Changing preamp tubes is easy and can be done by the newest of newbies.

Changing tubes

Power Tubes

1) Grip the base of the power tube with your finger tips, and cup the glass top with the palm of your hand.
2) Gently start removing the tube in a circular (clockwise) motion.
3) Do it carefully as to not to damage or break the tube.

4) Rebias new power tubes, bring your amp to a qualified tech if you can't do this!

Preamp Tubes

1) Carefully twist off the preamp covers.  It's common to break a preamp during this step.
2) Gently remove the tube in a circular (clockwise) motion.
3) Do it carefully as to not to damage or break the tube.