We have a small batch of used Tung-Sol 6550 tubes. All are tested and matched, and have our standard 90 day warranty. There were a few versions of the Tung-Sol 6550 tube over the years. We currently have the tube from the mid 1960's. This tube has grey 3-hole plates, and triple getter. They were made in the USA at Tung-Sol's Newark, NJ factory.
From Vacuum Tube Valley Magazine: "Detailed, even tonal balance, good bass, but not as much deep bass slam as the Winged "C"/SED 6550C."