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Re: [Phys-l] x-ray machine



For one
thing, you need to be somewhat careful about handling the
resistor, because fingerprints are conductive and might short
out the resistor. It's not a super-big deal; you just build
the circuit and /then/ clean off the fingerprints with alcohol.

The very high resistance resistors I have are in EHT meter multipliers. the are, probably metal film circular strips on ceramic, IIRC, all enclosed in a glass tube -- thanks for reminding me to keep it clean -- like Si-halogen lamps.

On 2009, Jul 08, , at 08:40, John Denker [also] wrote:

There are /some/ tubes that use a regulated (electronic) voltage
supply and then regulate the current using a grid, but I'm not
sure how widely deployed they are.



Of this I have some knowledge as I've repaired a GE XRD-5

Detail hazy as it's been 15 years -- the current is detected, likely by the PD "across" a resistor (obviously near the grounded terminal!). That EMF is used by a regulated P/S to control the power (temperature) of the Joule self heated (for "quick" response) kathode. In the case of the XRD, the current is variable from about one milliamp. to 50. The Anode potential is from, IIRC, about 10 to 50 kV. This potential for the reason previously (later?) given (emission limited current) is not regulated. Water cooled tubes easily replaced -- Cu Fe W, etc. anodes. Incidentally, the original P/S used vacuum diodes later models used SS diodes, the heater winding connections left dangling. I also have a Tesla type TEM P/ S. The primary driven by, I presume, a P-P tube oscillator -- one turn Cu tubing the secondary is several pancake coils. Again the whole thing was in an oil filled tank. It did have HT adjustment or at least regulation; I've lost the manual. Some one may perhaps explain why one side of the Al tank had a thick Cu plate, which I used as a non-depolarizing projection screen. (My stereo slides.) Now superseded by a daylite screen.

I have two dental X-ray systems. the very old one is the tube and a big oil filled can with a terminal for the heater and the HT one. The tube is huge (like the old Welch e/m). Some day I'll try firing it up (no reason to believe it is defective. Tother is claimed unrepairable, so I foolishly disassembled it. From the conversation I had w/ the company it's assembled in a glove box after the oil is vacuum prepared. The HV supply is only an 80kV transformer and filament winding. Everything is in one unit.

bc to quickly disassembles.


Recently Stanford surplused its XRD-5 to UCSC -- the bureaucracy was horrendous! They used it, inter alia, to duplicate Röntgen's historic "proof" they were transverse, i.e. double scattering w/ carbon blocks. I wanted to intro. this in "our" advanced lab. but never prioritized it sufficiently. We could do it safely as the machine is surrounded by a large SS shield w/ sufficient room inside.