This circuit is from a 1975 RCA tube manual. It appears elsewhere on the site. R5 depends on the voltage of the amplifier with which it is used and can be changed.
This circuit found in a French documentation from 1982 can be applied to any operator. The...
In this circuit of the 1980s, the T1 transformer has 30 turns of 28 primary wire and the secondary...
Found in American documentation from the 90s, this circuit triggers a buzzer with a Sonalert-type...