Valves are one of the earliest electronic components invented. They were an accidental discovery in the manufacture of light bulbs. In what you could call the defining lightbulb moment, around 1904 It was discovered that if you put another piece of metal inside the glass bulb, when the light was powered on and the filament was glowing, some electrons from that filament were attracted to the second piece of metal if it had the right kind of charge on it, but those electrons could flow in only one direction.

This exciting discovery opened up many possibilities, and led to lots of experiments being performed, and many more discoveries in regard to controlling that flow of electric current by adding other bits of metal inside the glass.

Over time, lots of different types of valves were made, each with a specific electronic purpose.

For audio amplifiers, we’re most interested in the types of valve that amplify small signals, and those that can drive the speakers.