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Considering giving up Electronics as a hobby - Geiger counter problems

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Proud Mary

Sun Nov 20 2016, 02:02PM

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

It will be great to get a really classic GM tube like TGC-2 working, so here are some thoughts:

Loose fitting of bakelite base. This is very common in old valves/tubes and is simply due to aging of the adhesive bond between bakelite and glass. This is not a gas seal, and it will not affect operation of the tube. Solution: secure the base to the glass with epoxy, but before you do this clip an ohmeter across all possible combinations of the pins and very gently wriggle the loose base a very small amount to see if there is any kind of crossed wires type intermittant fault. If you do detect a crossed wires type fault, all is not lost, as you can cut away the bakelite base with a Dremel cutting wheel or similar, and make your connections directly to the wires inside.

Value of anode resistor: 5.6MÎ© is all wrong for a GM tube operating at 1.3kV - it would mean 230 Î¼A pulses dissipating 300mW into your counting gas. I suggest trying it at 56MÎ© with a cathode resistor of 1.2MÎ©. (i.e. the rule of thumb for GM tubes where the cathode resistor value is the anode resistor value divided by 45.)

Your counting pulses appearing across the cathode resistor will be of some volts, so won't need much in the way of amplification.

Inducktion

Mon Nov 21 2016, 01:36AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I tried your suggestions stella, and the tube still isn't working properly which leads me to believe the tube itself is unfortunately, dead. Dremel'd the base off as well and everything looks fine down below...

To be 100% sure, is there anyone on here who would like the tube in exchange for postage, and making sure it's 100% dead?

Here's a link to some pictures I took of it.

Really unfortunate, but I tested it left and right and made sure that I have the proper voltage, and I'm getting absolutely nothing from it.

Proud Mary

Mon Nov 21 2016, 08:56PM

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

Well that really is a shame, as it would have been a great thing to get that historic tube going what with the 2mg window and all. I've bought several dud end-window tubes in the past, and now I have a rule that I won't buy an end-window tube unless it comes in an unopened OEM package - which some of the Russian sellers are able to do.

Anyway, we can't let your efforts go to waste here, so I can send you a Christmas present of a couple of decent Russian SI-22G (CÐ˜-22Ð“) gamma/hard beta tubes which are metal cased, 22cm long and like to see 390V on the anode. Here's a picture of Radu holding one:

If you put your address in a PM I'll get them off to you on Saturday.

Inducktion

Mon Nov 21 2016, 10:17PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Sent you a PM stella. <3

Only other thing I could possibly think of that's causing the tube to not work properly is that somehow my power supply might be too noisy... Do geiger tubes need absolutely perfect regulation to work correctly? Or is open ended regulation okay?

Proud Mary

Mon Nov 21 2016, 11:01PM

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

Inducktion wrote ...

Sent you a PM stella. <3

Only other thing I could possibly think of that's causing the tube to not work properly is that somehow my power supply might be too noisy... Do geiger tubes need absolutely perfect regulation to work correctly? Or is open ended regulation okay?

GM tubes are very tolerant supply-wise. Taking SI-22G as an example, it has a nominal operating voltage (that is, the centre of the plateau) of 390V with a plateau length of 100V. This means that the plateau of anode voltage needed for Geiger-Muller Townsend avalanche mode stretches from 340V to 440V, and so long as the anode voltage stays anywhere between these two figures the tube will work correctly. So the GM detector has high immunity to supply drift. As to noise immunity, well there is noise and noise, but I don't think your sort of circuit would have any significant effect on operation.

Inducktion

Tue Nov 22 2016, 02:18AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Mmmm... one last thing is that whiteish stuff I found under the protective cap. Like I said it almost looks like a mold, or fungus, do you think maybe that had something to do with the tube not working?

Proud Mary

Tue Nov 22 2016, 10:37AM

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

Inducktion wrote ...

Mmmm... one last thing is that whiteish stuff I found under the protective cap. Like I said it almost looks like a mold, or fungus, do you think maybe that had something to do with the tube not working?

It's hard to say without seeing it, but I suspect it is something like degraded polymer from the adhesive bond that sealed the glass bond on to the base. Early plastics and adhesives were not so stable as they are now, and your tube is likely 60 years old.

The white matter could even be flux used in tinning the wires where they exit the glass envelope, or other artifact of manufacture.

The condition of the glass seals where the leads pass through the glass is of much more importance, and would be a likely suspect were this not an end-window tube, where an invisibly small puncture becomes the most likely failure mode.

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