Special Purpose Tubes

Thursday, July 1st, 2021 - Basic Electronics

Special Purpose Tubes

Some special-purpose tubes are intentionally constructed with various gases in the envelope. For instance, voltage regulator tubes contain various inert gases such as argon, helium or neon, and take advantage of the fact that these gases will ionize at predictable voltages. The thyratron is a special-purpose tube filled with low-pressure gas, for use as a high-speed electronic switch.

Tubes usually have glass envelopes, but metal, fused quartz (silica), and ceramic are possible choices. The first version of the 6L6 used a metal envelope sealed with glass beads, later a glass disk fused to the metal was used. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The nuvistor is a tiny tube made only of metal and ceramic. In some power tubes, the metal envelope is also the anode. 4CX800A 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 8974, a water-cooled tetrode capable of dissipating 1.5 megawatts. (By comparison, the largest power transistor can only dissipate about 1 kilowatt). A pair of 8974s is capable of producing 2 megawatts of audio power. The 8974 is used only in exotic military and commercial radio-frequency installations.

Aplication Of Special Purpose Tubes

British Broadcasting Corporation (BBC) Research

Mazda of the UK produced a range of tubes for use in AC powered domestic receivers and other general purposes in around 1935 (the AC/ range). The British Broadcasting Corporation (BBC) used to maintain scrupulous records of equipment maintenence including the achieved life of all tubes. Their records show that a Mazda AC/HL (a triode) was removed from its equipment having achieved over 250,000 hours of service. When tested, the tube performed to the manufacturer’s specification. The BBC did not claim any record for this as this order of longevity of life was typical for this range of tubes. Repair shops stocked up on spares to meet the anticipated demand for replacement tubes, but few were ever required. Any AC/ series tube encountered today is most likely unused (and may well be in its original carton).

Transmitting Tubes

Large transmitting tubes have tungsten filaments containing a small trace of thorium. A thin layer of thorium atoms forms on the outside of the wire when heated, serving as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms diffuse to the surface to replace them. Such thoriated tungsten cathodes routinely deliver lifetimes in the tens of thousands of hours. The claimed record is held by an Eimac power tetrode used in a Los Angeles radio station’s transmitter, which was removed from service after 80,000 hours (~9 years) of uneventful operation. Transmitting tubes are claimed to survive lightning strikes more often than transistor transmitters do.

Receiving Tubes

Cathodes in small “receiving” tubes are coated with a mixture of barium oxide and strontium oxide, sometimes with addition of calcium oxide or aluminium oxide. An electric heater is inserted into the cathode sleeve, and insulated from it electrically. This complex construction causes barium and strontium atoms to diffuse to the surface of the cathode when heated to about 780 degrees Celsius, thus emitting electrons.

‘Computer’ Vacuum Tubes


Colossus’s designer, Dr Tommy Flowers, had a theory that most of the unreliability was caused during power down and (mainly) power up (nobody else believed him – but that didn’t stop him). Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the war time mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colussus Mk2 and the addition of another 500 or so tubes. Another 9 Colossi Mk2 were built, and all 10 machines ran with a surprising degree of reliability. The only problem was that the 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year – nearly all of it for the tube heaters.


To meet the unique reliability requirements of the early digital computer Whirlwind, it was found necessary to build special “computer vacuum tubes” with extended cathode life. The problem of short lifetime was traced to evaporation of silicon, used in the tungsten alloy to make the wire easier to draw. Elimination of the silicon from the heater wire alloy (and paying extra for more frequent replacement of the wire drawing dies) allowed production of tubes that were reliable enough for the Whirlwind project. The tubes developed for Whirlwind later found their way into the giant SAGE air-defense computer system. High-purity nickel tubing and cathode coatings free of materials that can poison emission (such as silicates and aluminum) also contribute to long cathode life. The first such “computer tube” was Sylvania’s 7AK7 of 1948. By the late 1950s it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours rather than thousands, if operated conservatively. This reliability made mid-cable amplifiers in submarine cables possible.

World War II

CV4501 subminiature tube

A CV4501 subminiature tube designed for use in a military radio set. The tube is a special quality type based on the EF72. It is 35 mm long and 10 mm in diameter (excluding leads).

Near the end of World War II, to make radios more rugged, some aircraft and army radios began to integrate the tube envelopes into the radio’s cast aluminum or zinc chassis. The radio became just a printed circuit with non-tube components, soldered to the chassis that contained all the tubes. Another WWII idea was to make very small and rugged glass tubes, originally for use in radio-frequency metal detectors built into artillery shells. These proximity fuzes made artillery more effective. Tiny tubes were later known as “subminiature” types. They were widely used in 1950s military and aviation electronics.

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