In amateur radio equipment, which is the major application for the silicon controlled rectifier (SCR)?

In amateur radio power supplies, the SCR is famously used in an overvoltage protection setup known as a "crowbar" circuit. If the voltage regulator fails and allows a dangerously high voltage to pass, a sensing circuit triggers the SCR's gate. Once triggered, the SCR acts like a heavy-duty switch that deliberately shorts the power supply output to ground (akin to dropping a physical crowbar across the terminals). This immediate short circuit blows the main fuse, completely protecting the expensive radio equipment from the overvoltage. Because an SCR operates strictly as a bistable switch (it is either fully on or fully off), it cannot be used for linear applications like a microphone preamplifier or a Class C RF amplifier.