Ed,
Thank you for the tip on "warm-up" time -- or rather in this case, time
for leakage to cool down. I will definitely give the 616 a chance by
leaving it on for the next 24 hours.
Last time I did a measurement, it settled in the +20 fA range, and I
confirmed this number by doing a dV/dT measurement in the charge mode.
In this configuration, the input current plus leakage gets to charge one
of the built-in capacitors (I selected the 1E-9 range) and the rate of
voltage change (dV/dT) is a good indicator of current.
I once had a K610B electrometer, which as you know used a differential
pair of 5886 tubes. It worked, but zero drift was very bothersome. I was
also weary of using a tube-based instrument, which at the age of 50
could fail at any time with a dead tube. I finally sold it and bought
the K616.
That's all for now, I'll write again tomorrow with measurement results !
Joel Setton
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