Tube Distortion

Tube or Valve Amplifiers

Tubes, also known as valve amplifiers, are audio processing units widely praised for their characteristic sound associated with embedded vacuum tube components. Before the commercial introduction of transistors in the 1950s, electronic amplifiers used vacuum tubes. One of the most famous examples is the combination of a tube amplifier with an electric guitar. This merger has led to a revolution in guitar sound.  

How Do Tubes Work?

To closely explain how tubes work, we have to turn to some basic principles of electricity. An electron is a negatively charged subatomic particle. In the vacuum, the state of the absence of air and matter, an electron will fly through space if attracted by a sufficient positive charge. Experiments over a century ago showed the possibility of controlling electrons. Scientists showed that, in a vacuum, electrons flowing from a heated metal element—the cathode—and being pulled toward a positively charged element—the anode—can be deflected by a magnetic field.  

Distortion

The ways that tubes distort when pushed to the edge are much more musical than the artificial sounds from transistor amplifiers when overdriven. Some transistor guitar amplifiers attempt to mimic tube distortion, but that's a different article. Producers often describe the tube sound as having "warmth" and "richness." However, the source of this is by no means agreed upon. It may be due to the nonlinear clipping. This occurs with its amp or due to the higher levels of second-order harmonic distortion.  

Harmonic Effect

Tube amplifiers have much more distortion than solid-state amplifiers, but most are second-order, which is quite musical. That's why it got its name - "harmonic" distortion. The second harmonic distortion is exactly the same note, an octave above. Ditto for higher-order even harmonics. They are also the same note, more octaves above. Even-order harmonic distortion can be so pleasant that back in the 1970s, the Aphex Aural Exciter was very popular in recording and broadcasting because it generated harmonic distortions.  

Reaction to Loudness

Not only is tube amplifier distortion harmonious, but it also increases as things get louder. This is exactly what happens in a musical performance. As instruments play louder, or as you hit a percussion instrument or piano key more strongly, they generate more harmonic content. As notes decay, the percentage of harmonic content drops again. Tube amplifiers mimic this. A good tube amplifier increases its distortion directly with the output level across three decades of voltage or a million-to-one power range. Tube power amplifiers sound their best at the volumes you want to enjoy. Solid-state amplifiers measure and communicate their worst at low levels, like digital systems. They also have their best performance close to their maximum output levels, where no one ever plays them.