In addition to Tom's comments, some additional inputs. Welcome to the history &
mystery channel...
The seminal theory and discussion of thoriated tungsten emitters was published
sometime in the late 20's - by an RCA guy I think. The basic R&D and tooling is
long since amortized! Indirectly-heated cathodes for tubes operating above
about 2.5 kV B+ didn't come along until after the end of WWII so far as I know.
Modern high-linearity, high gain cathode-type tubes including the 8874,
3CX800A7, 8877/3CX1500A7, and 3CX15,000B7 have "striped cathodes," picket-fence
style grids with conductors precisely aligned to the cathode stripes to reduce
electron interception and improve linearity, and very close spacing between the
cathode cup and grid - on the order of 20 mils in the 8877. Oxides used to coat
the cathodes emit electrons via a complex process, comprehension of which
requires a better understanding of solid state physics than mine. The oxide
preparation and application process seems to be partly science and partly black
magic - and susceptible to contamination not unlike some semiconductor
processes.
Around 1988 the manufacturer attempted to improve the 8877's cathode emission
without changing heater ratings. They changed the metal used for the "cathode
dam" - a very thin cylinder designed to support the cathode while conducting
away very minimal heat (to keep required heater power down). Unfortunately
they evidently didn't check thermal expansion properties of the new dam
material, resulting in a big mismatch between it and the cathode cup to which
it was welded. Result: at each heater turn-on the cathode dam was forced
outward by the expanding cup, and it didn't contract fully to its previous
diameter during cool-down. So it ratcheted radially outward toward the grid
(nominally ~0.020" away) by a fraction of a mil over each heater on-off cycle.
For six months to a year, evidently, 8877's in all kinds of service (ham, MRI,
etc.) failed enmass with grid-cathode shorts - typically after about 200 hours
of service... plenty of time to complete in-plant testing and burn-in,
installation in the field, and probably up to a year or more of amateur service
before the problem showed up. By the time we & our customers recognized the
failure pattern, we had at least 500 8877s of that "version" in field service.
And for nearly a year the only thing we could get to replace them was more
tubes with the same defect, doomed to fail at ~1/100th normal life. I guess
nearly all circa 1988 8877s failed this way - we know they did in MRI!
To make it worse all around, the initial tube diagnosis and "fix" took around 6
months and didn't improve the problem. Another 6 months or so passed before
good tubes started to ship again. Nothing similar has occurred since and to the
best of our knowledge the Eimac 8877 has been a model of reliability. If not
abused, 15,000-20,000 hrs is typical life. But that year-long-plus episode cost
at least one company its biggest customer and literally millions of dollars.
That's why W1AW (and anyone else) had problems with premature 8877 failures due
to shorted grids around 1988-92. 'Course we weren't making ALPHA 77s at the
time. And all this happened years before the new owner, CPI, took over Eimac so
it might even be news to them.
Can anybody shed more factual (maybe "inside") light on the 8877 cathode dam
episode? Some of us would love to know more.
73, Dick W0ID
> From: Peter Chadwick <Peter.Chadwick@gpsemi.com>
> At what emission level does an indirectly heated cathode become an
> impractical emitter?
It's mostly a function of surface area with indirectly heated tubes,
so they can have very high peak emissions. There are no hard and fast
rules for indirectly heated tubes.
They are permanently damaged (they lose emission) by either excessive
high voltage, or excess peak emission demands. Running low filament
voltage, operating the tube without enough warm up time, or drawing
too much current will ruin the emission.
> Or in other words, why thoriated tungsten for tubes
> like a 3-500Z?
Thoriated tungsten tubes saturate at about 125 mA or so per watt of
filament power. No matter what you do with other voltages, you can't
get more than that out of the cathode.
> So why thoriated? Is it
> much cheaper or what?
It is cheaper AND the tube is more forgiving. The grids are not gold
plated, so that source of trouble is gone. You can often heat the
grids until they glow without grid damage. The tube is not sensitive
to warm up time, you can apply full power and drive even before the
filament is hot without damage.
Not only are warranty expenses reduced, the production costs are
less.
73, Tom W8JI
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