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 2.GQ Geiger Muller Counter
 Sbt11a tube doesn't go over 6k cpm. Voltage?		  New Topic  Reply to Topic
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Distelzombie

Germany
202 Posts
Posted - 05/03/2016 :  11:32:02  Show Profile  Reply with Quote
Hi

I recently modified my 300+ with a sbt11a tube. I have a sample of uranium ore and the old tube measured about 80k cpm on it. (used app to go around the 65535 cpm limitation of the GQ GMCs.)
Now, I measured the same sample with my sbt11a tube and it stops at about 6-7k cpm. I expected it to go much much higher then the M4011 tube since it can also detect alpha radiation.
Is this a problem with voltage? Usually the 300+ doesn't need to be changed. It uses the same range of volts as the M4011.

What's wrong?

Thanks. :)
GMC-300E+ V4.20 with sbt-11a alpha tube

My statements are "stuff-a-hobbyist-says" and not in any way professional.
Reply #1

Distelzombie

Germany
202 Posts
Posted - 05/16/2016 :  15:07:46  Show Profile  Reply with Quote
OK I solved it. Even though I had no way to measure the voltage because my multimeter is dead. (RIP)
Fortunately, the little piezo speaker is beeping constantly with a specific frequency depending on the voltage going to the tube. That helped me deciding the right position of the little turn screw thing on the board.
Tone goes down while increasing the voltage, which is counter intuitive. I tested out max and minimum voltage while measuring a source (max means either constant detection of nothing or the same as -> min means dead silent tube.) and then chose a tone in the upper mid.

Anyway, after increasing the voltage I got a useful reading of about 73,000 cpm. (65535 + ~7420)

I would love to know the ySv/h. But I did not got a solution to the suddenly dropping ySv/h after the third calibration point yet.

I hope this helps anyone. :D
GMC-300E+ V4.20 with sbt-11a alpha tube

My statements are "stuff-a-hobbyist-says" and not in any way professional.
Edited by - Distelzombie on 05/16/2016 15:08:30
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