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  #16  
Old 11-19-2017, 08:52 PM
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RCA, as would any legitimate builder/rebuilder, would have labeled the crt as rebuilt in some way. I don't know when they started using the "Grade A", "Grade B" etc system. At least it would mention used envelope, new gun, etc.

Were the date code on a paper label I would speculate the date would indicate when it was packaged, but with it on the socket I think it more accurate. I have seen tubes with 2 dates on them (all the tubes in one of my Portacolors were like this.)
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  #17  
Old 01-18-2018, 12:57 AM
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Quote:
Originally Posted by Dave A View Post
Just my opinion. The whole 15GP22 production was so crude with the flat plate, early phosphoring, physical alignment, welds, glass to metal, etc., It had to be low production and the 21 series blew that out of the water to the bean counters. I cannot imagine RCA kept that assembly line around for long. As for studio monitors it was a quick jump to jeeped up 21CT55 sets as many photos document. I think the ones on Allied, etc. were happily gassing away in loneliness on the shelves.
Dave I believe you are right on the money about the length of time RCA would have kept the 15G production line.

It is my belief that RCA probably manufactured a certain amount of surplus for replacement tube warehouse inventory. In addition, tubes that were in the field and replaced due to low emission, which were still holding good vacuum, could have been a source of candidates for re-gunning at RCA after the original production inventory run had been depleted. I believe someone posted that they actually had seen a 15G with a sticker that stated it was a rebuilt tube. So that confirms that RCA actually did rebuild some 15G's.
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  #18  
Old 02-15-2018, 06:39 PM
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One of my 15GP22's has a 1959 date code.

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  #19  
Old 02-16-2018, 02:03 PM
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John, I think this makes your tube the latest known 15GP22, as the other 1959 tube that Pete references above is from week 26.

It's also interesting that this is now the 2nd '59 tube to show up and that no other tubes between '54 and '59 have surfaced to my knowledge. Unless this is purely coincidental, this makes it more plausible in my mind that Lancaster did a second run of these in '59 to replenish replacement stock. However, one would think they would have done so over a very quick timeframe (like a week or two), not 13 weeks. Or maybe they were intermittently reworking '54 tubes that had already gone to air while sitting in RCA stock? (However, I don't recall your tube showing signs of re-necking).
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Old 02-16-2018, 07:40 PM
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Are there only two known 15GP22's not from 1954?
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  #21  
Old 02-17-2018, 03:27 PM
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RCA rebuilding 15G's?

After acquiring CT-100 B8000173 in 1963, I considered buying a spare 15GP22. At a suburban Philadelphia supplier I checked, they were available, but the purchase-price required a dud be returned.

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  #22  
Old 02-17-2018, 04:57 PM
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Why would they have wanted a dud if they weren't rebuilding them in 1963?
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  #23  
Old 02-17-2018, 10:16 PM
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If RCA could rebuild them then, why can't we do it now?
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  #24  
Old 02-17-2018, 11:48 PM
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Because only a crt that is holding a PERFECT vacuum (not a leaker) can be rebuilt. The duds that are in the hands of collectors are leakers that will not hold a good vacuum after rebuilding. Rebuilding a leaker is a waste of time. It will only produce a picture for 30 days or less, and when the vacuum degrades, the crt will once again be useless.

