Vacuum tube


In electronics, a vacuum tube, an electron tube, or valve (British usage) or, colloquially, a tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
De Forest's original device was made with conventional vacuum technology. The vacuum was not a "hard vacuum" but rather left a very small amount of residual gas. The physics behind the device's operation was also not settled. The residual gas would cause a blue glow (visible ionization) when the plate voltage was high (above about 60 volts). In 1912, De Forest brought the Audion to Harold Arnold in AT&T's engineering department. Arnold recommended that AT&T purchase the patent, and AT&T followed his recommendation. Arnold developed high-vacuum tubes which were tested in the summer of 1913 on AT&T's long distance network. The high-vacuum tubes could operate at high plate voltages without a blue glow.
High-power tubes such as transmitting tubes have packages designed more to enhance heat transfer. In some tubes, the metal envelope is also the anode. The 4CX1000A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 4CM2500KG, a forced water-cooled power tetrode capable of dissipating 2.5 megawatts. By comparison, the largest power transistor can only dissipate about 1 kilowatt.
Vacuum tubes may develop defects in operation that make an individual tube unsuitable in a given device, although it may perform satisfactorily in another application. Microphonics refers to internal vibrations of tube elements which modulate the tube's signal in an undesirable way; sound or vibration pick-up may affect the signals, or even cause uncontrolled howling if a feedback path develops between a microphonic tube and, for example, a loudspeaker. Leakage current between AC heaters and the cathode may couple into the circuit, or electrons emitted directly from the ends of the heater may also inject hum into the signal. Leakage current due to internal contamination may also inject noise. Some of these effects make tubes unsuitable for small-signal audio use, although unobjectionable for other purposes. Selecting the best of a batch of nominally identical tubes for critical applications can produce better results.