Keithley 610B

The Keithley 610C Electrometer can measure voltages (with a very high input resistance), very low currents, high resistances and charge. There is a 610BR rack model as well.

Manuals – Specifications – Links – Pictures

The 610B used electrometer tubes at the input and transistors in the gain stages. The predecessor, Keithley 610, had used tubes in all stages, while the successor model, the Keithley 610C, was fully solid-state.

Key Specifications

Voltage	1 mV to 100 V full scale in 11 ranges (×1/×3), ±1% FS
Input resistance (volts mode)	>1014 Ω // 22 pF, or selectable 10 Ω to 1011 Ω in decade steps
Noise	noise 10 μV with shorted input
Zero offset drift	<2 mV in second hour after 1 hour warm-up
Current	10–14 A to 0.3 A full scale in 28 ranges (×1/×3), ±2% FS (≤3×10−12 A: ±4% FS)
Resistance	100 Ω to 1014 Ω full scale in 25 linear ranges (×1/×3), ±3% FS (≥3×109 Ω: ±5% FS)
Charge	10–13 C to 10–3 C (Ampere-seconds) full scale in 17 ranges (×1/×3), ±5% FS
Display	Analog meter with mirror scale; positive, negative and center-zero display modes
Outputs	Unity gain output, ±50ppm or 100 μV, max.1 mA; Recorder output, ±3 V full scale, polarity opposite to input, min. 300 Hz bandwidth; 3 kΩ source resistance
Power	105−125 V or 210−250 V, 50−1000 Hz, 10 W
Accessories	Model 6104 Test Shield Model 6105 Resistivity Chamber (test fixture for measurement of surface and volume resistivity) Model 2501 and 2503 Static Detector Probe (for measurements of charge on flat surfaces) Model 6106 Electormeter Connection Kit Model 6107 pH Electrode Adapter

Internals

At the core of the 610B is a bootstrapped operational amplifier with a pair of 5886 electrometer tubes at the input, three differential transistor gain stages, and a 12AT7 in the output stage on a +250 / –265 V supply.

Electrometer tubes

The 610B input-stage electrometer tubes are a matched pair and are known to go bad over time, not with emission in particular but because they need to have exceptionally small grid currents for the most sensitive ranges to operate in their specification. Maximum grid current limit is specified at 20 fA after the warmup period. However, the manual clearly instructs to also check the +21 V supply, and that high relative humidity may affect the reading as well.

As matched replacement tubes are of course no longer available from Keithley, it is important select replacements from a handful of tubes by matching them by gain and grid current, to allow the input stage to be balanced successfully.

Links

• Keithley 610B Electrometer @ radiomuseum.org

Pictures

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  Electrometer input tubes mounted in foam on vibration-damping rubber

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  Replaced the deteriorated foam and rubber standoffs

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  High grid current reading on a warm and humid summer day

Components