The TRIODE was invented in 1909 by Lee De Forest. The power triode is an "active" component that allows the amplification of an electrical signal capable of supplying the current necessary to drive strong loads; among the most common, especially today, are the loudspeakers. There are DHT and IHT triodes, that is, with direct or indirect heating.
Among the most famous are the majestic 2A3 Triode introduced by the RCA Corporation in January 10, 1933. It is probably the most famous BF (Low Frequency) tube amplifier and was actually designed for Audio amplification. Initially used in Radio-receivers and subsequently, given its linearity and sound performance, it was brilliantly used in pure amplifier circuits for Cinema and later in HI-END amplifiers, still today considered a TOP! This family also includes power amp valves such as 6A3, 6B4, 45, 300B, AD1, EBIII, 5930, 4683 and others.