The FX Files TG’S classic stompbox guide
From transparent tones to amp-like roar, the Fulltone OCD is one of the most enduringly popular modern drive pedals
The clipping waveform distortion on the OCD is delivered by two 2N7000 transistors after the first stage, with the Drive control determining how much the signal is amplified before it hits this point. After the first version, the drive pot had its value bumped for more dramatic distortion, and on later iterations of the pedal an asymmetric 1N34 germanium diode was controversially added to more dramatically shift the character of the clipping. In the DIY community, the pre-germanium version remains more popular to build.
The tone control is essentially a treble roll-off. Although a fair bit of treble is filtered at the main amplification stage, the tone control is used to remove treble above a fixed frequency right before the output of the pedal.
The HP/LP switch isn’t a high-pass, low-pass selector as the abbreviations imply, but instead selects between different frequencies to use at the shelving frequency for the Tone control. For most players, guitars and pickup combinations, the HP, or higher, frequency should be the better option, but like most things it’s about personal taste.