Spec sheets on such GM detector tubes usually quote a "suggested" operating voltage. Different tubes even of the same type and number have different GM region slope lengths and idealized voltage operating points. A precise voltage is never called for in such adjustments. Most portable GM meters and the GM tubes they use have a 50-100 volt sloppy slope from beginning of GM mode to the end of the Geiger region, (Townsend breakdown). I mention the need for this accurate calibration supply in my homemade 100meg input Z HV meter posting. This forces them to suggest using an "electrostatic voltmeter" to make adjustment of their bias pot (effectively infinite input Z)Īll such 1000:1 networks absolutely rely on a good stiff, accurately calibrated, usually digital read out, HV supply, (lower Z ), for proper calibration of such a network. Many portable GM counters love to place a series 22 meg resistor in series with the HV output to the GM tube's input circuit. Its suggested use was for bias setting readout on neutron and PMT tubes. My post here on the 100 megohm input Z conversion of the old $5.00 harbor freight digital voltmeter was similar but limited to 0-2kv using the 20 volt range. ![]() It has about $6.00 worth of components in it. A common circuit found in old TV 30kv add-on probes to VTVMs of the 50's on.
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