A triode consists of a heated cathode, an anode, and a control grid placed in a vacuum envelope. By applying a small varying voltage to the control grid, the device regulates the much larger flow of electrons from the cathode to the anode, achieving signal amplification. While largely replaced by transistors, they remain prized in high-end audio amplifiers and high-power radio transmitters.
The guitar amp uses a pair of 12AX7 dual triodes in the preamp stage to get that warm overdrive sound.
Related terms
Vacuum tube
An evacuated glass or metal envelope containing electrodes used to control electron flow.
Valve amplifier
An electronic amplifier that uses vacuum tubes instead of transistors to boost a signal.
Thyratron
A historic gas-filled tube used as a high-power electrical switch and controlled rectifier.