Guitar amplifier

The most common guitar amps are for electric guitars, hence also called electric guitar amplifier, but for the most just guitar amp. There are also Acoustic guitar amplifiers.
The standard guitar amplifier has one or two speakers, therefore also called combo amplifier. Most guitar amps has an input preamp gain, tone controls, a master gain/volume control and some have a reverb, nowadays digital reverbs with other effects have become common so that the old spring reverb has become more of a vintage feature even to that point where it is sometimes emulated as digital effects. Many guitar amps also have 2 channels, one for a clean sound and one for a distortion/overdrive sound.
The most popular guitar amplifiers are made by Fender and Marshall. Some other popular maufacturers are Vox, Peavey, Crate, Yamaha, Laney and Orange.

A guitar amplifier without a speaker, supposed to be used with a separate speaker cabinet, is called either rack or head amplifier depending on the size, forming an amp stack.

The first guitar amplifiers all used tubes. Later, some of the finer models had built-in spring reverb and tremolo though it was wrongly called "vibrato" (pitch modulation).
In the late 60s, transistor amplifiers were introduced. The transistor amplifiers did not color the guitar sound as the tube amplifiers. Transistor amplifiers had higher dynamics without any soft clipping like tube amplifiers, but any final amplifier distortion would sound more raw and unpleasant.
However, in contrary to the standard tube distortion sound, a harder distortion became used again, (since the first use of fuzz, mostly around 65-67), and developed in the 70s with distortion pedal transistor/ic circuits that were implemented in guitar amplifiers.
Fender Princeton Reverb guitar amp

Eventually, the "tube sound" again regained more popularity and tube amplifiers were put in production again, including reissues of old classic and trustworthy models. There are also hybrid amplifiers with tubes only in the preamps, usually 12AX7, and transistors in the power section.
Emulation of tube sound by analog circuit designs became popular as external effects and some guitar amp manufacturers integrated it in guitar amplifiers.
In a few years, digital signal processing advanced the possibilites of guitar modeling and in the last years, guitar amplifiers are available with many types of built-in effects units. These digital effects also provide reverb, chorus, flanger, tremolo etc. Genuine tube amplifiers are still produced.

Even though it is sometimes not possible to distinguish tube sound simulation from the "real" gear even by experienced users, there are people who stand by the point that the only real thing is real tubes.

Vox AC30 guitar amp There are probably several reasons why classic amp users refuse to accept that anything else than an amplifier with speaker(s), and tubes, can match up to the "real" tube sound. Tube simulation is said to fail to sufficiently capture the nuances with the different origins of the guitar, tubes, transformer and speaker(s). Besides, a less mentioned reason, nevertheless claimed to be impossible to disregard, is that an amplifier with tubes have a microphonic effect. Therefore, a tube amplifier with a speaker also gets a mechanical feedback as part of the sound. This effect depends on the volume. This mechanical interaction is probably not thought of by the tube emulation designers.

Moreover, two further things to be mentioned below;
- In a tube amplifier there is an output transformer between the power tube(s) and the speaker, that matches the impedance and voltage. When the ferromagnetic core of a transformer becomes electromagnetically saturated, the signal becomes compressed, and smooth clipping occurs. This distortion is added to the tube distortion.
- Compression effect called Sag, associated mainly to unregulated power supply of old class AB tube amplifiers. These had simple voltage regulation so when they were played loudly, the rectifier current increased, making a larger voltage drop over the rectifying tube. The voltage left to the amplifier would fall and so the amplifier's output resulting in a compressed sound. In this way, the player could achieve a certain sustain in a way that could be controlled. Sag will not occur with a Class A tube amplifier since it draws constant current regardless of the output.