S
The Tektronix Type S is a plug-in for measuring semiconductor diode recovery characteristics. It fits in 500-series scopes. It can be thought of as having two parts: a stimulus subsystem and a measurement subsystem. The stimulus comes from mercury switch that drives a 6GM8/ECC86 tube that operates in a current-steering mode. The measurement subsystem is a wideband amplifier that converts the single-ended voltage on the diode under test to the differential signal that the oscilloscope mainframe takes. One terminal of the diode is grounded.
Measurements with the Type S are typically made by applying a current step to the diode while observing the voltage across the diode.