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Keithley 610B: Difference between revisions
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==Internals== | ==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. | 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 stages electrometer tubes is 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. | |||
The maximum grid current limit is sepcified at 20 fA after the warmup period. However, the manual clearly instructs to also check the +21V supply and that high relative humidiy may affect the reading as well. | |||
When replacing the tubes, it is clear that they are no longer available via Keithley as the manual tells, so it is important to try and get a hand full of tubes and match them by gain and grid current, so that the input stage can be balanced successfully. | |||
==Links== | ==Links== | ||
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Keithley 610B input tubes 1.jpg | Electrometer input tubes mounted in foam on vibration-daming rubber | Keithley 610B input tubes 1.jpg | Electrometer input tubes mounted in foam on vibration-daming rubber | ||
Keithley 610B input tubes 2.jpg | Replaced the deteriorated foam and rubber standoffs | Keithley 610B input tubes 2.jpg | Replaced the deteriorated foam and rubber standoffs | ||
610BhighGrid.JPEG | High Grid current reading on a warm and humid summerday | |||
</gallery> | </gallery> | ||