There is more radioactive thorium in a coleman gas lantern mantle
than is in a transmitter cathode. Typically, the thorium in
a cathode is alloyed in the tungsten filament to make it emit
electrons at a lower temperature.
Unwarranted fright over thorium in vacuum tubes caused DRMO to
remove vacuum tubes from radios and test equipment, and toss them
in steel barrels... breaking them, rather than sell them to the
public...
-Chuck Harris
stevenhorii wrote:
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I found this online - a reprint of an article apparently from
Antiqueradios.com on rejuvenating vacuum tubes. There is a fair bit of
discussion on cathodes and their emitting surfaces.
A bit of a concern to me regards the use of thorium as a cathode emitter.
Some caution might be advised if scrapping (by that I mean breaking) old
non-working vacuum tubes (I know - considered a sin amongst tube
collectors). Thorium is radioactive and its decay products include radon.
The article mentions transmitting tubes which are usually high power tubes
that would not likely be in an oscilloscope (?)
I did research the old TV "tube brighteners" and apparently what they did
was to boost the filament voltage. One description of them (one of them on
eBay) does mention that some TV repair places had equipment that did more
than this (maybe that B&K 467?)
Anyway, I thought the discussion in this article might be useful for this
thread.
Steve Horii
On Mon, Mar 8, 2021 at 2:40 PM Roy Thistle <roy.thistle@...>
wrote:
Hi Charles:
I read through a two decades old PHd thesis on CRT cathodes. ( as anyone
can too! There are any number of people posting here who will first
humiliate one.. but, explain things one might not understand.)
I also read through the "B&K 467 Cathode Ray Tube Restorer Analyser"
manual. (There's even a nifty picture of Karl Corn inside the front
cover... yes, those were the days!)
In my opinion, people working with CRTs in the field (TV repairmen, video
game repairmen) contributed to forming a model that was sufficient to
explain a methodology... so that engineers (at B&K, Sencore and others)
could design and build things like the 467. But to my knowledge the "model"
that was used, by those designers, was never sufficiently explained
scientifically. And the thesis referred to above offers several competing
models: about what happens as CRT cathodes/filaments age, degrade, or
become poisoned.
Just the same, I've seen several topics and posts ..,here on TekScopes ...
about, "welding filaments", rejuvenating/brightening/rebuilding, old
oscilloscope CRTs.
As far as I cant tell, a lot of what is reported on the Internet, about
"rejuvenating" CRTs ... regarding "how rejuvenation works" and "whether it
works" is foolishness.
Yes! TV repairmen did "rejuvenate/brighten" TV CRTs (using self devised
methods, or gear like the 467); but, the old-timers that I knew ... they
related that it was a temporary strategy. It allowed them to "rejuvenate"
the CRT, on site, while they ordered in a new (or rebuilt) CRT.
Rejuvenation didn't last very long!
There might be something different about the CRTs Tektronix used (and
made) that might allow "rejuvenation."
But bear in mind that as far as I can tell, no CRT was designed to
reliably withstand the peak pulse power (or the relatively large average
current/voltages) that rejuvenation gear (or home-brew hacks) apply between
the cathode and grid (or to the filament).
Best regards.
--
Roy Thistle
