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Author Topic: Can you ID this mystery tube/bulb?  (Read 28938 times)

Offline Tim

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Can you ID this mystery tube/bulb?
« on: January 09, 2005, 08:56:38 am »

Hi Everyone,

I recently acquired a couple of strange looking tubes.? I would like to get the group’s thoughts on what these could be and what purpose they may have served.? I’ve posted more pictures here:

The tubes measure about 3 inches long and around a half inch in diameter.? One end is open and the glass is ground smooth with a cork placed in the hole.? The filament looks like coiled tungsten and I wonder if this really isn’t a light source of some kind rather than a vacuum tube?? There is what looks like a mica shield mounted below the filament, perhaps a heat deflector?? The four lead out wires through the glass pinch have the same look as Dumet wire.? Two leads go to the filament while the two outboard leads both go to the cylinder element inside the tube.

Can anyone help me ID this odd tube?

Thanks in advance!

Offline Chris W. Millinship

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Re: Can you ID this mystery tube/bulb?
« Reply #1 on: January 09, 2005, 09:10:40 am »
I don`t know a lot about them but that looks very much like a neon sign tube electrode. The fact that one end is open implies it is meant to be part of something else, and neon signs are all fabricated by hand out of their component parts, shaped and "welded" together with gas torch flame. The end electrodes are certainly pre-fabricated given their relative complexity and attached once the main "body" of the sign/tube is complete.

The coiled filament is the only mystery but possibly it could be for use following or during manufacture as a way of purging the main electrodes of impurities. It appears to be coated in something, could this actually be a getter that is vaporised by heating the coil after evacuating/backfilling the completed tube? Possibly once heated/vaporised, the lead wires are cut off and high voltage electrical connection made to the other two going to the large metal electrode. Maybe could also be used in a similar way to a fluorescent tube as a preheater to aid starting?

Just thoughts really but that`s what it seems to me.


Offline Tim

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Re: Can you ID this mystery tube/bulb?
« Reply #2 on: January 09, 2005, 10:17:50 am »
Hi Chris,

Many thanks for your quick response.  I think you may have the answer I was looking for..

Offline Alan Franzman

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Re: Can you ID this mystery tube/bulb?
« Reply #3 on: January 10, 2005, 09:46:23 pm »
I agree with Chris, it's almost certainly a sign tube electrode. Whether the filament is intended to be used only once to flash the getter, or also in normal operation to ease starting I don't know. My experience is limited to the two sign transformers I have, but neither has any filament connections.

I have to wonder just how well the cork protects the getter, and how the sign fabricator is supposed to remove the cork and weld the electrode section to his tubing without spoiling the getter. Is there enough of it on the filament that only a thin surface layer is contaminated, and vaporizing it will release sufficient fresh material to clean up the new sign section and still have some capacity to "get" any more outgassing that occurs as the sign is operated?

Offline mr_big

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Re: Can you ID this mystery tube/bulb?
« Reply #4 on: February 03, 2005, 03:54:51 pm »
I have seen these before they are most defenitly neon sign electrodes they are sealed at each end of the tube
I have seen one operating before at a store

Offline Mónico González

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Re: Can you ID this mystery tube/bulb?
« Reply #5 on: February 21, 2005, 08:01:11 pm »
Could be an electrode section for a green phosphor photocopier's tube?
These tubes often are preheated by filaments and excited by HV only when the image are scanned, being turned off again once the exposure have been done, but maintaining the filaments heated during the off time.
Neon (or mercury vapor) sign cold cathode tubes usually doesn't has any kind of thermionic emitter, because, as everybody knows, the arc strikes by itself in virtue of the high voltage drop between its electrodes that causes the ionization of the gas, being not necessary any kind of "help" to start.
But in copiers, things are quite different, due to the need of use lower voltages than in cold cathode tubes, so the need of certain "priming" prior to apply the voltage accross the tube, as in regular pre-heated fluorescent lamps does occur.
This method allows also a more exactly on-off timing and a higher efficiency in Lm/W because the presence of a coated incandescent filament previously to the arc striking.
But a close looking of it, seems to show a kind of "diode" structure, in wich there is a direct heating cathode (the filament) and the anode cylinder.
It's a curious detail to note that the lead-in wires are driven to the outside of the stem separately and isolated ones from anothers, this allows that both pieces (cylinder electrode and filament) could be connected also separately to different circuits and potentials. What for? (this seems to confirm my "diode theory").
Perhaps could be it a sort of Pirani Gauge for vacuum measuring? or perhaps the electrode for a high current tube (for copiers again) that needs a strong shielding to protect the emitter coating of filaments against sputtering and damage due to intense ionic bombardment?
These are only some ideas.
In any case, they are obviously pre-mounted pieces for ulterior tube assembling

High quality pictures too, Tim!.
M. Gonz?lez.
« Last Edit: February 21, 2005, 08:17:54 pm by M?nico Gonz?lez »

Offline James

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Re: Can you ID this mystery tube/bulb?
« Reply #6 on: February 24, 2005, 01:38:29 pm »
This is indeed the electrode for a neon sign tube.  Most neon artists can bend the glass for their tubes and gas fill them, but cannot make the critical glass-to-metal dumet seals at the tube ends.  Thus the entire component in Tim's photo is a standard mass-produced part which makes the neon artists life easier.

The cork is just there to keep dust out of the assembly during transit.  All the signmaker has to do is remove the cork and then simply seal the open end of this component onto his neon tube.  It saves them a huge amount of work.  One of the principal suppliers is Masonlite,  You can buy them off the shelf to suit a variety of different tube diameters and glass types.

Best regards,


Offline jon

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Re: Can you ID this mystery tube/bulb?
« Reply #7 on: June 19, 2005, 11:57:30 pm »
I remember seeng flourescant tubes often had filaments in them. I can't remember why this is, but it has something to do with allowing them to start without High Voltage.  they do look very similar to the electrodes used for neon lights, but I too am surprised to see all the individual wires insulated from each other.