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A guitar amplifier takes the audio signal from an acoustic or electric guitar and increases, or amplifies, the input to make it louder with minimal distortion. An amplifier is actually the device that changes the signal, but it usually includes a speaker or speakers to play the louder sound. Different kinds of amps, such as tube, solid state, and digital, differ in the quality of sound, loudness, distortion, and choice of audio effects.
The original guitar amplifiers were powered by vacuum tubes, just like a lot of electronic devices of the 1940s and 50s. Even when electrical technology replaced vacuum tubes with transistors, many musicians preferred the distinct sound and quality of a tube guitar amplifier. Thus, a huge market for vintage and reproduction amps remains alive today. They're more expensive, and might be difficult to repair, but they produce natural feedback loops and sweet tone.
Another kind of guitar amplifier is the transistor or solid state variety. With an electric guitar, the amplifier must have a preamp to initially boost the unamplified signal for the secondary power amplifier. Then it's strong enough to vibrate the speaker and become audible. Effects like distortion and feedback can be strictly controlled, rather than evolving organically in tube amps. Those effects can also be easily eliminated for a crisp sound. Solid state guitar amps rarely need repairs.
When selecting a guitar amplifier, make sure to consider volume, flexibility, portability, and how it sounds with your particular guitar. Bring your guitar(s) to the store to test them out on every possible setting. Listen for clean amplification, a responsive EQ, or equalizer, balanced sound between bass and treble, and the option for some sound effects like reverb, delay, or echo. You should be able to use it for solo practice, jamming with other amplified instruments, and larger venues.
Guitar amplifiers can be purchased with the power amp "head" separate from the speaker, but usually they are together as a "combo." The cabinet itself will affect the sound, depending on the type of wood, size, and whether the back is open or closed. For instance, closed-back cabinets increase bass. If you know exactly what you want, you can build your own combo guitar amplifier.
When out looking at guitar amps, do yourself a favor and test a modeling amp. Those handy things emulate other guitar amps and have a ton of built-in effects. Good ones are expensive, but they are cheaper than buying a standard amp and a bunch of foot pedals for effects.
While some may not like modeling amps because they are are based on transistor rather than tube technology, the better modeling amps can emulate that warm, tube sound quite well.