[header] How to fix or replace your preamp tubes and power tubes [title] Identify the problem. [step] The screws will be located on the side of the head, or on the back of the cage itself. [title] See if your preamp tubes are the problem. 
[step] Firmly grasp the tube with one hand and press down on the clamp to provide room for the tube to move. [substeps] Gently rock the tube back and forth while lightly pulling upwards.