This circuit, found in a 1977 documentation, indicates when the grid voltage goes through zero. Under these conditions, it produces an output pulse whose duration depends on the capacitor.
This basic configuration can be found in a lot of articles on this site. In this case, the relay...
This high pass filter was found in the February 1996 Electronics Now magazine. The circuit...
Found in General Electric documentation from 1967, this circuit uses an unusual PUT today. The...