This preamplifier is suiteable for Russian corona counters, it uses a Schmidth trigger to discriminate the noisy baseline typical of these detectors. The trigger point or the LLD is adjustable via a 25 turn trimpot, a second trimpot is available for adjusting the level of hysteresis required. These two adjustments make it possible to tune this preamplifier to work with different tubes.
Typical raw pulse from corona neutron counter
When the input signal (red trace) exceeds a set threshold, the schmidt trigger output goes high, simultaneously the input lifts to a new level thereby maintaining the higher level (grey trace) until it falls below the threshold, causing the output to go low. The resulting output is a clean on off signal.
(note: due to the slew rate of the opamp and the short pulse time, the actual output pulse will look narrower at the top)
Some knowlege of electronics and preferably an oscilloscope and a DVM is required to fine tune the preamp for use with your detector.