Modding a SD-1 Pedal

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I’ve had a Boss Super Overdrive pedal on my pedalboard for decades, it’s gives me a crunchy overdriven tube amp sound at lower volumes and now that I’m comfortable with modifying my guitar pedals I’ll take the soldering iron to this old classic.

Today I started with the Brian Wampler’s Cuervo Mod, which I found in his book How to Modify Guitar Pedals, more of a half-baked collection of mods than a book, but we should support the guy that almost single-handedly started the whole pedal modding community.

There’s a few popular mods for the SD-1, it’s an easy pedal to modify because the PCB is spacious and gives you lots of room to add components. The SD-1 circuit is a slight variation of the famous Tube Screamer (TS-808) pedal and many of the mods are about making more like the TS-808. I’ve built a few TS-808 clones and would rather make the SD-1 have it’s own character. I’d like to have my SD-1 have a tad more distortion and have a bit more headroom, it’s seems to compress the guitar’s signal a fair bit and takes away some of the dynamics.

a typical Super Overdrive to TS-808 Mod

How do you like your Clipping?

You can get different types of distortion by swapping clipping diodes in your Tube Screamer or Super Overdrive. For example, replacing the existing diodes with germanium diodes will yield a compressed, smooth fuzz sound. In contrast, silicon diodes (1n4148, 1n4001, 1n914, etc.) tend to provide a crisper, tighter, more focused sound. LEDs sound warmer, offering a great crunch, and usually making the pedal sound louder.

You can also experiment with different diode configurations. Two types of clipping can be achieved through different configurations: symmetrical and asymmetrical. Asymmetrical clipping—the type of clipping achieved in a stock Boss SD-1 circuit (see Fig. 2)—tends to yield a more dynamic and responsive overdrive resembling the feel and response of an amp overdrive. You can get asymmetrical clipping by putting two series-wired diodes in parallel with one diode oriented in the opposite direction (as shown in the mod instructions).

The Mid Hump
SD-1 Component Layout with highlighted capacitors that were replaced with different values

Tonal Changes

I’m not looking for the typical “Mid-range Hump”, an increase in the frequencies between 300hz – 3KHz, that the TubeScreamer give you. If you’re curious about that, there’s a great in-depth TubeScreamer Analysis by Electrosmash.

For my first mod, I chose to replace a few capacitors to different values to give me a wider frequency response. These changes are according to Wampler’s Cuervo Mod.

Capacitor 2 (C2) – 47nf
Capacitor 3 (C3) – 150nf
Capacitor 6 (C6) – 1uf Electrolytic

I tried out the pedal after these quick changes and was happy with the frequency response. The tone reminded me of the Timmy overdrive pedal, which I then put next the the SD-1 so I could AB test between the two pedals. The SD-1 Cuervo Mod gave almost the same amount of overdrive and tonal character as the Timmy pedal. A huge improvement.

Next Mod to Try

In the process of researching different mods I found the following SD-1 mod with higher gain, which can be dialled down for a transparent full-range clean boost. The main difference in this mod is an added pot which gives a ‘bright gain’ control. In combination with the original gain pot and the tone knob, this mod covers a lot of tonal territory, a bit like mixing normal and bright channels on an amp.

C2: increased to 0.056uF, for more bass
C3: increase to 0.15uF, more bass
R10: bypass or omit, for more volume
C6: removed, for more high treble
C5: decease to 0.018uF, to shift tone control up into the treble range for better control
R6: replaced by 2.2k in series with a 10k pot.

At a minimum setting the new pot sets a low gain range for boost/crunch with a wide transparent frequency response, from clean boost (add a bit of treble to get balance) up to a medium thick overdrive combined with the main gain pot. Turned up, the new pot kept the bass mostly unchanged, increasingly adding mids and then highs, returning the tones to more like normal SD-1 range, but with more gain available.

I’ll be posting the results from this next mod next.

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