Understanding Tubes in Audio Gear
In the first place, one should understand why tubes exist in audio gear in the first place. On the most basic level, tubes amplify electricity. Before the introduction of transistors into the market in the 50s, vacuum tubes were used in electronic amplifiers. In audio, vacuum tubes lend a characteristic sound when used as an audio amplifier. So, they’re often smooth out harsh sounding audio.
Tube based ( or valve based ) compressors were, in early days of audio, the only choice of machine-modifying gain. The principle on which they were working was that the level detector circuit sends the voltage to the tube. This voltage directly drives the gain of the tube. Fairchild and Altec compressors were the early devices built on this principle. Today Manley makes the state of the art update on this approach. Vari-mu (short for Variable Mu, also a Manley trademark) designs use a vacuum tube in place of a transistor, where the variable voltage input changes a tube’s bias instead of a transistor’s gain. It has a somewhat different action, generally claimed to be even smoother and more “creamy” or “organic”.
Other known vari-mu-style examples are the Markbass Compressore and the Lightning Boy Opti-Mu Prime. The sometimes musical and appealing distortion introduced by the tube based gain state is a part of the tube compressor sound. In addition, there is a reaction time of the tube as its gain changes by the level detector circuit. Any increase or decrease of the gain is not instantaneous. This gives tube based compressors their unique attack and release time properties. The rate at which the gain increases or decreases has a major influence on the overall quality of the compression effect.
Application in various devices
Beside compressors, tubes can be found a variety of devices, such as microphones, guitar amps, equalizers, etc. If we are talking about hardware units, it is important to understand that tubes need some time to warm up. Guitarists are mostly aware of this since these kind of amps are common. As the tubes warms up, they actually increase the temperature. They need certain amount of time to warm up so that they can become stable.
Before they completely warmed up, tube based audio processing devices can give you fluctuating outputs. Therefore this can mess up your sound. In order to be sure of the full potential of tube based gear one should power it on the beginning of the session and give it 10 to 15 minutes to warm up. On the other hand it is also desirable to let tubes cool down and then move it if you intend to. The reason for this is because the tubes are much more fragile when they are in the hot state.
Tube or valve compressors are mostly used after recording in order to add warmth to drums, vocals and basses. This effect is caused by small amount of the second-order harmonic distortion which occurs in the final gain circuitry of the compressor. The distortion is the result of the random movement of the electrons. In the cases of valves, this occurs at exactly twice the frequency of the amplified signal. Despite the fact that this distortion only contributes 0.2% of to the amplified signal, the human ear ( subjectively ) finds it appealing.
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