This circuit uses a resonant reed relay to recognize the frequency of an input signal. Adaptations can be made to obtain functioning as a shield. The circuit is from 1965 documentation.
This amplifier from 100 to 150mW was found in a Japanese documentation from 1967. The circuit uses...
This circuit is from a manual of JFETs and can be implemented with the BF245 and for the bipolar...
This source of some amps for voltages up to 40 V was found in an Italian magazine Elettronica...