When the subject came up on TCA a few years back I pointed to the published
statement... it was generally agreed there that all companies used improved
cathodes. I don't remember whether anyone came up with a black and white
statement like the one I had.
I'll ask again.
A quick look at Kohl [the 1960 edition] reveals that the cathode is
extremely complex. Interface resistance affected all tubes but only some
circuits were particularly sensitive to it. The problems experienced in
wideband scope circuits comes to mind.
As the component production methods and controls improved I think that
better quality materials came cheaper and were able to be used wider. That
effect can be seen in most electronics - eg resistors.
The broad statements about computer tubes and interface resistance aren't
correct.
Quoting from "Electron Tube Life Factors", US Army Signal Corps Development
Lab, Ft Monmouth, 1959.....
" Cathode Interface Resistance.
Cathode interface resistance was measured by Burroughs Corporation on the
tubes of the second life test run only for dynamic and static conditions.
The tubes of the cutoff conditions were life tested during the second run
only and all of them were measured for cathode interface resistance.
Burroughs reported that continual calibration of the McNarry bridge for
measuring interface resistance was made by artificial impedance networks and
by means of stabilized calibrated tubes. Although correlation with other
interface resistance tests were not fully satisfactory, the results were
consistent and reproducible on the equipment used.
The values of interface resistance measured at the end of the 5000-hr life
test are listed in Table 12-10 for the three test conditions in the order of
increasing silicon content in the cathode sleeves. The pattern of growth of
interface resistance is not uniform for all of the tube types. Only for
Types JAN-6AG7 and JAN-6AN5 did interface resistance grow to high values for
low duty factor and smaller values for high duty factor. The cathode sleeves
of these two types had the largest and the smallest amounts of silicon.
Those tubes having intermediate percentages of silicon departed from the
pattern of the high-silicon tubes and in two cases (Types JAN-5687 and
JAN-12AT7) the tubes on cutoff operation had the smallest values of
interface resistance. This is the reverse of what has come to be expected."
The paragraphs following this discuss the spectrographic examination and the
correlation to the actual tubes tested. Some of the data "should be assumed
to have questionable validity".
But a conclusion was still stated..." The evidence of this life test and
spectrographic analysis does not indicate any conclusive pattern of growth
for interface resistance."
It is interesting that the results of the JAN-12AT7 surprised them.
"The pattern of interface resistance growth was unusual in that th etubes on
cutoff life test conditions developed the smallest resistance, 2 ohms,
during the 5000 hr. The tubes life tested under dynamic conditions, which
were equivalent to the cutoff condition in this task, developed 46 ohms of
interface resistance. Surprisingly, the tubes life tested under standard MIL
conditions developed the highest interface resistance. This was 220 ohms at
5000 hr. The reason for this behavior is not readily apparent. The cathode
sleeve material was analyzed by Burroughs Corporation and found to contain a
small percentage of silicon, 0.016 percent. This amount is small enough for
the cathode to be considered to be made of passive material and not to
contribute greatly to the growth of interface resistance."
This brings me back to the complexity issue. The mechanisms were not fully
understood or characterised. For some tube type results the conclusion was
along the lines of eg JAN-7AK7 "The silicon content of the cathode sleeves
is relatively low at 0.015 percent, and this undoubtedly contributes to the
low interface resistance values in all of the tube lots."
It is an interesting 173 page book.
Interface resistance is a problem, to varying degrees, in dynamic circuits.
Manufacturers would have striven to produce the 'perfect' cathode - I want
to see more hard data. Obviously costs have to be considered, but
manufacturers can't afford to be plagued with erratic unexplained failures
and 'modern' production techniques rely on statistical control. The complete
short term failure of the very important proximity fuze program is an
example that did occur. One manufacturer was able to make reliable filaments
[much to the chagrine of the competitors].
However, when that particular single billet of tungsten was exhausted that
manufacturer began to suffer the same failures. The not-so-large piece of
tungsten made an incredibly large number of filaments; the wire was so fine.
John Kaesehagen
Australia.
jk
----- Original Message -----
From: "John Rehwinkel" <jreh...@mac.com>
To: <neonixie-l@googlegroups.com>
Sent: Friday, February 01, 2013 2:00 AM
Subject: Re: [neonixie-l] Re: MTX-90 in counting circuit?
I have a brochure from a British valve maker saying that once they solved
the interface resistance problem for computer valves they used the same
cathodes in all the valves.
That seems unlikely for a large valve maker, but maybe this was a smaller
outfit. The truth is that large valve makers added silicon to the cathode
sleeve mix to make the cathodes "activate" faster. Faster activation means
less time on the activation station, which translates to lower production
cost and higher profit. The drawback is that adding silicon was what caused
the "sleeping sickness" in tubes held in cutoff for long periods, so the
computer grade tubes cost more to make, as they were made without the
silicon. However, money could be saved by using old punches, winders, and
so forth, as linearity was not an issue (this is why computer grade tubes
are generally not a good bet for audio or other analog/linear use).
A smaller outfit, however, may not buy enough cathode sleeve alloy to have
two different formulations, and isn't as sensitive to high-volume
production, so it could afford to just let all their tubes activate more
slowly, and thereby use the the same allow for all their tubes. And
naturally, advertise this as an advantage!
- John
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