This amplifier with the EL84 tube is a European commercial type from Corpim mounted on a printed circuit board. The diagram appeared in a September 1958 Radio Electronics magazine.
This circuit prevents damage by accidentally reversing the power supply polarity. The circuit is...
This shield circuit for lamps and other charges was found in a Popular Electronics from the 90's....
Found in Philips' 90s documentation, this small source can use modern transistors like the BD135...