Vacuum tubes (also called thermionic valves) were the first active electronic components capable of amplification and switching. From the 1940s through the mid-1950s, they formed the heart of electronic digital computers, enabling the transition from mechanical and electromechanical calculation to true electronic computing. Though revolutionary, tubes were bulky, power-hungry, hot, and unreliable by modern standards.
Invented by John Ambrose Fleming in 1904, the diode is the simplest vacuum tube.
Lee de Forest added a control grid between cathode and plate in 1906, creating the triode—the first amplifying tube.
Vacuum tubes replaced slow, unreliable relays in digital logic.
Tubes required heated cathodes (like light bulbs). A machine with thousands of tubes generated enormous heat—ENIAC needed extensive air conditioning and consumed 150 kW of power.
Filaments burned out, cathodes lost emission over time. Mean time between failures was often just hours or days. Operators carried carts of spare tubes and replaced dozens daily.
Switching times were in the microsecond range—fast for the 1940s but millions of times slower than today's nanosecond transistors.
Thousands of glass tubes, sockets, and wiring filled entire rooms. Power consumption was huge compared to later semiconductor designs.
Vacuum tubes proved electronic digital computation was practical and sparked the computer revolution. By the late 1950s, transistors began replacing tubes—smaller, cooler, more reliable, and faster—ushering in the second generation of computers. Today, tubes survive in high-power RF amplifiers, guitar amps, and audiophile equipment, but their role in computing history remains foundational.