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Bad tube on Geiger counter?

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LAN_403

Conundrum

Sat Feb 21 2009, 04:39PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi all.

Just noticed a problem with my Bull Electrical counter (uses two SBM20 tubes), upon replacing the battery connector it still had the annoying "bursting" noise problem.

Looked at one of the tubes and its visibly corroded at one end on the metal casing.

Reckon its bad and the corrosion has eaten through, or its my counter? FWIW the contacts seem OK, and taking the tube out restored the counter to working order.

Was thinking about ordering an end window tube

adding series zeners to drop the voltage from 390 to 320V so my counter can also read alpha particles :)

regards, -A

Proud Mary

Sat Feb 21 2009, 05:51PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

SBM20 is - deliberately, for its design purpose - a very insensitive tube.

Why don't you just test the suspect tube? First, fix the ohmeter to both electrodes and give the tube a good shaking to see that there is no short circuit or ratttling sound. The reading should stay above 1G ohm, or infinity, during this test.

Apply a bias voltage to the anode through, say, 4M7, and collect an output signal off a 100K cathode resistor. Couple the top end of the cathode resistor to an amplifier with 50pf or 100pF. Very very slowly increase the bias voltage until you hear the tube begin to click. If the tube is healthy, this point will be the beginning of the Geiger plateau. Continue slowly increasing voltage until the tube suddenly starts counting like mad. This is the end of the plateau. Set your operating voltage at the mid-point along the plateau for best performance.

Avoid taking the output signal off the anode - which one sees in many bad amateur designs - because it shortens the tube life and blurs the pulses by adding capacitance to the anode. GM Tube manufacturers like Centronic strongly advise against it for this reason.

Lastly, I wouldn't add a costly end window alpha tube to an aging piece of Cold War memorabilia from Bull Electrical. You can make something much better yourself for not very much money using modern components.

Conundrum

Sat Feb 21 2009, 11:44PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Useful information, thanks Harry.

Also I have noticed that the "bad" tube looks decidedly grubbier than the good one. I wonder if it was actually a salvaged tube sent in error?

I will try your suggestion and see what happens :)

Regards, -A

Proud Mary

Sun Feb 22 2009, 12:28AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It's possible that some kind of corrosive acid flux was used in the former USSR, and that may have attacked the case.

Anyway, remember that GM tubes are high impedance devices, so you should keep the surfaces as clean as a whistle. Remove any corrosion with a wire brush or sand paper, clean it with white spirit/paint thinners and then with meths, and finally polish it up with a bit of silicone furniture polish. Then it will look good even if it doesn't work!

So long as the seal hasn't failed, there is not a lot to go wrong with metal GM tubes like yours.

Here is a link to the very simplest possible Geiger circuit. Set it up with what you think is the 'good' tube, and once you've got it to work, try it out with your newly cleaned 'suspect' tube. Then you'd be in a position to try out some of the more interesting - but still simple - GM circuits on the cosmic ray site:

Don't expect many background counts per minute with SBM20, as it was designed as a nuclear warfare device, and so made deliberately insensitive to low level radiation. It was produced and stockpiled in millions, and so has found its way into all sorts of hobbyist devices for which it is not really suitable.

Setting aside mica windows and so on, the sensitivity of a tube is largely (but not wholly) dependent on the volume of the gas it contains. A very small tube can never be very sensitive.

Conundrum

Sun Feb 22 2009, 08:21PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Cool pics. Swapped over spare tube and counter now works again.

BTW can someone please explain this? The paper has been in my shielded "watch" casing next to some sources for nearly a year and is one of those thermal paper bank receipts.

-A

Proud Mary

Sun Feb 22 2009, 09:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It would be interesting to look at those exposed areas under the microscope, to see if you could learn something about the nature of the process.

I'm glad you sorted out your tubes. Sounds like the corroded one is dud.

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