Tube amplifier

Vacuum tube amplifiers (valve amplifier - british english), are amplifiers that use so called vacuum tubes instead of transistors. Other names for vacuum tubes are thermionic valve or electron tubes (US english). Generally, the word tubes is used in US english and valves in british english.
There are different types of tubes such as triodes, tetrodes, pentodes etc depending on the number of internal electrodes, and for different voltages, for different types of applications such as for audio or radio frequencies and for each of these either low or high power.
Tubes are often easily replacable if needed. However, sometimes it can take some time to find the same or equivalent tubes for older products. A semiconductor/transistor amplifier though, needs more service to be repaired.
There are hybrid amplifiers too, with both both tubes and transistors.
Mostly the term tube amplifier refers to amplifiers in stereo systems or vintage stereo equipment, although there are tube amplifiers for other purposes too, as high power radio or tv transmitters. Some ham radio amateurs also use tubes in linear amplifiers, that amplify transmitter output power up to kilowatts.
Today, Class A tube amplifiers particularly, both old and new, are preferred by audio enthusiasts and guitar players. There are also other "classes" used that do not result in little or no even order harmonic distortion.

The reason why class A tube stereo amplifiers have gained popularity is simply because the sound is reviewed as getting more "warmth" and perceived clarity through the asymmetric even order harmonic distortion that occurs gradually towards higher volume. Class A has the lowest power efficiency so these amplifiers run warmer.

To understand the value of "vintage" sound, lets take a look back. In earlier years of music recording and production, the sound got even order harmonic distortion in subtle levels through the analog recording process. Music was recorded on tape recorders and in the 60s multitrack recording became the standard so there was actually two tape stages, first each of the tracks were recorded to the multitrack tape recorder. When all tracks were recorded, a mix of them was made and then recorded onto a new tape. After that it was transferred to a vinyl master. Finally, the music was played on the listener's turntable that further added some distortion. When good equipment was used all the way, the sound could be of a class that some audiophiles still value.

In the 60s, more solid state amplifiers were produced and used in home hifi systems, but it wasn´t until the mid 80s when the cd made its entry when a new standard was established for some years, trying to make "clean" recordings. Some qualified studios had digital recording equipment. The response to this direction changed after a couple of years. In recent times, simulation of analog sound in music production has become quite common but not always.

Therefore, a class A tube amplifier can make any music sound more "analog" even though the music may be recorded digitally dry.