After reading Part 1 of the M.I.Y series: Tools and Techniques, you should have a good grasp of what is required to get into your pedal and start modding. But before we flick on the soldering iron, I thought I’d write a few points about the diode, the key component to Part 2 of the M.I.Y series: Diode Clipping.
A diode is an electronic component that allows electricity to flow in one direction, much like a valve does letting water go one way but not the other. Diodes can be seen in nearly all effect pedal schematics and are noted as this symbol (left). They are integral in creating that overdrive or distortion sounds in your favourite dirt pedals, and come in different packages, materials and sizes. Each diode has a positive side (anode) and negative side (cathode). This polarity is what resembles a valve, and dictates the flow of electricity.
But how does a diode contribute to that overdrive sound? If we look at an audio signal (represented as a sine wave) we can distort the signal by “clipping” the tops and bottoms of the wave. This clipping produces harmonics that we hear as overdrive.
If a signal is clipped evenly on both top and bottom of the wave, this is referred to as symmetrical clipping and is used in pedals such as the Ibanez Tubescreamer. If a signal is clipped more heavily at one peak than the other, this is called asymmetrical clipping as heard in the BOSS SD-1 Overdrive. The more heavily a peak is clipped, the more a sine wave moves closer to a square wave, and closer to distortion and fuzz.
The location of clipping diodes has an effect on the sound as well. Soft clipping involves having two diodes connected on the feedback-path of an opamp or a transistor (Tubescreamer). Hard clipping is when two diodes shunt to ground, creating distortion as in the ProCo RAT. The material of the diode also affects the clipping. Silicon diodes have been described as being “sharper” and “tighter” whereas germanium diodes are described as being more”compressed” or “spongey”. You can also use LEDs (light emitting diodes), Zeners, Schottkys, MOSFETs etc… the experimental possibilities are endless.
So with all of that in mind, let’s crack open a pedal. We will need :
Firstly, unscrew the back so we can get to the PCB. You should be able to see a trace side, full of tracks and solder joins . Carefully lift up the PCB so we can see the component side, and look for a set of diodes . The markings (e.g D3, D4, D5) should correspond to your schematic. Desolder these diodes, marking down their original orientation, and clear the holes. Solder in a socket for each hole from where the diode was removed  and place the PCB back into the pedal. We can now experiment with different diodes, and combinations of diodes, and immediately hear the difference between them.
Place the diodes (using the same polarity as you marked down earlier) back into the sockets, power up the pedal, plug in and play. No need to close the pedal back up, it will just be a bit noisier. Now remove the power, remove the diodes and try another combination. Power, plug and play – hear the difference. Now try a symmetrical configuration, then asymmetrical.
In the place of one diode, try a MOSFET like an MPF102 (note in  how I bent only the ground pin to remove). Or you could try germanium and silicon diode in series, a silicon and LED, two silicons in series, two germaniums in series etc. To solder diodes in series, ensure that the cathode is soldered to the anode . Using diodes in series increases headroom and volume but sacrifices a bit of gain. Use MOSFETs for “tube-like response” and LEDs (reds are great) for crunch and higher gain. When you find a combination you prefer, desolder the sockets and solder in your killer selection. Close the pedal back up and you now have a diode clipping modded pedal. Common mods include placing different diode combinations on switches, or a symmetrical/asymmetrical diode switch. But we may leave that till next time…