Before the days of LCD, Plasma, etc., there were televisions that utilized vacuum tube technology to create the image that we saw. The efficiency and natural power generated by these tubes became very popular in a variety of appliances. When the electric guitar became a staple of modern music, the amplifier was then created, and soon it was discovered that the richest, most organic tone was created using tubes to both power the amplifier and serve as the sound processing units.
Russia has always been at the forefront of the engineering and technological landscape. Their space program was top-notch, their science program was unmatched for decades, and their appliances, vehicles, and other electronics are noted for their longevity and durability. In the guitar world, Russian-made Sovtek tubes are hailed as some of the best that money can buy. The depth of sound is unmatched, the response to a player's nuances and style is very accurate, and the overall feel and tone becomes more fluid.
There was a strong movement during the late 1990's and early 2000's to turn guitar amplifiers from the old tube power to newer, more efficient solid-state circuit boards and even computer chips. Interestingly, these systems are built not to have their own distinctive sound, but often to try and emulate the tone of a tube amplifier. However, in recent years, there has been a strong resurgence of tube amplifiers on the market, and the technology is even better than before.
Here is a video of one of my favorite blues guitarists, who uses an amp that makes use of Sovtek tubes.