Marco_esposito - 2800h is indeed a bit too much for the 6550 in Ref 3.
If you are about to change 6550 tube, I would strongly suggest trying NOS TungSol 6550 tube. The improvement is very substantial. It brought my Ref 3 to another level sonicly. There is a thread on this topic:
ARC Ref 3: Tung-Sol 6550 in power supply?
If you are about to change 6550 tube, I would strongly suggest trying NOS TungSol 6550 tube. The improvement is very substantial. It brought my Ref 3 to another level sonicly. There is a thread on this topic:
ARC Ref 3: Tung-Sol 6550 in power supply?