Ceramic/metal construction is usually very vacuum tight, with exceptions
due to mfr mistakes. Somewhere I have a paper from Eitel-McCoulough
extolling the virtues of their new "CX" tubes back then.
Glass to metal seals are the earliest type, learning first from the
Edison bulb ideas to Mr. Deforest, with improvements by Mr. Housekeeper
at Westinghouse and a long line of GE improvements. Sealing metal to
molten glass edge is an art, and requires good glass blowers/turner.
I don't believe that vacuum seal leaks are due to glass 'sagging' over
time, but just due to longevity of those seals themselves. Corrosion of
the metal is one factor. Temperature extremes between the metal and
glass is another (from overloading).
The major tube manufacturers like Eimac/CPI, RCA/Photonis/Burle and
Thales/Thomson have dropped all glass tubes from production in the past
decade for reasons that it was getting very difficult and costly to by
the glass blank cylinders from companies like Shott Glass of Germany. I
know one scientific user in UK that uses a big glass pulsed-megawatt
triode, they had to by a 20 year supply of the glass for a major tube
manufacturer to guarantee a supply of the tubes. Where I work, we needed
a source for 120 KV switch tubes, small planar triodes not much large
than ham sized. We requested glass, like Eimac used to make in Salt Lake
City, for reasons of improved HV standoff (when in oil), not with RF of
course. They did make them for us in Palo Alto, once we had need for
more than 50 of them.
For ceramic/metal, there has been many decades for improvement. The
biggest US-produced VHF gg triode still in production is the the RCA
7835, which has pressed seals of metal to ceramic. These tubes come with
2 l/sec vacion appendage pumps connected to them. No metalization, just
tremendous mechanical force keeps those tubes tight. Eimac makes the
4CM2500KG tetrode, their largest for RF, with metallized ceramic seals.
Thales does same for their big tetrodes for 500 kW and up, like TH558,
TH576, and Th628. Metallized ceramic, when done right, is very good for
vacuum tightness. A mechanism allows for flexure of the two components -
which are closely matched thermally, where RF current is carried through
good conductors inside. The metallization allows for the seal to be
brazed tight.
BTW, its amazing to see those big tubes being lit during their
production process, where they are run very hot. Thoriated Tungsten runs
about 1900 Kelvin. I saw a 7835 in Pennsylvania at the factory where the
ceramic was bright pink, almost white. It looked sort of transparent.
The tube has a 30 kW filament,no kidding, 5 Volts DC at 6800 Amps.
I love ceramic metal tubes. As others have said, they aren't as
beautiful as glass, but a lot more rugged.
73
John
K5PRO
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