Ideal Amplifiers are supposed to be "Linear". This means that the output signal is an exact replica of the input signal,
except that it is of a higher amplitude.
Guitar Amplifiers are far from linear. A fortuitous mixture of design
limitations, design errors, limits on the electronic components in the 50's and 60's, actual design features and economic
constraints lead to Guitar Amplifiers that are linear only for small signals and quickly enter non-linear sections of
response for slightly larger signals.
The Guitar is quite an unlikely candidate for lead work. Being a string instrument,
it outputs fairly pure sine waves devoid of rich harmonics. It also lacks sustain when compared to wind instruments.
amplifiers have two main effects :
INCREASE IN HARMONIC CONTENT : When an input signal
passes through a non-linear amplifier, the waveform is distorted. It is a mathematically accepted fact that any repetitive
waveform can be broken down into a mix of different sine waves. These sine waves are multiples ( or harmonics) of the main
frequency of the the original waveform.
The more distorted a waveform is, the richer in harmonics it is !
SUSTAIN or COMPRESSION : Most Non linear stages limit the maximum output signal. Suppose we pluck a guitar
string, we first see that the amplitude of the note rises sharply. This is called the "Attack"
phase. Then the amplitude falls a bit in the "Decay" phase. After this the note "Sustains"
for a while and then "Release" stage sees the signal drop to zero. This is known as the ADSR
envelope of the signal.
Suppose our amplifier cannot output amplitude higher than a particular limit as shown by the red line. It will clip of
parts of the signal that are of higher magnitude.
The resultant blue ADSR graph after the clipper shows that the Sustain part of the graph is extended
Early Amplifiers added a lot of tonal character and sustain to Guitars due to their non linearities. The aim of modern
Multi-effect units is to emulate those non linearities as closely as possible !!