Even though they are a fairly dated discovery, are rare
earth based hexaboride cathode coatings made possible ultra-long-life cathode
ray tubes in color television sets and even hi-fi thermionic vacuum tubes?
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, and given there are certain cathode ray tube based color television sets
manufactured in the mid 1990s that are still running, 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?
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
sound quality wise, 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?
