Modded Electra Distortion
The Electra distortion is one of the simplest distortion circuits out there. It lends itself highly to modification, and many versions of it have been made. There have been a few boutique overdrives based on it, such as the Lovepedal COT 50, the Lovepedal Woodrow, and the Nick Greer Green Giant. There have also been a few DIY takes on it, such as the Beavis Audio Trotsky Drive and Fred Briggs' Clock of Tone 50. Here's mine:
This circuit is similar to the original Electra Distortion, with a few main differences. I added a distortion control in series with the input. This works similarily to the clean-up effect you get with this circuit as you turn your guitar's volume knob down.
Updated June 25, 2011:
I took a look at the circuit again, and I've made a couple of changes. The fat switch is gone, for simplicity's sake. I didn't find it that useful. I also removed the emitter bypass cap to increase the input impedance. Finally, I made a few value tweaks to increase gain and to smooth out the distortion. The end result is a nice, ZZ-Top-ish overdrive.
Updated November 27, 2011:
Forgot the input capacitor. Smacks forehead
Mods For The Electra Distortion
The input and output capacitors in the original Electra distortion were both 0.1uF. They let through pretty much all of the inputted bass. By reducing them, you can roll off the bass. Decreasing the first input capacitor to 1nF-5nF gives a good treble boost tone.
Treble Rolloff Capacitor
C2 in the above schematic is a treble rolloff capacitor, taking off treble to reduce the harshness of the clipping. In conjunction with changing the input and output capacitors, you can get anything from a full range effect to a mid boost only. I personally find the distortion too harsh without any treble rolloff cap, but that's a matter of personal preference. This could also be controlled with a switch or with series resistance.
Series Input Resistance
As with many transistor-based distortions, the Electra distortion cleans up quickly with increased series input resistance. This partially expains the clean-up effect of reducing your guitar's volume, as this adds more series resistance with the input. This can be implemented, as I have, to reduce distortion.
The transistor's gain can be increased by a lot by bypassing the emitter resistor with a fairly high-value capacitor (4.7uF+). Or, a gain control can be added by replacing the 680Ω resistor with a 1k pot going to ground, with the middle lug connected to a capacitor as before. By using a lower value bypass capacitor (0.1uF-1uF), a treble boost can be added, although I find it more effective to just add a high-value bypass capacitor and reduce the value of the input capacitor for a treble boost.
By changing the clipping diode types, the overall output and distortion can be changed. Going from most clipping and least output, to least clipping and most output, diodes generally go: schottky, germanium, silicon, and LEDs. Germanium diodes and LEDs have softer clipping whereas silicon and schottky diodes clip more harshly.
Clipping Threshold Control
A clipping threshold control can be added by putting a variable resistance from 10k to 50k in series with the clipping diodes.