Probably the
Merlin Blencowe page.
He's updated the page numerous times since the first time I read it, but this seems to be the most relevant passage:
"In almost a century of valve technology the only audio amps ever to use standby switches were guitar amps. Why? The trend began with the more powerful versions of the Fender Bassman (the 5E6 I think). Some people have suggested that it was primarily intended as a kind of mute switch, allowing the amp to be silenced between songs without needing to wait for it to warm up again before the next song. This is not a compelling argument since there are much cheaper ways to mute and amp without needing a heavy-duty high-voltage switch. Also, you would expect competing manufacturers to have provided similar mute switches on their amplifiers, but none did.
Another theory suggests the switch was added to amps which used silicon rectifiers because they did not provide the natural 'soft start’ of the original valve rectified designs. This is obviously untrue, however, since the first Bassman amps fitted with standby switches were still built with valve rectifiers.
There are two plausible explainations for the added standby switch. One is that is helped to protect the power supply capacitors from excessive voltage before the valves start to conduct and pull the voltages down to normal levels. Fender often used a reservoir capacitor rated for higher voltage than the smoothing caps after the standby switch, which must have saved money and space. Take a look at the 5E6 Bassman for example. The 135 Bassman schematic even shows labelled voltages, exceeding the cap ratings during standby.
The other possibility is that the 5E6 was rhe first amp where Fender used a DC-coupled cathode follower. This stage will sometimes arc between grid and cathode at switch-on if the cathode has not yet warmed up. (These days you should put a diode or neon-lamp between grid and cathode to prevent this, not rely on the user)."
