Developing the Small Drive

When I was writing the description for the Small Drive listing, I realized that I had much more information on it than I should include there, so this post is the expansion on that.

So the Small Drive is a reproduction of the Smallsound/Bigsound Mini, an amazing drive pedal that has an incredible range. A few years ago, Brian from SSBS graciously shared his schematic for the Mini on FSB. Until I dug into it myself, I didn’t understand why I saw so few clones of it. The answer appears to be that it is a circuit that really needs to be dialed in, in more ways than one.

Before I go further, I wish to thank Mark Sescon for laying the ground work for me on this, and Runoffgroove for sharing their expansive knowledge on the Fetzer valve circuit.

The Mini utilizes JFET transistors in multiple Fetzer valve stages. The Fetzer valve is essentially a tube drive circuit, but the tube is replaced with the transistor.

To make this circuit work as it should, two things must be done: The JFET transistors must be measured and selected based on the desired outcome, and the JFETs must be properly biased.

I initially tried to make these with all 2N5458 JFETs, since I have so many, but they produced almost no distortion due to their low gain. So I then started measuring and experimenting with all of the JFETs I had available.

What I settled on matched what Brian had originally shared on his schematic: A J201 in the first stage, a 2N5457 in the second, and a J201 in the third.

The J201s each produce around 10dB of gain, and the 2N5457 produces 18-20dB. I have no idea what values were intended to be used, but these worked quite well. I suspect the 2N5457s I have are higher gain than most, but this may also contribute to being able to give the external bias control an extended range, since that control is directly tied to the second stage.

With everything properly internally biased using Runoffgroove’s calculator, the circuit works perfectly, allowing for everything from clean boost to crunch to dirt to gated fuzz.

I also plan on using this newly acquired knowledge of Fetzer valve circuits in future designs very soon. An extremely worthwhile project, to say the least.