We found this simple protection with zener diode in a 1967 Motorola documentation. The zener diode is 18 V, but the components can be changed for operation with other voltages.
This mechanical reverberator circuit for guitar was found in the February 1970 issue of Radio...
This circuit is based on the LM139, LM239 or LM339 series comparators and can be powered by...
Upon receiving a sound of sufficient intensity, the relay locks and remains so until the power is...