OLD TUBE SYSTEMS - LORON KNOWLEN

Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio communication and broadcasting, television, radar, sound reproduction, large telephone networks, analog and digital computers, and industrial process control. Although some of these applications had counterparts using earlier technologies, such as the spark gap transmitter or mechanical computers, it was the invention of the triode vacuum tube and its capability of electronic amplification that made these technologies widespread and practical.
In most applications, vacuum tubes have been replaced by solid-state devices such as transistors and other semiconductor devices. Solid-state devices last much longer, and are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes still find particular uses where solid-state devices have not been developed or are not practical. Tubes are still produced for such applications and to replace those used in existing equipment such as high-power radio transmitters.

Good review.