#16
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Response to 15GP22 Glass Frit comment
Charles,
Glass frit (you spelled it correctly) is essentially an extremely fine ground glass that flows into the microstructure of any adjacent surface (metal or glass). The frit is usually a lower temperature glass that has the ability to flow so that coalescence occurs during heating and bonds chemically to the adjacent surfaces. Temperatures as high as 500 degF (371 degC) not only helps the frit to flow but allows for rapid outgassing of the tube (mainly drive off water vapor - which sticks like hell to everything!). During manufacture high temperatures also help to anneal the glass structure. However, it could warp or buckle internal metal parts. As I recall, excess sealant exists during manufacture, so that during high temperature baking the sealant does not all pump out of the joint. However, reheating an old 15GP22 to extremely high temps would not be necessary since the original structure is already outgassed and frit seal created. If only the electron gun is replaced then outgassing may be accomplished at a lower temp, 392 degF (200 degC), within the limits even of any externally applied sealants, e.g. Varian 'Torr Seal'. The trick is to pump the tube for a long time! '73 Tom Ryan Last edited by Tom_Ryan; 12-02-2005 at 02:38 AM. Reason: Grammar and sentence structure improvements. |
#17
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Did RCA or anybody else rebuild the 15GP22 back in the 50's or 60's? I think we are also reaching the day when even 21FJP22 and 21FBP22's and other delta tubes will not be able to be rebuilt. But there are a lot more of them around so it will be a long long time before they rare and they don't seem to have the vacume loss problem as the 15G does. The way things are going in the CRT field there is a small window left for the rebuild to happen or the sad fact is there won't much of a chance in a few years. I hope people are able to rebuild some of the tubes before the art is gone.
Steve |
#18
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Last edited by andy; 12-07-2021 at 02:28 PM. |
#19
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Quote:
-Steve D.
__________________
Please visit my CT-100, CTC-5, vintage color tv site: http://www.wtv-zone.com/Stevetek/ |
#20
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I would think that bad 15GP22's in the 50's and 60's did not get rebuilt...since those sets were fairly rare there probably was enough new replacement tubes in RCA's stock to cover the need for replacements.
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Audiokarma |
#21
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Quote:
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#22
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Just out of curiousity...
What would a skid (10 or so) of 15gp22's NOS and PERFECT, in emissions and seal, be worth? (NO, I do NOT know where one is... sorry!!)
I am guessing, between 30 and 40k? |
#23
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There may be such a cache of 15GP22's somewhere so it always pays for all of us to check out old TV shops, etc. before it's too late.
Whenever I'm traveling through little towns in Texas and NM and I come across any TV shops I ask if they have any old round tubes. Last edited by Chad Hauris; 12-02-2005 at 11:08 PM. |
#24
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Quote:
In the fifties, for the motion picture industry, the Hazeltine Company developed a system for viewing motion picture internegatives as a positive on a tricolor CRT. It is known as a color film analyzer, and the first one used a 15GP22. The tube-type color film analyzers are now all gone -- the last all-tube survivor was a model made in the late fifties that ultimately used a 21FJP22. It was in India, still running in 1995. About 1996, a Mexican company traded in an old analyzer that used the 15GP22. It was totally inoperable, but had been used up to around 1975. The company from whom I got this information had an inventory of six NIB 15GP22's in stock until 1997, when management decided to toss all the tube-based analyzer inventory to make room. So along with all the other spare parts, there are six 15GP22's buried somewhere in a Los Angles landfill. So yes, chances are good that somewhere there are undiscovered NIB 15G's. |
#25
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The six spares in a landfill is a dreadful thought, but here is the flipside: Gary Hough (SoCal TV collector who passed away in '89) found two NOS 15GP22s back East. He bought them for a couple dollars, but found great disappointment in the fact that they had lost vacuum still new in the boxes. I had a similar experience with one that was in a working set, then the next time I fired it up it had failed to lost vacuum. The vacuum loss has nothing to do with how much if any time is logged through usage, so NOS is not applicable in the condition of a 15GP22.
