Marco_esposito - your findings seem to confirm what Guido has written before - the current production TungSol 6550 (so called "reissue") does not bring a net benefit compared to SED 6550C.
When I was talking about the improvment I got by inserting a TungSol 6550, I meant the NOS (New Old Stock) TungSol 6550 Black Plate from the early 50s - which is a totally different tube to the one you have tried.
I cannot recomment it (NOS TungSol Black Plate) highly enough.
When I was talking about the improvment I got by inserting a TungSol 6550, I meant the NOS (New Old Stock) TungSol 6550 Black Plate from the early 50s - which is a totally different tube to the one you have tried.
I cannot recomment it (NOS TungSol Black Plate) highly enough.