INTRODUCTION: If you have been following my articles you are aware of my experiences with the Svetlana directly heated triodes used in the Berning ZOTL SET.
WHY SVETLANA AUDIO TRIODES USE THORIATED TUNGSTEN FILAMENTS
by Eric Barbour
Svetlana Electron Devices
It has been known since the 1920s that this type of filament is simpler, more reliable, less prone to "poisoning" due to impurities, and longer-lived than an oxide cathode. The only real advantage an oxide coating offers is greater electron emission at lower temperatures than a thoriated filament, thus making it more practical for low-power audio and receiving types.
Thoriated filaments are resistant to intense electric fields. This makes them far better for tubes that operate at voltages above 500 volts. The vast majority of large RF power tubes, which operate at voltages of 2000v or more, use thoriated filaments, in spite of their greater power consumption.
Compare this to the oxide-coated filament or cathode. Its operation is VERY complex and remains poorly understood to this day. Essentially, it is like a semiconductor, with barium and strontium interacting with other materials and with the base metal (usually high-purity nickel or nickel-iron alloy). Oxide coatings are prone to "sputter" onto other tube parts, possibly producing stray leakage currents (a real problem in poorly-processed tubes). And oxides are not very resistant to high electric fields, which limits the tube's operating voltages. Special coatings have been developed for HV regulator and rectifier tubes, which are less efficient than those used in common "receiving" tubes, such as the EL34 or 12AX7.
Most oxide cathodes last about 1500-2000 hours in normal service. A really well-made oxide direct-heated filament, such as that used in the Western Electric 300B, can last 10,000 hours if operated conservatively. The 300B is an extreme case, as it was specially engineered for long life. Compare this to thoriated filaments, which ROUTINELY last 5000-10,000 hours.
The world record for lifetime of a power tube is held by a large transmitting tetrode. It was in service in a Los Angeles radio station's transmitter for 10 years, for a total of more than 80,000 hours. When finally taken out of service,it was still functioning adequately. (The station saved it as a spare.) Yes, it had a thoriated filament.
If you would like to read more about tube materials, let me suggest a book: HANDBOOK OF ELECTRON TUBE AND VACUUM TECHNIQUES, by Fred Rosebury, American Vacuum Society (reissued recently by the American Institute of Physics), 1964. ISBN is 1-56396-121-0. It is available through any large book dealer, such as Barnes and Noble or Tower Books. This classic text will answer all of your technical questions, and can be trusted as an authority.
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