Voltage amplifier with the 37 tube (T392E)

Found in a modern design book with tubes, this stage with the triode 37 has a plate current of 3.7 mA.

Optical isolator (CB8272E)
This sensor circuit from a 1967 Ferranti documentation uses components from the era. The circuit...
Light Relay (CB3866E)
When the sensor, an ordinary photo-transistor, receives light, the relay closes its contacts. The...
Frequency meter (CB7485E)
Found in a 1960 Sylvania manual, this circuit is intended to measure audio signals frequency....