I was told by someone that the manufacturing process involved some kind of procedure whereby the tube was cooked in a vacuum chamber without the front lens on it. Then the front lens and mask were assembled to the tube after cooking, still while inside a vacuum chamber. Zenith, not wanting to pay RCA, had to put visible screws in the mask of their 15GP22 clone because they couldn't figure out how to do it the same way RCA did... But the two known Zeniths still have full vacuum (KNOCK ON WOOD!!!) And one of the Zenith 43M20s with that tube survived being slammed repeatedly against a wall in the Northridge Earthquake... Enough to knock all the knobs off and break one of the wheels and still no glass-to-metal seal cracks! I was terrified to pull the back off that set for MONTHS after the quake, as I was convinced the tube had probably been wrecked. Finally I did and What a relief! I met one of the 15GP22 assemblyline girls about 10 years ago, and she did recall some strange details. I hate to admit that I can't remember her name. She lives (or lived) in Westwood, CA near UCLA. I did a TV repair housecall for her, and the subject came up. I will think hard to see if I can get a clue and try to pull her up on the database. But even if I find her, I think she was at least 80 then. She was a WWII widow who had originally started at RCA during the war and kept the job on due to loss of husband in war, as I recall her telling. Charles |
Audiokarma |
#26
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Quote:
Also, there are tales about RCA attempting to manufacture the 15GP22 in Puerto Rica and even Brazil. Did you glean whether the line she worked on was in Lancaster PA or perhaps there in CA? |
#27
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Last edited by andy; 12-07-2021 at 02:28 PM. |
#28
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Quote:
Charles |
#29
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15GP22 Picture Brightness
Quote:
The real question to ask is: How much residual gas pressure does it take to ruin a CRT, i.e. make the tube too dim to watch? Well, it seems to me that the question depends on both electron gun efficiency and ultor voltage field intensity accelerating the electrons towards the phosphor screen. Bright pictures require more energetic electrons, and more of them, with high kinetic energy to produce more photons as a result of slamming into the phosphor dots at high velocity, hence giving us a brighter picture. As pressure inside a 15GP22 rises to 10-6 torr the rate at which residual oxygen molecules inside the tube strikes the phosphor dot plate rises to 3.6 x 10 to the 11th power (molecules per square centimeter). That's a hell of a lot of garbage in the way of the electron stream trying to hit a group of phosphor dots while being scanned at horizontal line rates! As the pressure inside the tube gets higher towards the 10-5 torr range the picture begins to noticeably lose brightness. Above 10-4 torr the picture starts to get really dim. At higher pressure less than 10-3 torr you can expect the tube to arc over inside - especially when the ultor sits above 20KV. Now, the getter should be turning white too! Also, you don't have to be at room air pressure to internally arc the tube. Let’s put pressure into perspective: a new two-stage Edwards EM8 rotary pump (pretty common in most labs) after an oil change should reach 10-3 torr after 24 hours of continuous outgassing. To reach lower pressures requires either a diffusion pump or turbo molecular pump on the system to reach up to 10-6 torr to 10-7 torr range or maybe less (if a turbo pump). To get pressures lower takes either ION pumps or cryogenic pumps to reach the 10-9 range. Passive chemi-absorption techniques - hence the use of a 'getter' inside vacuum tubes can get things down to less 10-9 if the tubes seals are really good and keep it there. I recall one interesting incident as a graduate student; I once cranked the voltage up on an electron detector in a mass spectrometer quite high one day - just to see what would happen. Even at less than 10-9 torr I was able to strip enough electrons off a detector plate once it reached 40KV to create a massive arc inside the vacuum chamber! Ka-Boom!!! The SS chamber didn't implode - just made a heck of a loud crack ...I jumped, probably 5 feet and ducker under a table. Yet, it always amazes me how modern large CRTs can work at 35KV or higher and not cause severe ionization inside a tube over time - pressures must be very low and kept very stable inside these tubes - a definite a challenge for old tubes! ...of yes, I forgot to mention the 40KV supply I used could deliver over 100 milliamps of current ...several orders of magnitude higher than a TV chassis!!! Tom |
#30
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I know this is a bit offtopic for this particular forum, but while we're on the subject of rebuilding tubes... I've heard of people that had set up shop for rebuilding black and white tubes in their houses - from what I'd heard, it seemed simple enough, a car battery and some nichrome wire to cut the neck off the tube, replace the gun and reseal the tube, and pump the air back out. But what kind of pressures are required for a black and white tube? And I'm only referring to all-glass tubes here - the metal cone kind would have the same sorts of sealing problems. But just how low does the pressure need to be in order to have a serviceable B&W tube?
Another thing that confused me about tube rebuilds (B&W tube) - did they replace the phosphors or only the gun assembly? I always thought that the phosphors in black and white tubes degraded with use too. Kinda like the screen burn on computer terminals where something would get burned into the tube if left long enough, and occasionally you'd see a really well used terminal with 24 rows of 80 fuzzy little boxes from where the characters lined up. Wouldn't a black and white tube eventually get the same sort of phosphor degredation over the entire tube? Or does the relatively rapidly changing content on television prevent this? Basically, there's a lot that I don't know about picture tube rebuilds... Any insight? -Ian |
Audiokarma |
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