Rectifiers

Rectification of an ac signal will involve some kind of diode junction. In the ancient days, there were vacuum type diodes, "cat's whisker" germanium diodes, and finally silicon diodes. The base-emitter and base-collector circuits in a BJT can also be used as diodes. A BJT with its collector tied to the base is said to be "diode connected" and will behave largely like a diode.

Rectification means only passing the positive or negative swings of an ac signal. Full wave rectification means that we split the positive and negative swings then recombine them so that all the swings go the same way. This effectively doubles the frequency of the signal, so "octave up" effects that are analog are most likely made from a full wave rectification circuit of some type.

The Superfuzz and the Scrambler are notable in how they use discrete transistors in their full wave rectification schemes. Other octave up effects will use discrete transistors as a signal splitting device, and rely on diodes to rectify.

Usually, we aren't interesting in the octave up effect, though. Rectification is the first step in creating a dc voltage from an ac signal. The dc voltage can then control some effect. A great example of this would be the MXR Dynacomp. The signal is rectified, filtered by a capacitor to remove most of the ac "wiggle," and passed along to the CA3080 OTA as a control current. Any kind of "envelope" effect will have an ac to dc converter circuit. Some are made of discrete transistors.

Beyond guitar effects, the dc voltage from an ac signal can drive CV inputs on synthesizers, or drive VU meters or LEDs to indicate signal presence or strength on more sophisticated units.