Scythe Voice Mod (also applies to Gnat & Soul Vendor)

Chuck D. Bones

Well-known member
I applied this mod to my Tone Reaper (Scythe Fuzz). It will also work on the Soul Bender (Soul Vendor), Buzzaround (Gnat) and other circuits similar to the Tone Bender Mk III. This Voice mod was inspired by a version of the Tone Bender Mk III that I found on General Guitar Gadgets. It has a switch to select three different emitter bypass capacitors on the first stage. Here is a link to it.

Full Disclosure: I built a PNP version of the Scythe because I wanted to use a Russian PNP germanium transistor (GT402B) that had been languishing in my parts bin. It is the same circuit as the EQD Tone Reaper (Scythe Fuzz), with the polarities reversed and the mods described below. And I added an inverter to make -9V.

This mod adds a VOICE pot that allows you to roll back the bass and vary the gain of the 1st stage. The VOICE pot requires another hole in the enclosure. It's easier to apply this mod when initially building the pedal as opposed to retrofitting it later.

The Voice Mod is centered around the 1st stage emitter bypass capacitor, which is C4 on the Scythe, C2 on the Gnat and C6 on the Soul Vendor. These capacitors all do the same thing: maximize the 1st stage gain by eliminating negative feedback at the emitter. The size of this capacitor determines how the gain drops off at low frequencies. This mod replaces C4 with two capacitors and a pot to pan between them. The idea came from a Buzzaround circuit I saw on the 'net that has a switch to select different caps for the emitter bypass.

You will need a 10uF capacitor, a 1uF capacitor and a B1K pot. I prefer tantalum caps, but aluminum works fine. Remove or omit C4 from the circuit board. Install a 10uF capacitor from pin 3 to case on the VOICE pot. Install a 1uF capacitor from pin 1 to case on the VOICE pot. Both caps have their minus lead soldered to the case (GND). Run a wire from pin 2 of the voice pot to the C4 + pad on the circuit board. Make sure that the VOICE pot will make metal-to-metal contact with the enclosure when mounted. If you don't trust this method of grounding, then run a wire from the VOICE pot case to any convenient ground, such as C4's - pad or the input jack's ground lug. For positive ground circuits like the Gnat & Soul Vendor, reverse the capacitor polarity. The Gnat uses a 4.7uF capacitor; you can use 4.7uF and 470nF if you want to retain the stock voicing.

Turning the VOICE pot full CW provides the stock 1st stage gain and frequency response: 42dB with bass rolling off below 260Hz. Backing the VOICE off lowers the gain of the 1st stage and flattens the frequency response. The lower gain reduces the tendency for the 1st stage to saturate. At noon, the 1st stage gain is minimum at 33dB. Rotating further CCW increases gain, but with less bass and mids. Setting VOICE to full CCW increases the 1st stage gain back up to 42dB, but with bass and mids rolling off below 2.4KHz. Feel free to experiment with different capacitor values, I find that something in the neighborhood of 10:1 is a good ratio. If you want more bass, increasing the caps to 22uF & 2.2uF moves the cutoff freq down an octave. That's what I ended up with in mine.

I performed two other mods that are applicable to the Scythe only:
Change C9 to 2.2nF for a little less treble when TONE is dialed down.
Insert a 220K resistor in series with pin 3 of the LEVEL pot to move the unity setting from 15% up to 35% rotation. Still has plenty of volume on tap. Some versions of the Tone Bender Mk III have this resistor.

Here are some pix. Front panel graphics are pending. The Sputnik device in the middle of the board is the GT402B. I tried a 2N1309 there, but its leakage current was too low and it sounded too fizzy. That green electrolytic was a little too big for the layout, hence the tipsy stance.

Stoned Bender - front 03.jpg

Stoned Bender - innards 03.jpg
 
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