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Today I safely evacuated a CRT. I simply applied heat with a propane torch. The hot spot weakened and the vacuum sucked a small bubble into the tube until it popped. The entire thing was uneventful. I wore a full face shield, welding gloves. I probably should have had leathers too. A strategically positioned chair was used to protect the ability to procreate.

Were I to do this again, I would chose a spot farther from the weld where the gun is attached to the rest of the tube. I might have a different (more exciting?) result in that case so choose your method carefully.

I now have an electron gun. I'm not sure what I am going to do with it just yet. I expect I will try to use it to shoot protons in more controlled fashion than my current setup.

Regards,
Jason C. Wells

Nice work, Jason. Thanks for sharing the picture. Did you practice with light bulbs?

In this context, "evacuate" means putting the vacuum in, not letting it out. Don't know if there is a good one-word verb for your accomplishment. Belljar.net talks about "the safe way to leak up a picture tube".

Here are a couple of other methods that I've used. No need to remove yoke or remove tube from cabinet (which offers some implosion shielding).

1. As described in belljar.net. Expose the glass tubulation through which the CRT was evacuated at the factory. File a tiny notch across the top of that [horizontally oriented] tube near its tip, as one does for cutting glass tubing. A sharp downward tap on the tip makes the tube break at the notch. If you are gentle the broken-off tip is held in place by air pressure. I used to deliver the tap to a vertical steel rod whose bottom end rested on the glass tip. Did that while standing on the screen side, and reaching across the top of CRT.

2. Much easier and faster: Punch or drill through the metal of the anode connection button. It's no thicker than a tin can. I bet a push pin or thumbtack is strong enough to do the job.

Venting is the most common term for bringing a chamber to atmospheric pressure.

Long time lurker, first time poster. (Sorry I haven't followed the rules and made an intro post.., I will eventually)

Not that safety is a bad thing, but you may have taken more precaution then necessary. A while back I worked for a recycling company and used to take apart a whole bunch of these for the flybacks (And would then normally just break the vacuum seal for fun, sometimes hitting off the tube with a hammer. I wouldn't ever recommend doing it this way, but there was never any kind of explosion that warranted a full face mask.) As rich said, it is very easy to tap the notch near the tip with a screwdriver to break the seal, and then listen to the atmosphere get sucked in.

Anyway, what prompted me to actually post something for once:

Back in my days of dismantling CRTs I eventually tried to salvage an Electron Gun to use in a future experiment (Didn't have any plans at the time). During the take apart of the CRT my employer walked by and asked what I was doing, to which I explained I wanted to use the electron gun for something. He then told me, that once the Electron Gun from the CRT is exposed to the atmosphere, it becomes useless due to oxidation of the material. At the time I just accepted what he told me, and discarded the electron gun, not to think about it again until seeing your post.

I did a very quick google search ("Reusing Electron Gun from CRT"), However from those results I can't quiet tell whether or not my old employer was wrong? (It looks like he might have been, maybe he just meant that I would have had to put it back under vacuum)

This is a black and white CRT i'm assuming?

The electron gun filament is the issue in an atmospheric regassing, (venting), as the rich electron emitting coating has its original function chemically disturbed. The filament will still work when placed back under vacuum it is just that the special coating on the filament will have its original emission grossly reduced, in most cases. It will still supply electrons once back under a proper vacuum again in your project, it will just not as much emission as it orginally did. Boosting the filament voltage to 7 or 8 volts would help, of course.

Richard Hull

Back in the old days of TV repair, I would just simply cut off the tip that the tube was originally evacuated from if the tube was going to be discarded. If it was going to be rebuilt, the rebuilder would only accept tubes still under vacuum.

I don't think I wanted to crack the fill port (woops, backwards again) because I would have to disturb the connector on the gun to do so.

I thought that the anode button was beefier than Rich reports. It looks like a sturdy co-molded chunk of metal to my inexperienced eye. I think that would be preferred. You wouldn't get any debris inside the tube if cleanliness was a concern.

I really enjoy filling things with vacuum. Emptying the vacuum out of things is fun too.

