The vacuum tube has been used for longer than stereophonic sound has been around. A vacuum tube is somewhat like a light bulb: constructed of glass and inside this glass envelope there is a heated filament. The interior of the tube has all the air evacuated (thus the vacuum). The heated filament gives off light, which we can see, just like a light bulb.
Vacuum tubes contain other elements which are the real reason for their function. In a basic tube, there is the filament (often called the “cathode”), and a metal strip (called the “anode” or “plate”) situated near the inside edge of the tube.
The most common amplification tube is the triode. The triode adds a third element inside the tube, called a grid. The grid has a negative charge applied to it. Because of the grid, electrons from the cathode are repelled and not allowed to flow from the cathode to the anode plate.
So, if the audio signal (a very small voltage) is applied to the grid, and this very small voltage fluctuates with the sound, there will be a fluctuating amount of current flowing between the cathode and anode, in time with the music. Since the anode plate voltage is very high in comparison to the grid voltage, we have amplification! There are other types of amplification tubes such as the pentode, and tetrode. But most audio purists choose the triode for its simplicity and musicality. Multi-grid tubes usually produce higher powered amplification, but, along with it, much more distortion. Triode based tube amplifiers were the first, simplest, and perhaps still the best amplifiers.
Their sound is wonderful, in part, because they are simple. The best in this category is called a single ended Class A triode amplifier (otherwise known as an SET). Usually one output tube is used, and the plate is connected to one wire on the primary winding of an output transformer (for your information, the secondary winding of this transformer supplies the speakers), while the other wire of the output transformer primary winding is connected to the power supply.
Usually only few watts of output can be obtained from these tiny mites. This is why they are recommended for anything but two-channel stereo listening in relatively small spaces and they are best used with very sensitive loudspeakers.
SET's have a very well-known pattern of distortion that produces what is known as a second harmonic distortion. Second harmonic distortion is musically equivalent to adding the same tone one octave higher, to form a chord. In this case the added tone is at a lower level (not as loud) but the effect is to "fatten" the sound. This second harmonic tends to be very pleasing to the human ear, thus making SETs extremely pleasant to listen to for long periods of time.
Unfortunately tubes slowly wear out any time that they are in use. Like a light bulb, they gradually lose their performance until they suddenly fail. If the frequency extension and dynamics that they once had are no longer there, then they have worn out. Power tubes will require regular replacement; it is recommended in many cases to replace the output tube once per year under normal usage, or every three months, if the amplifier is used continuously.