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Old 04-10-2017, 04:49 PM
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Crt cathode meterial

So crts have cesium alloy for the cathode meterial what's to say that the cesium if changed or recoated would the tube work again. I know there should be a coating over the cathode meterial to keep it from being contaminated by air so if this is done and pull a vacuum , would it work, without a getter.
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Old 04-10-2017, 05:20 PM
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With no getter, would you be able to achieve sufficient vacuum for the electron beam to function?
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Old 04-10-2017, 05:45 PM
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You'd probably have gas. Especially if you don't bake the tube during evacuation...Also HVAC vac pumps can't pull enough vacuum to be acceptable even with a gether in most cases (if that was what your planning).
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Old 04-10-2017, 05:55 PM
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You'd probably have gas. Especially if you don't bake the tube during evacuation...Also HVAC vac pumps can't pull enough vacuum to be acceptable even with a gether in most cases (if that was what your planning).
Well I will try anything once if it didn't work that's ok too. I know the tube should be hot for awhile and it helps the vacuum when it cools. I don't even know what the cathode meterial is coated with to be heated off later after its done because it cannot come in contact with air so how would this cesium be put on, brushed and would there be a specific time it can be handled in air befor having to be coated. And what exactly is the getter made of, does it stick on the inside of the tube glass and then hit with high megnetic field to burn it on.
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Old 04-10-2017, 07:17 PM
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The getter is a reactive metal like magnesium....I hear some (home made radio tube) experimenters have taken ones from junk tubes and re-flashed them with RF induction heating rigs.

They coated the cathodes with some mix of the emissive material and another substance that prevents it from reacting with air (do some searches on vacuum tube and CRT making for the recipe). The stabilizer is burned off a during the final stages of evacuation or after IIRC.
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Old 04-10-2017, 08:33 PM
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You can bet that tube factories did not waste any money on doing unnecessary processes, so the answer to "will it work without (x)" is generally no.

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Old 04-10-2017, 09:06 PM
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The cathodes didn't use Cesium, AFAIK. The cathode in most tubes is a high purity nickel structure coated with an active layer of barium and strontium (and sometimes calcium) carbonates. During exhaust, the cathode is heated, converting the carbonates to their corresponding oxides.

Cesium was occasionally used as a getter material in some tubes, and it is used as part of the photocathode in some camera tubes, phototubes, etc.
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Old 04-10-2017, 09:26 PM
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The cathodes didn't use Cesium, AFAIK. The cathode in most tubes is a high purity nickel structure coated with an active layer of barium and strontium (and sometimes calcium) carbonates. During exhaust, the cathode is heated, converting the carbonates to their corresponding oxides.

Cesium was occasionally used as a getter material in some tubes, and it is used as part of the photocathode in some camera tubes, phototubes, etc.
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Old 04-11-2017, 02:22 AM
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This is interesting actually. We really need to have a comprehensive source of information for making/rebuilding tubes... Enough of the knowledge has been lost already, it wont be long before it's all gone forever.

Nickel for a cathode seems easy enough, but is it plated with barium or like just painted on?
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Old 04-11-2017, 11:02 AM
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Quote:
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Nickel for a cathode seems easy enough, but is it plated with barium or like just painted on?
I believe a dot of a emissive paste is applied to the nickel cathode. Once in a vacuum the cathode is heated, the CO2 (+ H2O, etc.) is removed by the pump, leaving oxides behind. If exposed to air the oxides re-combine to for carbonates again but can also be contaminated but other things.

Rebuilding a CRT is very complicated, like rebuilding a bathroom from scratch. Many different skills to get right.
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Old 04-11-2017, 11:11 AM
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Well while the materials may be available I think I will just let this thought of mine go since there seems to be a check list of things to do while other things are being done rebuilding a tube and if one is left out or forgotten then it was time lost including the tube .
It's not as simple as it may seem, that's for sure so maybe I'll wait for E T F maybe to rebuild any tubes I may have which is only 7jp4 tubes but there are no parts for them which then the gun would have to be rebuilt rather then just changed. from what I understand the cathode meterial was said to last a hundred years so why are there so many tubes with low or no emission due to cathode meterial.

Last edited by timmy; 04-11-2017 at 11:14 AM.
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Old 04-11-2017, 11:30 AM
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from what I understand the cathode meterial was said to last a hundred years so why are there so many tubes with low or no emission due to cathode meterial.
That may be the shelf life, but CRTs and most vacuum tubes in general only have a service life (hours powered on) of 10-30k hours IIRC.
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Old 04-11-2017, 12:44 PM
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I would suspect average receiving tube life to be substantially less than 10,000 hours.

Tubes with a 10,000 hour guaranteed life were available (RCA "Special Red", Mullard "10M series", etc.), but they were expensive premium stuff reserved for critical industrial/military applications.
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Old 04-11-2017, 12:49 PM
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No, no and no.

Won't work without a getter, cesium is not part of the equation, and there's nothing you can do to reactivate a cathode or getter once fired/activated.

This question belongs in the rebuilding forum anyway, perhaps a mod can move it.
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Old 04-11-2017, 12:51 PM
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DuMont rated the 20BP4, 15AP4, amd 12JP4 CRTs at about 5000 hours life. This would correspond to 5-6 years, if used for 3 hours a day 300 days per year. That seems about par for the course based on CRT replacement tags I've seen.
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