It’s pretty common knowledge that guitar distortion and overdrive tones were discovered by accident. And enterprising guitarists the world over have been improving on those accidents ever since in the never-ending quest for tone.
Dave Davies of the Kinks ripped his speaker cone and the result was the unforgettable “You Really Got Me” and “All Day And All Of The Night”.
Link Wray dislodged one of his guitar amp tubes and liked how it sounded so he kept that configuration when recording “Rumble”.
What exactly is happening when you use an overdrive pedal or amp? Technically, distortion refers to any aberration of the waveform of an electronic circuit’s output signal from its input signal. In the context of musical instrument amplification, it refers to various forms of clipping, which is the truncation of the part of an input signal that exceeds certain voltage limits. Because both vacuum tubes and transistors behave linearly within a certain voltage region, distortion circuits are finely tuned so that the average signal peak just barely pushes the circuit into the clipping region, resulting in the softest clip and the least harsh distortion. Because of this, as the guitar strings are plucked harder, the amount of distortion and the resulting volume both increase, and lighter plucking cleans-up the sound. Amps that are good at this are referred to as “touch sensitive”.
Great early distortion effect pedals include the Ibanez Tube Screamer that tried to cram an amps power stage clipping tone into a pedal. Early TS-808 Tube Screamers are highly in demand. A great modern day pedal that utilizes the same tones as the Tube Screamer is the MJM Phantom Overdrive Pedal. The Phantom Overdrive delivers some very warm, amp-like overdrive sounds.
Leo Fender of Fender guitars and amplifiers observed these trends and engineered many of his amplifiers to “compress” and/or “overdrive” slightly without drastically distorting the signal. The early Fender “Tweed” and “Blackface” amplifiers are considered a good example of clean electric guitar tone. Many later amplifiers are based on these designs, such as the Tone King Imperial and Metropolitan.
Tube distortion is commonly referred to as overdrive, as it is attained by driving the output tubes in an amplifier at a higher level than can be handled cleanly. Multiple stages of tube gain/clipping can be “cascaded” to produce a thicker and more complex distortion sound. In some modern tube effects, the “dirty” or “gritty” tone is actually achieved not by high voltage, but by running the circuit at voltages that are too low for the circuit components, resulting in greater non-linearity and distortion. These designs are referred to as “starved plate” configurations, and result in an “amp death” sound.