Tube Amplifiers

A tube amplifier, or valve amplifier, increases the strength of an electrical signal using vacuum tubes as the active devices. In an audio amplifier, a weak signal from a source is applied to the control grid of a tube, where it modulates a larger current flowing from cathode to plate. Several stages are usually cascaded: input and driver stages build up the voltage swing, and an output stage delivers the power needed to drive a loudspeaker.

Because tubes work at high voltage and relatively low current while loudspeakers need the opposite, most tube power amplifiers use an output transformer to match the high plate impedance to the low impedance of the speaker. Designs vary in how the output tubes are arranged. Single-ended designs use one tube or set of tubes operating over the whole signal, while push-pull designs use pairs that handle opposite halves of the waveform, increasing power and cancelling certain distortions. Triodes, tetrodes and pentodes each give a different balance of power, gain and tonal character.

Solid-state amplifiers replaced tubes in most consumer equipment, being smaller, cooler and cheaper, but tube amplifiers remain popular in high-fidelity audio and especially in electric guitar amplification. Many players prefer the way tubes distort when driven hard, producing a warm, gradual overload rather than the abrupt clipping of transistors. Building and restoring tube amplifiers is an active hobby, supported by reissued tubes, transformers and published schematics.

Frequently asked questions

Why do tube amplifiers use an output transformer?

Tubes operate at high voltage and low current, while loudspeakers need low voltage and high current. The output transformer matches the high plate impedance to the low speaker impedance.

What is the difference between single-ended and push-pull designs?

A single-ended stage uses one tube over the whole waveform, while a push-pull stage uses a pair handling opposite halves, giving more power and cancelling some distortion.

Why do musicians still prefer tube amplifiers?

When driven hard, tubes overload gradually and produce a warm distortion that many guitarists prefer to the sharper clipping of transistor amplifiers.