From MetroAmp Wiki
Vacuum tubes need to be internally heated to work at spec, which requires some warm up time. When you power on your tube amp, current begins to flow through an internal heater in each tube. Generally, operating temperature is reached with 30-60 seconds. Because the resistance of the heater is higher at room temperature than operating temperature, initial current draw is high and levels off. To prevent strain on the power supply, many tube amps incorporate a "standby switch" which interrupts high voltage while tubes warm up.
Theory
The idea behind the standby switch is to allow tubes to reach normal operating temperature before applying high voltage. Accomplished by powering on with the power switch, while leaving the standby switch in the off position for an arbitrary amount of time.
The standby switch can interrupt the flow of high voltage in different circuit positions. The most common ways are to:
- Interrupt the AC voltage between the power transformer and the rectifier(s). Which requires a Double Pole, Single Throw (DT/ST) switch. This method is common in Marshall 100 watt amps.
- Interrupt the DC high voltage between the rectifier(s) and the rest of the circuit. Which requires a Single Pole, Single Throw (SP/ST) Switch. This method is common in many Fender amps and Marshall JTM 45's.
Tech
Since the highest voltages in the amp flow through the standby switch in many cases, a high quality, well rated switch is necessary.
Some amp designs also incorporate a capacitor connected to the standby switch to suppress the large, momentary transient than can occur when switching high voltages.