A method of fixing the leaks must be found before a successful rebuild will be possible.
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  #25  
Old 02-22-2018, 01:53 PM
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Bob, this is where I always get a little confused. If it's true that ANY tube found today that has now gone to air will "only work for 30 days or less" after rebuilding (which I have to believe, since you've been studying these things for years), that must also affirm that ANY such tube only began leaking after some finite event along its lifetime (failed weld, failed glass to metal bond, stress crack in the glass, etc.). For whatever reason, I can't get it out of my head that at least SOME of the tubes found today that have gone to air did so at an even rate from the very moment they were made, and only at some point along their lifetime (twenty years ago, two years ago, or two minutes ago) enough air entered to finally consume all of the getter material and cause the filament to fail. If it took ten or twenty years for that to happen, then why couldn't it be assumed (unless the leak were made worse during the rebuilding process) that the rebuild would not last equally long? Not doubting you here - just wanting to understand why that's not possible or even likely for some tubes.
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  #26  
Old 02-22-2018, 02:45 PM
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Bob's the expert, but I believe part of the problem is that the stresses change irreversibly when the tube is processed the first time, and subsequent cycles just make things worse.
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  #27  
Old 02-22-2018, 03:06 PM
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I fully agree - to the point that I wouldn't lay any money that a tube with "perfect" vacuum prior to rebuilding doesn't stand a substantial risk of becoming a leaker afterwards. People talk about using vacseal, not subjecting a good tube to temperature or humidity changes in situ, etc. Yet, these stresses pale in comparison to the beating these weak points will need to survive during a successful rebuild process.
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  #28  
Old 02-22-2018, 09:02 PM
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WRT: the tubes still under vacuum; We have rebuilt tubes that were under vacuum, and they developed a leak big enough to spoil the vacuum withing 30 days. Another builder out in California tried the same thing many years ago and ended up with the same result. They will go to air in about 30 days.

Tubes that have lost vacuum are even more problematic. It does not matter if the tube was good for 50 years and then suddenly sprang a leak, or it leaked very slowly for 50 years to the point where it had to much air in it. The fact is that it has a leak. A leak is a leak and even if it is a slow leak when you put it in the oven to evacuate, the heat and stresses of pulling a vacuum will make the leak even worse.

At some point down the road perhaps we might be able to seal the tubes with a special epoxy that we have experimented with in the past. The issue is that we will need to do a very long low temperature pump down so that the special epoxy does not burn up. But to do that we will need a facility that can do the experimental work. When Hawkeye closed it's doors several years ago, we lost our last good rebuilding facility where we could go and conduct experiments.

Until we have a facility with good equipment in which we can conduct experiments, we are going to be at a standstill with the 15G project.
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  #29  
Old 02-22-2018, 09:18 PM
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I wonder if it would be possible to complete all the steps of evacuation short of pinching off the evacuation tube then connect a helium leak detector and inspect the tube/try to correct new leaks before the final pinch off?
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  #30  
Old 02-22-2018, 11:03 PM
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Tom,
Even if no leaks were found, When you finally put the tube under vacuum and heat, this is the point where the leaks are created. The heat and physical vacuum stress, causes weak areas (usually in the glass) where micro cracks form at the juncture of the glass and the ultor ring.

It is an easy matter to find out if the envelope is leak free before evacuating. The leaks are created during the evacuation process. Even if you could test for a leak during evacuation during pump down, prior to pinch off, What good would it do? You could shut down, but you still have a new gun installed in a envelope that has a leak. I guess you might be able to salvage the gun and stem assembly by cutting it of and possibly re-using it on another tube. But we are better off finding a method of reliably sealing the envelope. Without a method of reliably sealing the envelope so that it doesn't leak during evacuation, attempting rebuilds of 15G's is pointless and we are just crossing our fingers that maybe 1 in 10 attempts might yield a good tube.

The one tube that RACS was successful with was a tube that did not have any leaks to begin with. It was pure luck that It did not develop any new leaks during the evacuation. It was just pure dumb luck. But it does demonstrate that it can be done under the right circumstances. RCA was rebuilding duds that were not leakers.

That's why we are hoping that a long, slow, low temperature pump down, combined with a special high temp epoxy that we have worked with, may help to lessen the possibility of causing new leaks in an envelope that is leak free prior to evacuation. It may even be able to seal micro fractures that exist in tubes that have a slow leak.

The epoxy would be applied in a manner similar to what RACS did with frit glass in France. However their attempts to seal with the frit were utter failures. IMHO frit was not the correct approach. Frit is designed to be placed between 2 precission ground glass surfaces. RACS thought it might work by slathering it on the outside of the tube. Frit was never meant to be used that way.

And there is no assurance that we would have any better luck with a low temp pump down and the special high temp epoxy. We will never know unless we are able to do some more experiments. Perhaps some day when Nick gets his crt rebuilding facility up and running.

As for me, until I am free of the responsibility of caring for my 102 year old mother, I rarely have any time for pursuing my hobby. I have care giving responsibilities 7 days/week, 365 days/year.
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