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 Measuring Tube Voltage
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ZLM

1069 Posts

Posted - 05/13/2013 :  20:36:23  Show Profile  Reply with Quote
There are two ways to measure the voltage on GMC-080, GMC-200, GMC-280, GMC-300, GMC-320 tube:

1. Use a 11M ohm mutimeter 1000V range to check the voltage on tube two ends. For 380-450V, the meter should read about 180V.

2. To get better accuracy voltage reading, use following way. This is a generic method and it should work on most high voltage citcuit:

You need a regular 11M ohm internal resistance multimeter. Serial a 1G ohm resistor on your multimeter prob. Read the voltage frm the multimeter with 20V range.

And then calculate the actual voltage by following formular:

V(actual) = V(DMM read) x ((R(meter ohm) + R(probe ohm)) / R(meter ohm))

Here:
Voltage = V(reading from multimeter) * 91.9

if your multimeter reading is 4.2V, then the actual voltage is 4.2*91.1 = 382.62V
Reply #1

Frank

USA
39 Posts

Posted - 10/10/2013 :  15:21:18  Show Profile  Reply with Quote
When it says check the voltage on the tube ends, is that installed, powered up and for a required tube voltage of 380-450, you should measure around 180V? I have a 4011 tube from here, voltage range for this tube ? so I should be setting it too ?
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Reply #2

ZLM

1069 Posts

Posted - 10/20/2013 :  21:02:57  Show Profile  Reply with Quote
To get accurate voltage, use above formular to calculate.

The 140V-150V is measured directly by a regular 11M digital multimeter on a 1000V range position.
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Reply #3

Craig

USA
20 Posts

Posted - 10/28/2013 :  21:40:44  Show Profile  Reply with Quote
* I don't know about the M4011, but some G-M tubes are very touchy about added external capacitance. Legend has it that the capacitance between your meter's test leads (as much as 20pf) can take out some G-M tubes. _That_ is the reason for the "minimum anode resistor" specification (frequently: 4.7 meg-ohms), since any capacitance on the other side of that won't hurt the tube.

Consequently, the suggestion here to measure anode voltage through a stand-off resistor at the end of your meter's probe is excellent advice.

If you have a 100 meg-ohm resistor (or make one out of 10 10M resistors), keep it clean and calculate what's on the anode side of that resistor per the ratio: meter resistance + 100M divided by the meter resistance. Example{ 10M + 100M / 10M = multiply display by 11x.

For better accuracy, add another 4% to the example --due to the 110M load against the 4.7M circuit board anode resistor (if that's what it is, and assuming you're touching down on the G-M anode).
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Reply #4

Benezi

15 Posts

Posted - 05/16/2014 :  07:54:34  Show Profile  Reply with Quote
Do I have to remove the tube for the exact method?
Regards, Benedikt
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Reply #5

ZLM

1069 Posts

Posted - 05/20/2014 :  15:39:08  Show Profile  Reply with Quote
No. The tube has much higher resistance. You do not have to remove it.
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Reply #6

F_Schro

USA
4 Posts

Posted - 11/10/2015 :  09:53:45  Show Profile  Reply with Quote
Craig; is it that the tubes are "touchy" about capacitance or is it the loading on the DC-DC converter that drops the voltage ?

In either case ZLM's method should work fine (1G series)as long as the
resistor doesn't have too much parasitic capacitance. Some HV resistors with the single serpentine type element can be 5-10pF.

You can decrease the effective cap by several R's in series but hard to find higher than 20M in the smaller resistors.
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Reply #7

Nudnik

40 Posts

Posted - 03/08/2016 :  00:03:22  Show Profile  Reply with Quote
Can I use a oscilloscope with a 10:1 probe? E.g. DSO nano?
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Reply #8

rogerroentgen

USA
10 Posts

Posted - 05/01/2016 :  21:56:44  Show Profile  Reply with Quote
quote:
Nudnik Posted - 03/08/2016 : 00:03:22 Can I use a oscilloscope with a 10:1 probe? E.g. DSO nano?


a 100x probe would probably be safest to check voltages at the tube.
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