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 CI-3BG tube on 300Eplus
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rogerroentgen

USA
10 Posts

Posted - 04/02/2016 :  19:52:23  Show Profile  Reply with Quote
I just got a cheap CI-3BG to use with a 300Eplus.

A while ago, I broke the LCD on this 300Eplus and I ordered a replacement main board. I fixed my 300Eplus, but I was thinking of using my 300E main board with the broken LCD to connect to the computer for measurement.

I tried to install this tube, but it is much shorter. I used a wire with alligator clips to connect the - terminal to the tube. When I powered up the unit, I did not hear clicking noise as with my working 300Eplus. I understand CI-3BG is less sensitive than the original 4011. Can I adjust tube voltage or do I have to calibrate to the less sensitive tube? I may not be able to do that with the broken LCD...

Any help appreciated!
Reply #1

rogerroentgen

USA
10 Posts

Posted - 04/03/2016 :  09:13:20  Show Profile  Reply with Quote
I checked my tube voltage with a multimeter. I found 170v with the USB cable connected to power,and 154v on battery. What is the actual voltage if I'm reading 170v at the tube?

Also, can I increase tube voltage by adjusting the R25 trim pot? Clockwise to increase?
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Reply #2

rogerroentgen

USA
10 Posts

Posted - 04/05/2016 :  21:44:25  Show Profile  Reply with Quote
Can anyone help? I want to increase the tube voltage to see if this SI-3BG tube works. I tried a thorium mantle and I'm not getting much response at all- 3 to 4 CPM with mantle right under tube. I'm getting less than 1 CPM background. I found a spec sheet for the tube and I should be getting higher readings than this.

I just need confirmation R25 trim pot on the GCM-300E Plus board is tube voltage adjustment.
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Reply #3

ZLM

1038 Posts

Posted - 04/06/2016 :  07:48:17  Show Profile  Reply with Quote
SI-3BG has similar voltage. However, the tube is the lowest sensitivity I have seen. So, it is not a good idea to install the SI-3BG to monitor the regular background. You may not see any reading unless the high level radiation source.
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Reply #4

rogerroentgen

USA
10 Posts

Posted - 04/06/2016 :  08:36:41  Show Profile  Reply with Quote
Yes, I know it is a low sensitivity tube, but doesn't the tube voltage seem low @ 170v. How do we calculate full tube voltage from the measured voltage? Spec sheet for SI-3BG says tube voltage operates at 380-480v.

With thorium mantle I should get more than 4 CPM?

I guess I'll buy another different tube since this one isn't a good choice.

Can you at least confirm how to adjust tube voltage so I can test? if I blow up the tube, it's not a big loss since it isn't very good to start with.
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Reply #5

ZLM

1038 Posts

Posted - 04/06/2016 :  19:31:49  Show Profile  Reply with Quote
You can not use regular multimeter to measure the tube voltage.

See:
http://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=3745

Adjust the tube voltage as little as possible and then check with the meter. You can adjust the voltage before you install the tube.
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Reply #6

rogerroentgen

USA
10 Posts

Posted - 04/09/2016 :  21:20:21  Show Profile  Reply with Quote
Thanks ZLM. I tried changing the voltage. Initially it seemed to get a little more sensitive at higher voltage, but when in the presence of a source,the readings did not increase much. This tube is incredibly insensitive.

For anyone else wondering, the R25 trim-pot does change tube voltage on the 300Eplus board. Adjust VERY gently to change voltages.

I've ordered SMB-20 tubes and will be going with that instead for now.

I read on the forum it might be ok to connect two tubes to the GMC. Do you think connecting two SMB-20 in parallel would be ok for higher sensitivity reading?
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Reply #7

ZLM

1038 Posts

Posted - 04/10/2016 :  20:21:50  Show Profile  Reply with Quote
The GMC-300E Plus is able to handle the two tubes in parallel. But the sensitivity may not be doubled. It increases the CPM reading.
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Reply #8

rogerroentgen

USA
10 Posts

Posted - 05/01/2016 :  13:19:46  Show Profile  Reply with Quote
I received my SBM-20 tubes and connected two to my 300E+. I did some readings with 1 and 2 tubes connected. I found that CPMs were almost exactly double with two tubes connected. In my experiment, I placed the two tubes parallel to each other about 2 inches apart. I placed a thorium check source between the tubes about center. This provided nearly exactly double CPM readings when compared to the single tube counting the same check source.

I'm very happy with the multiple tube setup, but I ran into a problem once I connected the 300E+ to the USB of the computer. Once the 300E+ has a DATA connection with the two tubes installed, the CPMs virtually double while the data connection is made. If I only connect the USB cable to a power supply (not computer), the CPMs are accurate. Once the data connection is made, CPMs are doubled. It takes 30 seconds to see the change, but after a full minute, the CPMs are 2:1.

For example, with a single GM tube, the background count is ~35cmp. When I connect two tubes, background increases to ~70cpm. When I connect the GMC300E+ with two tubes installed to the PC, suddenly background is ~150cpm. This change does not happen with only a single tube installed. I would like to be able to use the two tube setup connected to the PC for data logging and network reporting. Any ideas what may be causing this result of 2x readings?
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