Found in a 1967 publication by General Electric, this control is intended for motors up to 2 A according to the SCR used. Higher current equivalent SCRs can be used.
This circuit is a from publication 1980, but still can be mounted with BC548 transistors for Q1...
This circuit for testing zener diodes by lifting their curve on an oscilloscope was presented in...
This high quality circuit with complementary symmetry was found in an Electronics World of 1966....