How a tube amplifier works

A tube amplifier sits in the same place in the chain as any integrated amplifier: it takes the small signal from your source and makes it strong enough to move the speaker cones. What does the amplifying is a row of glass tubes. Inside each one a heated cathode gives off electrons that fly through the vacuum to a metal plate, and a fine wire grid in between, carrying the music, opens and closes that flow. The heaters make the amber glow you see, and the tubes are why the amp runs hot.

The tubes work at a few hundred volts and a small current, and a speaker wants a lower voltage at a much bigger current, so each channel has an output transformer between them. Pick the 4 Ω or 8 Ω tap nearest your speaker’s impedance. A pair of KT88s makes about 35 to 50 watts a channel, less than most transistor amplifiers, so a tube amp suits sensitive speakers best. The tubes wear slowly and are replaced after a few thousand hours, and each new one needs its bias set.