Q: I have a tube guitar amp made in 1978 that is kind of noisy; can I use it’s standby switch as a mute switch between songs?
The short answer is: well, technically you could, but we don’t recommend it.
A longer explanation is that with most pre-1980 amp circuits with capacitors that have not all been updated to modern higher voltage types, when you first turn on the amp the voltage may rise above the safe rated voltage of the capacitors until the tubes warm up.
This is why you want to use your standby switch on start up. However, using a standby switch as a mute could actually shorten the life of the tubes, not protect them! Simply turn down the amp’s volume control or use your guitar’s kill switch if it has one, or use a pedal such as a tuner or noise gate that can mute your signal during breaks.