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Hi to All,
Hello Jeffrey,
Thanks! for your message.
"The problems seem to split into 2 categories. Those which may be difficult but can be solved with craftsmanship and practice. And those that are a lot harder."
"In the former are glassblowing skills, including the graded seals needed for Pyrex CRTs, Also the vacuum, baking and gettering techniques. These are all well known technologies, still in use for other purposes."
RACS does not do glassblowing. From their days from working with Corning Glass (RACS's founder was originally a Senior Engineer at Corning) they've kept an assortment of NOS glass bulbs, alas mainly in the 8-10 inch sizes so no "big" bulbs to simulate 12 inch CRTs. I've asked them to keep this glassware while huge quantities of bulbs for 19 inch & above (90° to 110° deflection angle) went to the recycler.
When several of the Pyrex CRTs developped cracks around the EHT contact, i asked them to get a quote from SIVE to know what it would cost to make a master mold for instance for a 12BP4 bulb. SIVE is a Paris company which specializes in laboratory glassware and who are experts at working with Pyrex. From previous experience with SIVE, Philippe quoted around 2000 Euros for the mold, but i never saw a formal quote. The shallow insert of the EHT metal contact is difficult to do and would probably impact the price. If the demand was high enough, it would allow to make new "Repro" 12BP4s or any other tube in popular demand.
Personally, i do not relish the idea of a new Pyrex bulb, why not use much easier to process modern electronic glass. I guess the answer is that SIVE is a Pyrex specialist and they tend to stick with what they know. They are probably not the only glass molders/blowers to exist...
"Things get a lot harder when replacement guns are not available. A spot welded gun can, in theory, be rebuilt using fairly simple equipment and a lot of skill. The gun of an EMI 6/6 was rebuilt this way in 1986. This is a complex hexode gun. It was rebuilt at Thorn-EMI Electron Tubes, now long gone, who specialised in photomultipliers. They certainly had the skills but not any specific jigs for that CRT. A gun assembly fused on to glass rods is more difficult. I don't know if a heater/cathode assembly can be replaced without dismantling the whole gun."
You've pinpointed the problem: RACS has scores of new ready to install guns but most of these are for tubes of the 60s (a few), the 70s & 80s (a lot), so they are pretty useless for our specific vintage needs.
Still, having an inventory of parts allows "to be creative" when unforeseen difficulties happen with a rebuild. I remember François R. showing me a couple of UK tubes where gun parts had been grafted from a donor NOS gun because the original was too far gone to rebuild properly.
Once you master from scientific training + decades of experience the interactions between electron beam focussing, acceleration, deflection angles & EHT, yo can juggle the parameters to make it work.
"It strikes me that the hardest problem is when new heater/cathode assemblies cannot be obtained. As Jerome says, you can't remove and replace a heater from a cathode cylinder. Are h/k assemblies still available? At least they are small so storing them (in vacuo?) is not such a big problem. Making new cathodes with good emissivity and long life seems difficult. The knowhow for doing this must be almost lost."
RACS has a huge inventory of filaments/cathodes/heaters & getters. As for new production, it has really ground to a stop for consumer CRTs. There are still specialists producing parts for high power Broadcast tubes, particle accelerators used in physics - heavy use of Klystrons here, etc.
Actually over time, the industry of CRTs simplified the product range. Philippe told me that today there are only 2 sizes of cathodes, so it it quite possible to salvage NOS ones from much more recent guns if you need a part and have no inventory.
In their opinion, there might be suppliers of new parts in China and India, after all China and Russia produce today copies of favourite audio output tubes for the hi-fi market. The hitch is that with the company scaling down to near closure, they have no incentive to meet companies in faraway lands & assess the quality of unknown parts. Also, any order would have to be in high volume, probably on the order of a 1000 at least.
They also have experience in re-coating the electron-emissive paste when some really oddball tube occasionally came in. Think Mil-spec Radar/Sonar/Cockpit Display CRTs.
I don't know anything about re-screening and aluminising.
In a nutshell, you clean the entire CRT down to transparent glass, then you rebuild the entire screen assembly:
- wash out all phosphors, Dag, etc. - many wash cycles
- clean with a diluted mix of hydrofluoric acid.
- deposit new screen - a wet suspension
- tilt CRT, pump out excess liquid
- dry
- apply a layer of cellulose for adherence of the aluminum layer
- dry
- vacuum pump & sputter aluminum
- cook to evaporate the cellulose - nasty (flammable solvents here!)
- apply Aquadag to inside of tube - not easy
By then, the bulb assembly is now ready to receive its new or rebuilt gun.
Sme nuisances that can ruin your day:
- uneven screen coating - restart from scratch
- micro-roughness of the glass on the inside surface of the screen: leads to streaks once you light up the finished tube.
- Inside Aquadag too close to gun's end: EHT flashover.
"Colour CRTs add a whole new set of problems."
Gun wise, it's triple the amount of work. Screens are inspected prior to any work because if the phosphors are damaged, nothing can be done.
No rebuilder ever deposited colour screens. Only the OEM supplier had the lighthouse projection systems for depositing the phosphors.
"I think others have mentioned an additional practical problem. We have a limited number of CRTs to use for learning the techniques. Each one is precious. When the original makers were making CRTs they could easily write off many CRTs as part of the development process."
Very true. However practical experience could be gained from rebuilding cheap oscilloscope tubes such as 3PB1, 5BP1 of which there are still ample Mil supplies dating back to WWII.
Rebuilding these screens with P4 (B&W) phosphors would certainly make Pilot TV-37 owners happy.
One last piece of news about RACS: they are hoping for a Navy order of Radar/Sonar rebuilds. This would allow them to reactivate the screen coating line. As it hasn't been used in over 6 months, the entire plumbing must be flushed in formaldehyde and re-brought up to the stringents specs needed to ensure contaminant-free screen deposition.
"It's a very brave venture. I wish all of you every succcess in keeping the craft alive."
Thank you! we will need every help we can get.
Best Regards
jhalphen
Paris/France
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