Did you know you can add a simple circuit to your guitar or bass that produces onboard distortion — without using a battery?
The idea is pretty simple — you attach a pair of diodes to the guitar’s output via a pot or switch. With the diodes connected to ground, you get a fairly nice distortion sound. With the connection to ground broken, you get your regular tones. It’s the same method used to create distortion in a several ’70s distortion pedals, notable the MXR Distortion+ and the DOD 250.
You can purchase the necessary components in a sleek black housing under the name Black Ice for $27.50, or you can do a DIY version for about $2 in parts. I’ll explain how after this brief video demo:
My reactions: It’s a pretty cool effect, though the volume drop when engaging the distortion might be an issue. If would be great, however, in any context where you need a crunch sound at restrained levels, like when playing in a stage pit band. It’s not a terribly refined tone, but definitely a usable one.
Before recording this demo, I tried a Black Ice in a vintage-voiced Strat. It definitely created a distortion effect, but it’s much more gratifying here with the higher output P-90s and humbuckers.
I rewired the demo guitar — which you may recognize from this post about makeovers for cheapo axes — for two volume knobs and a single tone, freeing up the fourth pot for the Black Ice circuit. But I don’t think I’d repeat this method.
The blended settings just don’t do much for me — the pot acts more as a clean/dirty blend than a distortion control per se, and the mixed settings strike me as a thin trickle of distortion suspended over a clean sound. Furthermore, the recommended A250K pot seems like a poor choice, since all the usable sounds are clumped together at one end of the pot’s range. In the future, I’ll experiment with other pot values — or more likely, just attach the diodes to a SPST switch and consider it an all-or-nothing effect. This wiring will work great for the latter approach:
I like the Black Ice package — it’s a snap to install, and I even bought a second one to try in my P-Bass. But you can definitely get a similar effect using a pair of low-voltage Schottky diodes like these or these. I have no idea which exact diodes the Black Ice uses, but the two types I mentioned will get you very close. (FYI, I tried a dozen or so diode types, and none sounded nearly as good as the two I linked to.)
Conclusions: Interesting, inexpensive, and possibly useful. Worth a try! 
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