Tube or Valve amplifiers
Tube, also known as valve amplifiers, are audio processing units that are widely praised for their characteristic sound associated with vacuum tube components embedded in them. Before the era of commercially introduced transistors in the 1950`s, electronic amplifiers used vacuum tubes. One of the most popular examples is the combination of a tube amplifier with the 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 of matter, an electron will fly thru space if attracted by a sufficient positive charge. Experiments that took place over a century ago showed that there is a 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 that come 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 amps, or due to the higher levels of second-order harmonic distortion.
Harmonic effect
Comparing to solid-state amplifiers, tube amplifiers have much more distortion, but most of it is second-order, which is quite musical. That’s why it got its name – “harmonic” distortion. 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. Specifically, 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 at which you actually want to enjoy them. Just like digital systems, solid state amplifiers measure and sound their worst at low levels.Additionally, they have their best performance at close to their maximum output levels where no one ever actually plays them.
Additional Resources & Source Texts
http://kenrockwell.com/audio/why-tubes-sound-better.htm
