Many guitarists do not really understand the correct use of the standby switch (if fitted). This explains how to use it correctly.
Valves have heaters which make them glow hot. They also have very high voltages across them (400V or more) to make them work. Ideally the valve should be allowed to get to working temperature before the 400V is applied as this will extend the valve life. This is the function of the standby switch.
In the OFF position, there is no high voltage (e.g. 400V) applied to the tubes. In the ON position, the high voltage is applied. So to use the standby switch correctly, first turn it OFF, then turn the amplifier ON, then wait about a minute, then turn the standby switch ON. Note: When you have finished playing, you do not need to turn the amp to standby before switching off—just turn the amp off as normal.