Tube Amplifiers and Standby Switches
The standby switch has been a controversial component in amplifier design, with some modern amps forgoing it thanks to advancements in technology and consumer education.
Historically, not all tube amplifiers had standby switches. Leo Fender was likely the first to include a standby switch on a guitar amp to protect power supply capacitors from high voltages caused by cold tubes. However, changes in rectifier and capacitor technology have negated the absolute need for these switches.
A common misconception around standby switches is that they can extend tube life. However, this only applies to transmitting tubes, not receiving tubes as used in guitar amps. Receiving tubes do not need to have the B+ or plate voltage removed while the filaments warm up.
And finally, since traditional standby switches handle very high DC voltages, they can sometimes develop internal arcs when opening or closing that come through the speaker as a loud pop. While this is more of an irritation and anything, it often requires replacing the switch or adding components to the switch to remedy.
The PRS Standby Switch
Some PRS models feature standby switches, such as the MT 15 and Sonzera 20. This circuit is designed to mute the preamp section of the amp circuit, eliminating possible noises while standby is engaged during any short breaks.
Standby switches were never meant to be used while taking a long break on stage--if you take a long break, turn the amp off.
Amps, like well-tuned gas-powered cars, will have an idling sound before you hit the gas and rev it up. If you hear a faint white noise from your amp in Standby mode, it's not a problem, but rather an indication that your amp is ready to rock.