Terry Buddingh

Those mystery tubes in the photos above are Tung Sol 6550s, rebranded with the RCA logo. RCA-labeled Tung Sol 6550s were fairly common in the mid-'70s. 7552 is a date code indicating they were made the 52nd week of 1975. Look carefully at the top of the tubes. You should see a hexagonal graphic surrounding the number 6550. This graphic is often very faint, and sometimes nonexistent, if it's been rubbed off. Cool tubes.