Before transistors were invented in 1947, all amplifiers ran off of vacuum tubes. The invention of the simple valve or tube is credited to John Ambrose Fleming, who created it while working for the Marconi Company in London, 1904. It was called the diode due to its dual electrode structure, which conducted electricity in one direction only. Lee De Forest then added a third electrode, creating the Triode, which he called the "Audion." It was the first electronic amplifying device. Its first audio application was boosting the signal of telephones. In the 1950s, valves began being used for music (and later TV) amplification.
In the early days of the electric guitar, general-purpose audio amplifiers were used. But as the instrument grew more popular, specialized tube guitar amplifiers were designed. In the 1960s, as rock and roll became the dominant popular music, manufacturers experiment with many different types of tube amplification, seeking variations in tone and greater flexibility of volume and gain (the signal drive which creates distortion). Many different types of tubes were used, with many different characteristics; some types included KT66, EL34, and KT88. EL34 and KT88 tubes are still in production today.
Tube amplified sound is generally considered to be superior and preferable to transistor amplified sound, especially for the electric guitar. The tube sound is generally much warmer and more organic sounding than solid-state. Tube amplifiers are also far more responsive to the dynamics of a guitarist's playing (how hard she hits the strings). Tube amplifiers, when played hard, produce some desirable grit that solid state amps do not. The tube amp is the easiest way to get a classic sound out of your guitar.
The main drawbacks of tube amplification are expense and durability. Tube amplifiers are much more expensive than solid state amplifiers, as solid state transistors are much cheaper to produce than tubes. Guitarists' demands for the more "authentic" sounding tube amps also drive up the price. Tube amps are also more fragile than solid state amps (the tubes can break) and are sensitive to heat and cold where solid state amps are not. Tube amps are also much heavier than their solid-state counterparts.But with process development of tube manufacturing, its durability has been improved significantly and working time is up to ten thousands hours or above. Also the manufactures are able to provide cheaper tube amplifier with high quality.And, a protective shield is added to amplifier for pretect the tubes.