Thermionics
Newbie level 2
Electron guns, anybody found a source of new unused guns for CRTs?
There was a thread here about getting an electron gun out of an old TV/monitor picture tube.
Ideally, I'd like to find a source of new unused, old stock electron guns for my experiments.
I have obtained guns from old CRTs and my preferred way is to wrap nichrome wire round the neck where I want to cut the tube and arrange to
pass a current through the wire till its red hot. The tube cracks in a very fine line all the way round. Air leaks in quite slowly,
so the phosphor isn't disturbed on the screen. I always do this remotely, at a safe distance with the tube covered. Imploding tubes are dangerous.
I did this recently really as a test of my high vacuum system. This comprises an Edwards EO2 vapour diffusion pump, backed by an Edwards E2M2.
The system evacuates an 18" high 12" diameter glass bell jar. (Yes it IS behind an acrylic safety cylinder!) The jar is mounted on a thick steel base plate with electrical feed-throughs and there is a large diameter cooled Edwards high vacuum valve between the chamber and the diff pump.
Once pumped down (which takes about an hour) I turned on the heater and applied an EHT of 1kv. Initially I connected the focus anode to the EHT, so the
spot I got was about 1/4" in diameter. I was quite surprised that the oxide cathode had survived being back at 1 atmosphere.
From this experience, it seems that ex CRT guns can be used for experiments, though how long their cathodes will last remains to be seen.
The particular CRT I used is an old oscilloscope tube with a 4" screen and an unusually open and large electron gun. There is just about enough room
for me perhaps to be able to fit a replacement directly heated cathode when and if the original one fails.
I am able to get very reasonable emission from
nichrome wire that has been coated with a 50/50 mixture of strontium and barium carbonates. These carbonates are not particularly hazardous. I apply the
50/50 mixture as an aqueous slurry to a slightly heated nichrome wire so it dries quickly and leaves a nice even white coating. Once a high vacuum is established the wire is strongly heated and, although completely dry, a considerable rise in the diff pump's foreline pressure is seen. This indicates the breakdown of the strontium and barium carbonates to their respective oxides, liberating a small amount of CO2 (carbon dioxide). After that, when heated to dull red, the wire is highly emissive and a thermionic voltage can be seen between the wire and an adjacent plate.
The attached pic is of my ex CRT gun in the bell jar. The glow from the heater is on the left and the bright green spot is the resulting electron beam hitting
a ZnCdS screen.
There was a thread here about getting an electron gun out of an old TV/monitor picture tube.
Ideally, I'd like to find a source of new unused, old stock electron guns for my experiments.
I have obtained guns from old CRTs and my preferred way is to wrap nichrome wire round the neck where I want to cut the tube and arrange to
pass a current through the wire till its red hot. The tube cracks in a very fine line all the way round. Air leaks in quite slowly,
so the phosphor isn't disturbed on the screen. I always do this remotely, at a safe distance with the tube covered. Imploding tubes are dangerous.
I did this recently really as a test of my high vacuum system. This comprises an Edwards EO2 vapour diffusion pump, backed by an Edwards E2M2.
The system evacuates an 18" high 12" diameter glass bell jar. (Yes it IS behind an acrylic safety cylinder!) The jar is mounted on a thick steel base plate with electrical feed-throughs and there is a large diameter cooled Edwards high vacuum valve between the chamber and the diff pump.
Once pumped down (which takes about an hour) I turned on the heater and applied an EHT of 1kv. Initially I connected the focus anode to the EHT, so the
spot I got was about 1/4" in diameter. I was quite surprised that the oxide cathode had survived being back at 1 atmosphere.
From this experience, it seems that ex CRT guns can be used for experiments, though how long their cathodes will last remains to be seen.
The particular CRT I used is an old oscilloscope tube with a 4" screen and an unusually open and large electron gun. There is just about enough room
for me perhaps to be able to fit a replacement directly heated cathode when and if the original one fails.
I am able to get very reasonable emission from
nichrome wire that has been coated with a 50/50 mixture of strontium and barium carbonates. These carbonates are not particularly hazardous. I apply the
50/50 mixture as an aqueous slurry to a slightly heated nichrome wire so it dries quickly and leaves a nice even white coating. Once a high vacuum is established the wire is strongly heated and, although completely dry, a considerable rise in the diff pump's foreline pressure is seen. This indicates the breakdown of the strontium and barium carbonates to their respective oxides, liberating a small amount of CO2 (carbon dioxide). After that, when heated to dull red, the wire is highly emissive and a thermionic voltage can be seen between the wire and an adjacent plate.
The attached pic is of my ex CRT gun in the bell jar. The glow from the heater is on the left and the bright green spot is the resulting electron beam hitting
a ZnCdS screen.
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