Question about 15GP22 production

[Electronic M]

On application of the 15G that lasted a LONG time was color film viewing machines (I forget the correct name) used for doing color correction/editing of films.

I remember a page on one of the big CT-100 sites many years ago told of how a few years before a collector contacted the company that maintained these machines, the company had given up servicing the 15G equipped machines and tossed 6 NOS 15Gs in the dumpster.

[Pete Deksnis]

Originally Posted by Electronic M

On application of the 15G that lasted a LONG time was color film viewing machines (I forget the correct name) used for doing color correction/editing of films.

I remember a page on one of the big CT-100 sites many years ago told of how a few years before a collector contacted the company that maintained these machines, the company had given up servicing the 15G equipped machines and tossed 6 NOS 15Gs in the dumpster.

(1) the last machine using a 15GP22 operated until around 1975 in Mexico.
(2) Your remembrance is on my CT-100 site; here's my painful posting: What's the chance that serendipity will rescue a pallet of 15GP22s from a dusty warehouse somewhere? I've always believed it would. This next email shows it can happen and hopefully will.

"Date: Sat, 03 Nov 2001 14:16:55 -0800
Pete,
I am sick. I had no idea that anyone was interested in these old tubes.
I make color film analyzers and had the old stock from Hazeltine when
they discontinued production. There were six of those old 15 inch CRT's,
but since there were no more analyzers in the world using the tubes, we
chucked them to make room.
Regards,
Ric "

(6 Nov 01 update: Just missed these by a couple of years! The six 15GP22's had their vacuum broken, then were sent to a Los Angeles landfill sometime in 1997 in their original RCA cartons. We'll keep looking for a pallet with the next batch of 15G's.)

[old_tv_nut]

Thanks, Pete!

[bgadow]

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.)

[Scooter76]

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).

[vts1134]

Are there only two known 15GP22's not from 1954?

[Pete Deksnis]

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.

Pete

[Tom9589]

Why would they have wanted a dud if they weren't rebuilding them in 1963?

[mrjukebox160]

If RCA could rebuild them then, why can't we do it now?

[ohohyodafarted]

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.

[Scooter76]

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.

[old_tv_nut]

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.

[Scooter76]

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.

[ohohyodafarted]

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.

[Electronic M]

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?