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A cathode is the electrode of an electrical device in which the electrical current flows away from the device. A hot cathode can be a cathode that’s heated directly or indirectly. The cathode’s filament is the source of the electrons in the case of a directly heated cathode. The filament is electrically insulated from the cathode in the case of an indirectly heated cathode. A hot cathode emits more electrons than a cold cathode with the same surface area.
The first hot cathodes used direct heating. They were made of pure tungsten and heated to white incandescence. Later filaments were covered with a material that emits electrons more easily than tungsten, which reduces the operating temperature of a hot cathode. Further improvements to hot cathodes that use indirect heating include filaments made from sintered tungsten, thorium-tungsten alloys and tantalum. Some hot cathodes are also shaped like parabolic mirrors.
Hot cathodes that use indirect heating have an advantage when using alternating current, because this configuration produces less hum than a cathode that uses direct heating. The filament in a hot cathode that uses indirect heating is commonly called the heater. The heater is usually made of tungsten and surrounded by a tube made of nickel.
An oxide coating on a hot cathode’s filament can reduce the filament’s operating temperature. The first choice was pure barium oxide, and later formulations used a mixture of barium oxide, calcium oxide and strontium oxide. Hot cathodes coated with these compounds have a maximum operating temperature of 1,832 degrees Fahrenheit (1,000 degrees Celsius). They are subject to rapid degradation in high voltage conditions, and they are most often used in low-power vacuum tubes.
Hot cathodes that must handle high current commonly have coatings of hexaborides such as cerium hexaboride (CeB6) or lanthanum hexaboride (LaB6). Other types of hexaborides commonly used to coat hot cathodes include barium hexaboride, calcium hexaboride and strontium hexaboride. These types of hot cathodes burn more brightly and have longer lifetimes than tungsten hot cathodes, but they’re also more expensive.
Filaments that contain a small quantity of thorium are another option for hot cathodes. The filament is heated to white incandescence in an atmosphere containing hydrocarbons, which causes the thorium to migrate to the filament’s surface. Thoriated filaments can handle high voltage and have long lifetimes. The primary disadvantage of thoriated filaments is that thorium is radioactive. Alternatives to thorium in hot cathode filaments include cerium, lanthanum, yttrium and zirconium.
@miriam98 - I don't know about the old amplifiers, but the fluorescent ballasts of today use hot cathodes from what I understand. In those cases, I think they do have protective coatings. Industrial lighting gets very hot.
I had one of these things go out on me at work. It was an overhead fluorescent lamp and one day it started flickering like it was about to die any day.
The flickering was annoying, but what was even more annoying was the electric snap, crackle and pop sounds that it made as it flickered. I thought I was going to get zapped any day.
Eventually we had the repair man come out and he said it was actually the ballast that was bad, but he just went ahead and replaced the whole unit.
I remember owning an old stereo amplifier that used cathode ray tubes. This was a hand me down and it was well before they used solid state circuitry.
This thing would work – very well actually – but the cathode tubes certainly did burn hot, and on one occasion in particular one of the tubes burned out.
I don’t know if those tubes had any of the protective material coatings mentioned in the article, but I kind of doubt it. The amplifier still continued to work, with its signature whines and squeals, but of course the day came that I had to part with it, sentimentality aside. I sold it dirt cheap at a garage sale.