SBM-20: Do I really need the Anode resistor and what does it do?

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3 months 2 weeks ago #4402 by Burak27
Hi Guys, hope to get some answers here.

So I'm building a geigercounter with a sbm-20 tube and already built a circuit which transforms 9v to 400v.
So usually I would connect the tube to that 400 volts using a 4.7MOhm resistor, but when I do that the tube does not have enough voltage (about 220 volts).
Without that anode resistor i get the recommended 400volts and the tube counts just fine.

So why should I use an anode resistor?

I use this circuit here: mrks.de/static/image/blog/adce3ad6c475-288-full.jpg but without these two resistors on the left side of the tube

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3 months 1 week ago - 3 months 1 week ago #4403 by Alpha01
Hello,
The anode resistor is there to limit the current when the tube fired.
It allows also the tube to correctly "quench" (recover) after a detection: the voltage drops across the tube and the conduction linked to the detection can not be sustained.
Without these resistors, you will very quickly destroy definitively the tube.
Of course, it may take some time if you're only recording background events, but finally you will get a bricked tube.
In fact, it is the HV power supply of your design (related to the blog page of your link) that is quenching the tube!!! Making very poor and unreliable reading.
The risk is also to continuously fired your tube (short cut), that will finally destroy it.
Take great care when you measure the tube anode voltage: you must take in account the internal resistance of your voltmeter. Assuming you're using a 10M voltmeter (quite generic), the real voltage at the anode is 220V x (10 + 4.7 + 0.56) / 10 = 335 V
Of course, you have to adjust the above formula with the real value of your voltmeter input resistance (replace the 10 number by the real value in mega ohms)
You also have to take great care to not overload the HV part of your design: usually, it can only provide some µA and a low input resistance voltmeter could drop the available voltage.
Also to mention: if you have additional high value resistances in the filtering part of your HV generator, you have to take them in account while assessing the real voltage at the tube anode with a classical voltmeter.
Another, not directly related question, concerning the capacitor across the 22k+100k network. I believe it's pF and not µF, Otherwise you will get another problem...
Regards,
Last edit: 3 months 1 week ago by Alpha01. Reason: Adding some more explanations and typo corrections.
The following user(s) said Thank You: Burak27

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3 months 1 week ago #4405 by Burak27
Thanks for that very detailed answer.

I will build the circuit again on monday and inform myself about the internal resistance of the voltmeter I'm using. Then I will use your formula to calculate a better pair of resistors to get to 400V as accurately as possible.
So when I built the circuit using the 4.7M and 560K resistor I could not measure any pulses. Maybe I had not enough voltage on the tube. Gonna try it out

I'm really grateful

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3 months 1 week ago #4406 by Alpha01
Hello,
It seems very strange, because you have already measured 400V without resistor. So I believe your HV generator is OK. Do not focus too much on that.
If the schematic of the HV generator is the one that we could see in this blog article , it has plenty of reserve of power. You can directly measure the HV across the 22nF capacitor without any thing connected to it.
But I must strongly insist that the recommended value for the anode resistor for the SBM20 is 5.1 M ohms. This value is tailored for the best performances of the tube. In any case, never choose a value below 1M.
If your tube doesn't work with the recommended value, that could mean it is dead (or almost)... Check also the 220 pF (picofarad) capacitor: you could safely remove it for a try. Theoretically, this capacitor should be adjusted if you modify the anode resistor. But at a low background level, it doesn't matter.
If you have an oscilloscope, you can check if there are any positive pulses (around 6 to 9V on the cathode of the SBM20 or on the 22K resistor if it's more convenient.
Check also the DC voltage with the voltmeter (tube installed and powered) across the 22k+100k resistor: it should be 0V, if not the tube is dead.
Take also in account that the wires used to connect the tube must be short to avoid increasing the total capacitance of the tube section. Ideally, the anode resistor should be as close as possible of the tube anode. (Personally, I solder it on the clip connected to the anode).
Hope this could help you,
Regards

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