A valve amplifier, or amp, is also called a tube amplifier due to the use of vacuum tubes to power it. The vacuum tube was originally produced in the 1930s and powered early telephone systems, computers and radar systems in World War II. The powerful tube was first used in a musical application when 1950s era rock-'n'-roll guitarists discovered the raw power and subtle nuances the valve amplifier was capable of producing. While the valve amplifier is very powerful, which equates to loud sound for most bands, the tubes generate incredible amounts of heat and are very costly to replace.
There are both pros and cons to using a valve amplifier. The tubes are expensive, generally driving the cost of a tube amp up to several times that of a comparably-sized transistor version. The tube amps also produce the best sound when played at the maximum volume settings. This often creates noise issues with even a small tube amp being able to produce the volume to drown out a large transistor unit. This, however, can be both a pro and a con, depending on the view of the player. Tube amps also produce a telltale buzz when sitting idle, creating frequent problems when recording.
The subtle warmth of a raw valve amplifier has forced manufacturers of transistorized amplifiers to attempt to copy the sound of not only a tube-powered amp, but significant tube-powered amps from particular manufacturers and specific periods. Many transistor-equipped amplifiers claim to posses the sound of a well-known vintage, tube amp. Some players feel that the transistor amplifiers are a great-sounding amplifier, while others continue to scout many avenues in search of a vintage valve amplifier to produce a particular sound. Some of the producers of the world's most sought-after tube amps continue to offer genuine, vintage power tubes to customers at premium prices.
In an effort to solve musicians' dilemma of only being able to produce a particular sound when at full up on the amplifier's volume knob, some manufacturers have created a device intended to soak up the power of the amp and allow a lower volume. Marketed under many names, this type of brake is plugged into the amplifier and works as a type of sponge. The amp noise is reduced significantly while the nuances of the valve amplifier remain. For many classic rock-'n'-roll guitar aficionados, nothing but a screaming, tube-powered stack amplifier will suffice.