Applications Of Vacuum Tubes

Friday, November 2nd, 2018 - Basic Electronics

Applications Of Vacuum Tubes

Tubes were ubiquitous in the early generations of electronic devices, such as radios, televisions, and early computers such as the Colossus which used 2000 tubes, the ENIAC which used nearly 18,000 tubes, and the IBM 700 series. Vacuum tubes inherently have higher resistance to the electromagnetic pulse effect of nuclear explosions. This property kept them in use for certain military applications long after transistors had replaced them elsewhere. Vacuum tubes are still used for very high-powered applications such as microwave ovens, industrial radio-frequency heating, and power amplification for broadcasting.

Tubes are also considered by many people in the audiophile, professional audio, and musician communities to have superior audio characteristics over transistor electronics, due to their warmer, more natural tone. There are many companies which still make specialized audio hardware featuring tube technology.

12AX7 tubes inside a modern guitar amplifier12AX7 tubes inside a modern guitar amplifier.

Tubes’ characteristic sound when overloaded (interchangeable term with overdriven) is widely used in electric guitar amplification, and has defined the sound of some genres of music, including classic rock and rhythm and blues. In this regard, tube amplifiers are typically desired for the warmth and natural compression they can add to an input signal.

Cooling Of Vacuum Tubes

All vacuum tubes produce heat while operating. Compared to semiconductor devices, larger tubes operate at higher power levels and hence dissipate more heat. The majority of the heat is dissipated at the anode, though some of the grids can also dissipate power. The tube’s heater also contributes to the total, and is a source that semiconductors are free from.

In order to remove generated heat, various methods of cooling may be used. For low power dissipation devices, the heat is radiated from the anode – it often being blackened on the external surface to assist. Natural air circulation or convection may be required to keep power tubes from overheating. For larger power dissipation, forced-air cooling (fans) may be required.

High power tubes in large transmitters or power amplifiers are liquid cooled, usually with de-ionised water for heat transfer to an external radiator, similar to the cooling system of an internal combustion engine. Since the anode is usually the cooled element, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator. Otherwise the high voltage can be conducted through the cooling water to the radiator system; hence the need for de-ionised water. Such systems usually have a built-in water conductance monitor which will shut down the high tension supply (often kilovolts) if the conductance gets too high.

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