested in building one, I have the usual board layout and build documents on github (here). Now go build one and get some bass grooving happening!
MKII Update
Please note in the schematic below that there are only 2 JFETS and no MOSFETs. The symbols are for the purposes of using either an SMD or through hole JFET in each stage and the Eagle CAD model from Mouser for the MMBFJ201 shows a MOSFET symbol instead of JFET. Confusing, I know, but you only need two J201's for this thing. I have also omitted the power section, as it is dead simple, and there is no board layout or accompanying files available for this guy. That's ok, it's super easy to put on perf board, vero, or whatever.
