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 2.GQ Geiger Muller Counter
 GMC-320-Plus and LND7231 tube		  New Topic  Reply to Topic
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temnik

3 Posts
Posted - 05/08/2017 :  13:23:37  Show Profile  Reply with Quote
Hello all,

I recently bought the GMC-320-Plus and was thinking to add LND7231 thin mica end window tube to it. I was going to leave the internal glass GM tube untouched and install the LND7231 in parallel with the M4011 via a quick-disconnect harness. However when I tried to temporarily attach the LND7231 to the poles of the M4011 and checked it with the unshielded Am-241 (strong alpha source) it responded with very modest increase in counts, almost as much as the M4011 if I put the unshielded Am-241 next to it. The same Am-241 source easily maxes out a Mini900 radiation counter (weird british Geiger counter with logarithmic scale graduated in counts per second) with the LND7231 hooked up to it.

Apparently, the LND7231 requires 500V and 4.7Mohm anode resistor for operation however the GMC-320 works on 400V and 6Mohm. Would this difference explain the very low efficiency of the LND7231 or I am missing something basic?
Edited by - temnik on 05/12/2017 10:57:13
Reply #1

ZLM

1261 Posts
Posted - 05/10/2017 :  10:28:44  Show Profile  Reply with Quote
The GMC-320 tube voltage can be adjusted to about 500V. There is a small metal variable resistor near USB port.

Remove the M4011 is easy. You may try that. The tube resistor on bar is about 3Mohm and should work with the LND7241.

Make sure the tube + / - should be correctly connected to the voltage.
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Reply #2

temnik

3 Posts
Posted - 05/10/2017 :  12:06:05  Show Profile  Reply with Quote
quote:
Originally posted by ZLM

The GMC-320 tube voltage can be adjusted to about 500V. There is a small metal variable resistor near USB port.

Remove the M4011 is easy. You may try that. The tube resistor on bar is about 3Mohm and should work with the LND7241.

Make sure the tube + / - should be correctly connected to the voltage.


Thanks a lot for your response! I'll try removing the original tube and playing with the high voltage. I'm a bit confused about the value of the anode resistor. Is it 3 or 6Mohm as you've mentioned in one of your earlier post? See https://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=4060
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Reply #3

temnik

3 Posts
Posted - 05/12/2017 :  10:52:12  Show Profile  Reply with Quote
Mystery of the non-responsive LND 7231 tube solved! It was not a 100V difference between recommended voltages for the two tubes (400V for M4011 vs. 500V for the LND 7231). It was a hidden resistor wired to the anode pole of the LND. If I decoded it right (yellow-black-green-brown) it's a 4M Ohm resistor. Once I bypassed the resistor the tube now goes crazy when exposed to unshielded alpha-source!
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