“2nd or 3rd best 5751 ranking up there with the Sylvania & GE black plates.”
“Well to my ears its a more refined and yet more lively tube than those in group one. It has a good linear frequency response with fairly good HF extension, if not quite as palpable in the mids as a few others in group 2, but not by much.”
These are 1956 era tubes. A matched pair is available.