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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Thanks for the link, Les.
I think I want something like a GU5B (from memory, I've been doing a bit of research, but I'm new to valve technology).
Around 5kV and greater than 1kW. I may buy something from Ebay, but I don't want to run it close to it's limits. Knowing my luck I'll either buy a dud one, or blow it the first time I use it. The idea of a rebuildable one does appeal to me, though, and I will be running a vacuum system for other parts of the same project.
It's for a magnetron project I'm working on. (which is part of a bigger project.) I'm not sure what the peak power will be yet, but average power probably won't be much more than 1kW.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Ash Small wrote ...
I think I want something like a GU5B (from memory, I've been doing a bit of research, but I'm new to valve technology).
Yes, something like that with a massive anode design, should be ideal for conversion like that. Be aware that the tungsten filament may be a coated one, which will be interesting. Personally I am not looking forward to a filament replacement on my tube, I'm sure it will be entertaining to say the least. It is quite a massive filament compared to other tubes, but it will still be a very fiddly job.
Also be aware of the amount of work you are taking on. It looks straight forward enough, but it is the product of many mistakes and failures. This was a project born out of 'last resort'. If I had a choice to just plug in a new tube I would have.
It was a fun project though, and one that few amateurs seem to have undertaken, and the possibilities are endless.
Standard vacuum flanges are the way to go with a project like this. Brass flanges are easy to silver solder, stainless is a little more difficult, but it is ok once you get the hang of it(you must use phosphoric acid flux).
I used Aluminium flanges for the glass tube section, since that is what I had lying about, If I could do it again, I would use Stainless, especially for a hot tube (I worry it may cause problems in the future). Aluminium expands too much on heating. If I had real money, I would have had a glass section made with moulded NW-40 ends on it, as a single piece.
In fact, thinking about it, I might ask for a quote on a bespoke glass section.....
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Yes, I have seen that, or at least a table similar to it. Still waiting for the day I can order carbon nanotube velvet to play with! The experimental tubes in all the papers, used an accelerating grid, since all the various field emission tips used have only a small working voltage range.
I observed, when playing with home-built carbon fiber cathodes, that once the voltage gradient between the cathode, and first anode becomes too large, the emission becomes very unstable. However I also found that with careful construction of the carbon fiber cathode, that you could 'tune' the working voltage within certain limits.
If you pulse the tube, i.e. in flash x-ray mode, you can use what ever voltage you like.
Unlike commercial 'velvet' cathodes, I cut a small section of narrow gauge pyrex tube, into which I pulled a carbon fiber bundle, using a loop of nickel wire. It is possible to form really tight bundles in this manner, and the tighter the bundle, as well as the closer you trim the fibers to the end of the tube, the higher the working voltage will be.
It would be nice to assemble a miniature x-ray tube with just two electrodes, whether it be pulsed or CW.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I was wondering how you'd set up your carbon fibres. I'd guess it was possible that some of the broken fibre ends might taper down to the nano scale in places, but I don't know much about this material.
Here - on the off-chance that you haven't already seen them - is a paper describing the use of a grid - creating an X-ray triode - to produce fast pulses in a CNT X-ray source - - and another on 'intense emission from carbon fiber cathodes' -
I imagine that the X-ray triode could be configured as a blocking oscillator to produce its own pulses, but I haven't tried this yet.
Registered Member #4861
Joined: Fri May 11 2012, 06:21PM
Location:
Posts: 1
hi Looks a great tube modification. Do any of these tubes include the following; I am trying to get hold of; a filament transformer, 3V a.c. 50/60 hz 10A 100KV insulation, and a 100KV Triode 10mA for current control. I am new to triode choices for our project , have you ever seen a hacked one?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Anode current in constant potential X-ray tubes is almost always controlled by varying thermionic emission from the filamentry cathode - in a word, by controlling the filament current. In advanced set ups there may be a feedback loop from the anode to a filament current controller, to achieve dynamic regulation.
I said that tube current was almost always controlled by varying filament temperature/current, because there are X-ray triodes, where a grid between cathode and anode can modulate the electron flow, or switch it on and off to produce pulse modulation of the X-ray output. To the best of my knowledge, these triodic X-ray tubes are produced for low power analytic tubes - certainly the ones I have seen from Svetlana are.
Nothing would be gained by connecting the X-ray tube in series with a triode valve, which would become a serious X-ray hazard in its own right at the voltages you mention. Effective shielding and dosimetry can often cost as much or more than all the other parts put together in an amateur X-ray outfit.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Nice project Leslie, have you had a chance to do any radiography with it after the rebuild?
Info on x-ray tube design can be hard to come by, but there are some old books on archive.org with some good info. Some of it is pretty outdated, but a lot of it is still relevant. Check out "X-ray Technology" from 1930 and "X-ray studies" from 1919 . The former contains some interesting info on x-ray tube design and the latter is a collection of very interesting papers, including Coolidge's original paper on the hot cathode x-ray tube. Do you have any good tips on interesting x-ray litterature?
Also, as good beryllium window tubes are hard to come by, have you done any experiments with mammography tubes? They are totally overkill current-wise, but they are pretty common and the fine focal spot is usually 100µm.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Wolfram wrote ...
Nice project Leslie, have you had a chance to do any radiography with it after the rebuild?
Info on x-ray tube design can be hard to come by, but there are some old books on archive.org with some good info. Some of it is pretty outdated, but a lot of it is still relevant. Check out "X-ray Technology" from 1930 and "X-ray studies" from 1919 . The former contains some interesting info on x-ray tube design and the latter is a collection of very interesting papers, including Coolidge's original paper on the hot cathode x-ray tube. Do you have any good tips on interesting x-ray literature?
Also, as good beryllium window tubes are hard to come by, have you done any experiments with mammography tubes? They are totally overkill current-wise, but they are pretty common and the fine focal spot is usually 100µm.
Thanks!
I haven't done any radiography with this tube.... yet! Although, it has produced x-rays, at more or less the same specification as the original.
I have in fact run into a little hiccup, that I had been aware of from the outset, which was that I didn't trust the glass section I had built. The reason being the expansion co-efficient of Pyrex is far less than the sockets into which it was torr sealed. I figured if there was any point of failure, then that would be it. During re-activation of the filament the glass cracked :( This is pretty unfortunate, but not unexpected, and there is a plan B in progress, which I noted earlier up the post.
plazmatron wrote ...
In fact, thinking about it, I might ask for a quote on a bespoke glass section.....
I have located a glassware company that will produce a single glass piece, with NW-40 flange ends, at a really reasonable price. Being all glass it requires specialist Teflon centering rings, and Teflon chain clamps, but they supply those too, also at a reasonable price.
Incidentally, since the filament was cranked way up at the time, it (although intact) is probably ruined, so will need replacement, but I have found a supplier of annealed Thoriated Tungsten wire too, so it will probably be up and running in a month or so. I will post pics, once the parts arrive!
Thanks for the links, I will check those out!
Regarding literature, I mostly look at patents, and any engineering docs that show up on google. My interest after this rebuild, is centred on carbon fibre cathodes, and low kvp (1-3kvp) radiography. I have a load of .pdf files somewhere, that I will get round to uploading to my web-server at some point.
I have never had a mammography tube, though it has crossed my mind. However that would require a total rebuild of the cabinet that houses the tube.
I am pretty determined to make this thing work, and work it shall!
Lessons have been learned from this experience, and though inconvenient, will ultimately refine the design, and provide experience that you just couldn't get anywhere else!
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Rebuilding an x-ray tube.
