Valves and internal components: do not replace by guesswork

A valve should not be replaced merely because it is old, and not every noise should automatically be blamed on valves.

Loss of power, microphonics, noise and instability can have different causes. Some amplifiers require specific procedures after certain valves are replaced.

If you are unfamiliar with the circuit and safety procedures, limit checks to the exterior and use a technician. Keep removed parts where they have historical relevance or can help document the service history.