Any 6550 will work fine! That tube is a voltage pass-regulator in the power supply. It is used to regulate the B+ voltage. The music signal does NOT go through it! No effect upon sound quality!
Best modern 6550 tube for ARC per-amp
There are a lot of modern 6550 tube in the market such as Reissue Tung-Sol 6550, GE-6550A (Green letter), Svetlana 6550, Russia Gold Lion 6550/KT88........etc. What is the best modern 6550 tube for ARC per-amp in your opinion? (PS: GE 6550A is not newly made but still a lot in the market).
I used to own a NOS black plate 3 holes Tung-Sol 6550 but which has leak and now I’m using the stock Svetlana 6550. I want to buy a nice sound 6550 for my ARC Ref 6 and my stock Svetlana 6550 will be used as spare. Will not consider NOS black -plate Tung-Sol again because of pricy and not sure the condition.