Gibson GA-20 Owner's Manual page 13

smoother reverb effect. A clockwise turn increases the reverb "tail;" a counter clockwise
turn decreases the reverb tail. At 7 o'clock, the Reverb effect is off.
Tremolo: Creates an intermittent signal effect (think surf music) with variable Speed and
Depth Controls.
Speed: Determines the rate at which the signal fluctuates in volume.
Depth: Determines the subtly or severity of the signal fluctuation.
On/Off: Turns your amplifier on or off. We recommend that you have your "Play/Standby"
switch in the "Standby" position when powering up. This helps preserve tube life.
Play/Standby: Allows you to idle your amplifier in "Standby" mode (no signal output) or Play
your amplifier in "Play" mode (signal output).