Cheers,
Jason

If your proton gun is to be used in a Van de Graaff you may want to build your own rather than use the unit from an old TV. I say this because a gun inside a Van de Graaff needs a very high voltage to eject the ions (positive) into the already positive electric field around the VdG dome. Otherwise, your current output will be very low to maybe nil, I'd think. Not sure an old electron gun from a TV could handle that job well. Considering the level of work to get the e-gun and modify it, you might be better off making a simple gun used in the original Sci. Am. proton accelerator project (yes, better guns can be mde but that isn't the critical part of the project as you and I have discovered the hard way (lol.) I do not really know what you desire for this gun but if you are still trying to get your proton accelerator (with a VdG system) to work, consider building your own proton gun. Also, consider enclosing the gas supply in the dome to reduce current lose from the VdG (you can see my pic. That was a rather easy job and will pay dividends once I ever get my VdG to produce enough current ...ugh.) From experience, I've seen my original gas line from the dome glow like a neon sign when the VdG was on thus draining away current from the dome.

Like Richard mentioned, the cathode is a problem.

If you vented it with dry argon or nitrogen it might be ok, but the BaO, SrO, CaO coating is hygroscopic, forming BaOH, SrOH and CaOH, reactivating won't work that well.
When new, these cathodes have a coating of Ba, Sr, Ca carbonate, they are activated by some extra heating, this forms CO2 and BaO + SrO + CaO. (there are some other variants, e.g. without Ca).
Tungsten and thoriated tungsten cathodes (directly heated) can handle normal air, they are not hygroscopic.

If you need an inactive CRT cathode I can send you one, but they are only practical if you replace them every time you vent your chamber, or vent with argon, but even at E-5 mbar these oxide coated cathodes get contaminated.

I use a simple tungsten cathode (12, 5W car lamp), emission of 20mA, but it gets a bit hot and it is sputter-coating my anode, I will switch to thoriated tungsten soon.

While you will get more emission from thoriated tungsten, they "wear out" after awhile. pure tungsten filaments last until they finally burn open thousands of hours later. Then again, I don't expect you will be operating the filament 24/7/365, so the lifetime of thoriated tungsten will probably be acceptable.

Bob, is it possible to reactivate thoriated tungsten after "wearing out"?

You need to heat them a white to create the thorium oxide layer, I don't exactly know what process causes the "wearing out" you mention. Is it contamination, diffusion of the thorium oxide back into the cathode? Or something else? Would reactivation help?

Although thoriated tungsten filaments are first a uniform composition of tungsten powder and thorium oxide a subsequent step is to "carburize" the filament by heating in a low pressure atmosphere of hydrocarbon vapor. This process creates a thin surface layer of metallic thorium which is a better electron emitter. However this thin layer will evaporate though ion bombardment. A poor vacuum will hasten the process.

Incorrect filament voltage will also hasten the demise of a thoriated tungsten filament. The procedure that I follow to get the most out of a transmitting tube with a thoriated tungsten filament is to run the filament at the manufactures rated voltage (as measured right at the tube socket) for the first 200 hours. Then I drop the voltage until I note the output power dropping (or in the case of a modulator tube, the distortion going up). From that point turn it up 0.1 volts. This will typically be 7.2- 7.3 volts for a nominal 7.5 volt tube. Two to three years later, for a tube in 24/7/365 service, when the emission starts dropping I turn it up. When I'm back up to rated voltage, that's when I order a spare if there is not already one at hand. When I'm at this stage, I usually can get another 6 months out of the tube before it won't do rated power anymore.

The big transmitting tubes are rebuildable. The rebuilders don't try to "fix" the filament, they just install a new one.

Bob,

isn't the carburization process creating a tungsten-carbide layer to minimize evaporation of the filament?

This text: http://electrontubestore.com/thoriated-tungsten.pdf mentions the metallic thorium forming a mono-atomic dipole layer on the surface, the thorium coming from reduction (where is the oxidizing part? the tungsten? is it ThO2 + W -> Th + WO2? )
It also mentions "the right temperature for the diffusion speed of thorium to the surface to compensate the evaporation of the thorium", so the reduction of ThO seems to be an ongoing process.

Anyway, the idea seems to be creating a tungsten carbide layer, not to reduce the ThO to Th with carbon.

If the ThO2 is reduced by tungsten, maybe the creation of WO2 is the "wearing out", or (seems more likely to me) the depletion of ThO is the causing the performance degradation, or something else...