This circuit was found in a Signetics documentation from the 1980s. The integrated is a PLL and the SCR is a general purpose type The pitch frequency is given by R1 and C1 as formula next to the diagram.
This circuit was found in a Popular Electronics from the 90s. Transistors are general purpose...
This circuit works with only 1.5 V and can be powered by a solar panel. The circuit is from 1991....
This circuit has a maximum output current of 5 A, and the 2N3055 must have an excellent heat sink....