This circuit with unijunction transistors was found in a documentation from the 80's. The trigger times of the SCRs depend on the capacitors and are adjusted in the 5M (4M7) potentiometers.
This circuit can use 20 to 100 mA relays with a general-purpose transistor such as the BC538. The...
The tone control in the figure below can be implemented with almost any operational amplifier. The power...
This circuit was found in a 1967 General Electric documentation. The components are from that...