Gibson GA-20 Owner's Manual page 17

To change the line voltage setting:
1. Disconnect the power by removing the power cord from both the power outlet and the
amplifier.
2. Open the fuse compartment by inserting a screwdriver blade gently into the tab at the
bottom of the compartment and prying until the tab can be removed with the fingers.
3. Remove the line voltage selector assembly by gripping the line voltage indicator tab with
pliers and pulling it straight out of its connector.
4. Rotate the line selector assembly to the desired voltage and reinsert.
5. Verify the appropriate fuse for the selected line voltage and reinsert the fuse
compartment by pushing it back into place until the tab locks.
SWITCH IT TO "STANDBY"
Standby allows your amplifier to "warm up" before you activate its output as well as sit in an
idle position (no output but the amp is still on). This preserves tube life and allows you to
bypass output when desired.