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Old 05-29-2018, 08:06 AM
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Potential alternative to frit glass and Epotek?

So I've been sort of looking into potential methods of sealing a 15GP22, 15HP22, or 19VP22 that don't involve John Yurkon's method of essentially splitting the tube in half and eliminating the ultor ring (which is admittedly the most elegant solution, but also probably not going to work with the CBS tubes), and that doesn't involve frit (a known failure) or Epotek epoxy (which may burn up in the oven).

I'm a theorist, not an experimental physicist, so I went and asked some of my colleagues what they would use to seal such a tube. Some of the guys working on next generation X-ray sources for computed tomography machines HIGHLY recommended using Pelco No. 16026 ceramic adhesive just as one would frit or epoxy, as did our research instrumentation specialist, and a friend now working in some capacity for NASA.

Has anyone looked into this as an option? It seems to be more than capable of withstanding normal bakeout temperatures, and I've been told it makes an excellent hermetic seal between most metals and glass. Thoughts?
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Old 05-30-2018, 08:44 PM
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Certainly sounds promising:
https://www.tedpella.com/technote_html/16026-TN.pdf
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Old 05-30-2018, 10:09 PM
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It says the recommended thickness is 2-8 thousandths of an inch, are the mating surfaces of a CRT flat within those tolerances?
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Old 05-30-2018, 11:01 PM
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It sounds like a decent potential solution. One thing I'd be curious about is its stability long-term. It is safe to say most to all 15GP22s are going to survive (as physical objects, but not necessarily as working parts) the next 1-2 decades, barring accidents and failed rebuild attempts, possibly the next 1-2 centuries for some number of examples. If treated with this adhesive do we know that it will last over a decade (and if so how much) without breaking down and or failing in some way?

Also, some good tubes were coated with VacSeal that IIRC is known to be ineffective...Can this apply to those CRTs too?
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Old 05-31-2018, 12:35 AM
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VacSeal

VacSeal Only works when actively pumped on during the cure process. Just applying it is like putting on a Band-Aid without removing the wax-paper strips.
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Old 05-31-2018, 06:46 AM
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Quote:
Originally Posted by Electronic M View Post
It sounds like a decent potential solution. One thing I'd be curious about is its stability long-term. It is safe to say most to all 15GP22s are going to survive (as physical objects, but not necessarily as working parts) the next 1-2 decades, barring accidents and failed rebuild attempts, possibly the next 1-2 centuries for some number of examples. If treated with this adhesive do we know that it will last over a decade (and if so how much) without breaking down and or failing in some way?

Also, some good tubes were coated with VacSeal that IIRC is known to be ineffective...Can this apply to those CRTs too?
Well this is just alumina and silicate. It's hard to imagine it breaking down to any appreciable degree given that it is essentially just a "paste" of rock. I do think the biggest threats to a tube treated with Peltron would be water and the solutions of acids or bases, but I can't imagine anyone taking their 15G with them for a swim so that's kind of a moot point.

Of course a low humidity environment for a successfully sealed tube would be advantageous, but given the propensity of the ultor flange to rust, low humidity has always been ideal.
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Old 05-31-2018, 06:49 AM
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Quote:
Originally Posted by Eric H View Post
It says the recommended thickness is 2-8 thousandths of an inch, are the mating surfaces of a CRT flat within those tolerances?
The surfaces we're trying to mate really aren't flat to begin with. The ultor flange sticks out from the glass portions at a 90 degree angle. We'd be filling in the cracks that occur near those joints. I doubt very much we'll ever stumble across a perfect material for sealing these tubes, so there is going to be some kludging in any case. The properties of the Epotek still look a bit more promising to me, except for the rather low temperature it can tolerate without breakdown.

Last edited by benman94; 05-31-2018 at 09:05 AM.
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Old 06-12-2018, 11:02 PM
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Quote:
Originally Posted by benman94 View Post
The surfaces we're trying to mate really aren't flat to begin with. The ultor flange sticks out from the glass portions at a 90 degree angle. We'd be filling in the cracks that occur near those joints. I doubt very much we'll ever stumble across a perfect material for sealing these tubes, so there is going to be some kludging in any case. The properties of the Epotek still look a bit more promising to me, except for the rather low temperature it can tolerate without breakdown.
It sounds like the Pelco material is designed to be used in between precision mating surfaces, like vacuum flanges or precision ground glass mating surfaces, however that is just conjecture on my part. It sounds like this material is similar to the VacSeal, which is also used between precision mating surfaces. Using VacSeal to just coat external surfaces is totally useless. Perhaps the Pelco product is pretty much the same.

The advantage of the 353ND from Epotech, is that it not only has a very high affinity to mating with metal and glass but it can be applied as an external coating. Although the 353ND has the disadvantage of not being able to withstand a 650F evacuation oven, it is possible to evacuate a crt at temperatures that the 353ND can tollerate, you just have to pump for a lot longer time.

Of course this has not been proven because so far we have not had the opportunity to try a low temperature pump down in practice with 353ND sealing a 15G. Sure would be nice if someone with an oven could give this theory a try before I die.
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Old 06-20-2018, 09:41 PM
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You don't have to test it on a 15GP22. Any old tube will do
for a first test. IF it fails on that tube, it fails. If it passes well,
then test further.
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Old 06-26-2018, 02:17 PM
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At the end of the day, ANY material used is going to be used outside of the intended use. The problem of the ultor flange and associated cracking isn't one that comes up a lot. For example, when building new Coolidge tubes to use an experimental cathode assembly, or more often, a new type of target, all the pieces being assembled have been built with some particular assembly process and method in mind, and then any residual leaks are sealed with another material. Even then, the system usually doesn't have to hold a vacuum for more than a couple of months at most before its useful life in terms of research is over and it heads to the scrap heap, or a cabinet or drawer somewhere.

I still think Vac-Seal is deserving of another look, as is the frit glass. Nick Williams suggested unwelding the ultor flange, separating the two halves of the tube, removing the portion of the ultor flange that was attached to the funnel end of the tube, leaving the face plate flange in place, and then using frit to seal the whole mess back together. That seems like a sensible approach.

A very long, low temperature bake out should work. I seem to recall a writeup on a pre-war British CRT rebuild in the 1970s or 1980s with a very low temp bake out, done out of necessity. If I can find it again I'll link to it or post it here.
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