Science Buddies: "Ask an Expert"

I am making a homemade x-ray machine for a school project. I am using a 2X2A vacuum tube with pins that look similar to the tube used in the experiment done on this website. On the tube I am using I have a cap by the anode and 4 pins by the cathode, 2 of which are heater pins and the other 2 have no connection. I have made a voltage multiplier circuit that is producing about 35 kV dc and I have two output wires coming out of my circuit. For shielding I am using 1/16" lead sheets. I am ready to test my tube but I am not completely sure how to attach my output from the multiplier to the tube. Could you please assist me in connecting my tube in either cold cathode mode or with heated filaments? Any help is greatly appreciated.

ayaz15m - An over-abundance of caution is warranted in this experiment. The voltages are lethal and the emission of x-radiation has a lot of potential to damage human tissue and organs. That said, I wasn't too familiar with this experiment but have a modicum of vacuum tube and high voltage multiplier circuit experience. The multipliers were typically cascade Cockroft-Walton circuits. I found some interesting things in doing some research. As you suggested, the filament can be heated or left 'cold'. One experimenter suggests that the high-voltage across the anode-cathode can be reversed. When reversed, he makes the analogy that the tube is acting similar to a zener diode once a voltage threshold is reached. I've included links to these sources. The range of 20-40 kV seems to be the required supply when using one of these half-wave rectifier tubes. One of the sources of the 'experimenter' grade tubes (United Nuclear) suggests that they sort thru many 2X2A type tubes looking for examples that emit significant X-rays. Sounds like 'your mileage may vary' with an off-the-shelf tube. I've also included a link to the tube diagram showing the connection to the cathode (pin 4). The anode (or plate as we used to call it) is the top cap.

http://www.radiomuseum.org/tubes/tube_2x2a.html

http://unitednuclear.com/index.php?main ... cts_id=592

http://uzzors2k.4hv.org/index.php?page=tubexray

Be safe! and good luck.

Rick Marz

Thanks Rick. I have decided I want to operate in cold cathode mode. I am pretty sure that I connect the positive output of my multiplier to the cap. I am uncertain about the cathode/heater pins connections. Do I connect my negative output from my multiplier or ground it?

ayaz15m - The connections to the tube should be as follows. If you look at the base of the tube, orient it so that the two smaller diameter pins are at the top, and the two larger pins are at the bottom. Starting at the larger diameter pin at the '7 o'clock' position, this is pin 1. Clockwise, at the '10 o'clock position, a smaller diameter pin is pin 2, continuing to the '2 o'clock' position is pin 3, and finally, at the '5 o'clock position (large diameter) is pin 4. Here are the assignments.

Pin 1 = Filament
Pin 2 = No connection
Pin 3 = No Connection
Pin 4 - Filament and Cathode

If you are going 'cold filament' you only have to connect your power supply to pin 4, cathode, and the anode cap connection at the top of the tube. Depending on which polarity you want to use, the standard conventional connection would be HV+ to the Anode, and HV-, or ground, to the cathode at pin 4. For reverse polarity, just reverse these two.

If you are going to use the heated filament, simply attach a separate 2.5 VAC, 1.75 amp source to pins 1 and 4.

Here is a data sheet for the 2X2A tube.

http://www.datasheetarchive.com/dl/Scan ... 173908.pdf

Exercise caution! Wear safety glasses and perhaps even gloves when firing this machine up.

Rick Marz

Thank you Rick. I tested it out with about 12 kV and got a faint blue glow.

I also have a 6BK4B(or 6BK4-B) tube which I also tried to test out in cold cathode mode. I connected the HV positive to the anode and the HV negative to the cathode which is pin 1( about 7 or 8 o'clock position) but got nothing. Am I connecting this tube correctly?

Ayaz15m - Your polarity is correct, positive to anode, negative to cathode. In a conventional application as a rectifier, with a heated filament, the tube would conduct DC current in this setup. What I don't understand totally is that the tube, in this x-ray application is simply driving the gas in the tube into a plasma condition,with the internal resistance of the power supply limiting current. That is the blue glow you observed. Also why a cold filament/cathode is often used. The comment from the United Nuclear site was that they tested up to a hundred 2X2A tubes to get one that produced acceptable emissions. It would be interesting to know what the supply voltage was reduced to in this experiment (or what the current through the tube was) to see what the loading conditions of the tube was, but very HV voltmeters are tricky. The current capability of your multiplier power supply may only be a few milliamperes,if that.

Rick Marz

Okay I connected 4 AA batteries to the heater pins and it seems to work. I have an intensifying screen, do you know how can use the tube to possibly take x ray pictures? Thanks.

ayaz15m - The Science Buddies experiment doesn't really detail how to take x-ray pictures. They cover how to use a Geiger Counter to verify that the tube is outputting X-radiation. The other site I noted above shows a setup that appears to use Dental X-Ray film to capture the X-Ray image of the integrated circuit. By the way, the filament power required as shown on the data sheet was 2 volts at 1.75 amps. A battery pack made up of 4 AA cells will have 6 volts open circuit voltage, but will not produce 1.75 amps for more than a few seconds at best. You may have to provide a different power source if you intend to run a hot filament. Here is that link, again.

http://uzzors2k.4hv.org/index.php?page=tubexray

Rick Marz

Rick,

Thanks for all your help. Both tubes worked perfectly!