I wholeheartedly agree with John and Tom's points about transmitting
tubes and how manufacturers can easily loose the formula, sometimes
they do regain it. In my own 25 years experience in the end use of
large tubes, it would be difficult to write about all the instances
of where an Eimac or RCA/Burle tube has suddenly experienced short
life. Each was traced to a single batch or period of manufactuing
where a process was changed, a test set was modified, a step was
missed, a worker retired, or a plant was sold and relocated. Working
with these companies, (and the others are probably similar, i.e., E2V
- Marconi, Thales - Thomson, Amperex - Covimag, Toshiba, the
Russians, Czechs, etc) one has to be a detective, an evesdropper, an
engineer, and learn about tubes and their design, in order to stay on
top of the curve and have success.
Some of the mistakes cost us hundreds of thousands of dollars in lost
time, rework, warranty claims (including filling out those little
forms packed with each tube and getting an RMA # to return them), and
lost confidence of our end users of the RF amplifiers. But in the
end, many did finally come clean as manufacturing problems. I am not
blaming them entirely, though, as I have my share of boo boo, some
with exploding results.
73
John
K5PRO
TR said:
>As difficult as it is for some to believe, transmitting power tubes are very
>troublesome components. They are very much material sensitive, and
>manufacturing process sensitive. It is almost like a black art, as anyone
>who has been around tube manufacturing would know. Even small receiving
>tubes operating in low-voltage systems were the most likely components to
>fail in old tube equipment.
JB said:
>Just a few years ago there was a problem with some 3CX800A7s. I kept
>hearing from Eimac that we (while I was at Alpha/Power) were the only ones
>having problems. Couldn't find any reason why ~30% of the new tubes
>would fail within about 100 hours of use. They just got OLD very
>fast with low
>emission and high grid current. Fortunately (or unfortunately--depending on
>your perspective) K1FO at Lunar Link was having the same problem and we
>started comparing notes. Steve did some excellent "CSI" detective work
>and discovered that the tubes that would fail early would also put out full
>output
>at a more reduced heater voltage than the long-lived ones. In effect the
>heater
>was enough different that the tube would act as if the heater was run
>at excessively high voltage. Eimac subsequently solved the problem
>(to the best of my knowledge--I dropped out of the debate to have
>open-heart surgery!) and I believe that the current 3CX800A7s are
>much better. But this issue persisted for more than a year--resulting
>in Alpha amps working fine during their 48+ hour burn-in, but failing in the
>field within weeks of being delivered.
>
>But the issue added a lot of GRAY hair--although I cannot make a claim
>that the heart condition was Eimac's fault! ;-)
>
>73--John W0UN
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