Other likely causes of problems Open cathode resistors are a common cause of failure. This is usually a result of running the amp with bad tubes, or improper bias setting. Tube sockets with splayed pins or extremely heavy oxidation are also common in old amps. Cleaning may seem tempting, but replacement of the sockets is by far the best route in this case. Newer sockets will have tighter tolerances and better metallic materials for conductors.
Some people replace internal wiring with newer, higher quality stuff.
Commercially available wire from companies like KimberKable can yield significant audible improvements. If you need to replace sockets, take a good look at the wiring for signs of oxidation. Older wire did not have insulating materials as durable as what is available today, and the purity of the copper is often less than current materials.