Lots of kinds of diodes are used in amplifiers.
In the power supply section, diodes may be used in groups of two
or four, as rectifiers to convert AC (from the mains supply, via a
transformer) into DC for the amplifier. These diodes are usually
rated at several hundred volts, and several amps. They are
classified as rectifier diodes. A typical example is 1N5404.
In the amplifier circuitry, diodes may be used to provide a
known (approximately) voltage drop where required by the design of
the circuit. They can also be used for temperature feedback,
although for direct temperature compensation a transistor is more
common. These diodes are generally small-signal types like
1N4148.
In logic switching, diodes are often used for simple OR-type
functions. Look on Wikipedia for "diode OR". These are small-signal
types.
If the amplifier has a switching power supply, or is a Class D
amplifier, high speed switching diodes are used extensively. They
are used for rectification (though they must be high-speed devices
because of the high switching frequency), suppression and clamping
in the Class D output stage (ditto), and miscellaneous functions in
the control circuitry. These are high-speed, high-current
diodes.