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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
At the swap meet yesterday I acquired a back-up x-ray tube from a familiar trader. This one has 3 threaded posts on the cathode end, implying a separate grid (focus or on/off) terminal. An ohmeter (and hasty tests forcing 1 ampere) showed all 3 terminals "shorted" and no evidence of filament glow. Looking through a hole in the base which accomodates glass seal-off tip, I could see two of the wires twisted together. Did the tube leave the factory this way?
Perhaps too hastily I removed the base. (which was slightly loose to begin with).
Mystery solved. The twisted wires tie the grid to one end of the filament. Two of the base terminals are tied together by an internal jumper. With the base off, I lit the filament (it took more than 1A to glow, and about 1.5V at well over 2A to get a microamp of anode current).
Before I put the base back on, some questions: 1) is there any sort of standardization as to which pin should be the grid, if I choose to bring it out separately? (obviously it'd be one of the two that are jumpered today). 2) any hints about DIY formulas for tube base cement?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The hole in the base is to admit the free ebb and flow of dielectric oil, both when first filling the apparatus, and afterwards through thermal cycles of expansion and contraction.
There is no standardized protocol on base pin outs for X-ray tubes, which are, at times, manufactured only in tens and the low hundreds, or made to order, to suit a particular application.
Extra Not Connected (NC) pins are often added for socket location, mechanical stability, or simply for the economy of being able to use the same base across a number of products. Some tubes have flying leads, rather than plug and socket connection.
In your case, the manufacturer has twisted the focusing cup together with one side of the filament so they can offer an alternative version where the focusing cup voltage can be independently controlled.
Have a look at oven/stove door glass rope adhesives. Some are rated to 350 deg C if you want something cheap. Personally, I would use Waleset ceramic cement from Wale Apparatus.
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Base pinout for tube with grid? DIY tube base cement?
