Cathode Poisoning: Death Knell Of Thermionic Devices?

 Though largely forgotten when virtually all consumer electronic devices turned solid state at the start of the 1970s, is cathode poisoning still a concern for those who still use thermionic vacuum tube based electronic devices? 

By: Ringo Bones 

Though now virtually forgotten when virtually every consumer electronic devices we have are solid state based – including the now “affordable” organic light emitting diode (OLED) video display monitors, there was a time when cathode poisoning was of grave concern – like back in the days when the first programmable digital computers still use thermionic vacuum tubes. But as thermionic vacuum tubes returned in the high end audiophile scene, could the concept of cathode poisoning – in a strange twist of fate – inspire consumer electronic companies to design longer lasting thermionic vacuum tubes, even ones rivaling the longevity of solid state transistors and integrated circuits? 

Back in the 1950s, when vacuum tube technicians were still concerned with their tubes developing “sleeping sickness” whenever it was kept in soft-start mode for a prolonged period of time in radar and digital computer applications, cathode poisoning was of a grave concern on how to prolong the life of their banks upon banks of vacuum tubes when they are mostly switched to low current mode in switching applications. In short, cathode poisoning is the failure mode of a thermionic vacuum tube where the emissive layers degrade slowly with time and much more quickly when the cathode is overloaded with too high a current – which usually results in weakened emission and diminished power of the vacuum tubes or brightness of the cathode ray tubes – i.e. CRTs. Given what every “thermionic vacuum tube experts” had learned through such first hand events, were there any progress made in prolonging vacuum tube life and making cathode poisoning less of a concern? 

Even though there are various rare earth and halogen compound based cathode coatings that prolong thermionic vacuum tube life that make cathode poisoning much less of a concern, in the rather conservative world of thermionic vacuum tubes primarily designed for audio applications, manufacturers are choosing the tried-and-true from a sound quality perspective but older thoriated tungsten filaments which was discovered in 1914 and made practical by Irving Langmuir in 1923. A small amount of thorium is added to the tungsten filament. The filament is heated white hot at about 2,400 degrees Celsius and thorium atoms migrate to the surface of the filament and form the emissive layer. Heating the filament in a hydrocarbon atmosphere carburizes the surface and stabilizes the emissive layer. Thoriated tungsten filaments can have very long lifetimes and are resistant to high voltages. They are used in nearly all big high power vacuum tubes for radio transmitters and in some tubes for hi-fi audio amplifiers. Their lifetimes tend to be longer than those of oxide cathodes. 

Due to concerns about thorium ionizing radiation emission – i.e. radioactivity – and toxicity, efforts had been made to find alternatives. One of them is zirconated tungsten where zirconium dioxide is used instead of thorium dioxide. Other replacement materials include various rare earth oxides like lanthanum (III) oxide, yttrium (III) oxide, cerium (IV) oxide and other mixtures. 

Various rare earth borides had been used to prolong the life of thermionic vacuum tubes but there’s no news yet on how they affect sound quality of vacuum tubes when used in audio applications. Like boride cathode vacuum tubes that use lanthanum hexaboride and cerium hexaboride as coating of some high-current cathodes. Hexaborides show low work function around 2.5 eV. They are also resistant to cathode poisoning. Cerium hexaboride cathodes show low evaporation rate at 1,700 Kelvin than lanthanum hexaboride, but becomes equal at 8,800 Kelvin and higher. Cerium hexaboride cathodes have one and one half times the lifetime of lanthanum hexaboride cathodes due to its higher resistance to carbon contamination. Hexaboride cathodes are about 10 times as bright as the tungsten ones and have lifetimes up to 10 to 15 times longer. They are used in electron microscopes, microwave vacuum tubes, electron lithography, electron beam welding, X-Ray vacuum tubes and free electron lasers. However, these materials tend to be expensive. Other useful rare earth based hexabordes with long lives are yttrium hexaboride, gadolinium hexaboride and samarium hexaboride. 

Even though rare earth based hexaboride cathode coatings for thermionic vacuum tube devices may not yet be a hit in the hi-fi audio world, but I think these might have contributed in making extra long life CRTs or cathode ray tubes in television sets. Back in 1995, I’ve bought a 14-inch GoldStar color TV manufactured by LG Collins Electronics Manila, Inc. It’s a model CN-14A146 with serial number 60524212 which I bought for around 150 US dollars and it is still running until this very day. I wonder if this particular GoldStar 14-inch color TV uses a rare earth based hexaboride cathode coated CRT?