Thanks. LOL, I'd bookmarked that, but failed to comprehend
"Alternatively two letters were used. This was allowed up to January 1st 1965. In this scheme the first letter A = 1945 and so on, and the second letter A=January and so on. Letters I and O were not used. Thus AA = January 1945, NF = June 1957."
That "allowed up to" throws a spanner in the June 1968 date from the eBay link. The 'F' for June makes sense, but if that format started in 1945, less the use of I and O, ZF equals June 1969, but still conflicts with "allowed up to." But UG would be July 1964 (U is 21st letter minus 2, so 19th... 1945 + 19 = 1964). I can live with that
The ZF is the odd tube out, so I'm less concerned about its pedigree, but "enquiring minds want to know"
This
http://www.tubecollector.org/cv/1963/cv2462-cv2549.pdf
makes me wonder about the 'AD'. Not sure what the "GOV. SPEC." column refers to.
ATM I'm really groovin' to a pair of Ediswan 6BQ7As I snagged a while back from a Spanish eBayer, IIRC. They've passed 200 hours, and I listened at the beginning, was forced to take a break, then finally got back to them today. Euphonic is a good word for them. I like their sound stage presentation, and they are quite mid-centric. Guitars and vocals take center stage, and overall I'd say they have a warm signature. Can't recall what I paid, but I hope it was under $100. I really wanted something that said EDISWAN (
), and these were the only ones that came up on eBay that were compatible. Wasn't willing to throw $200+ to Uncle Kev for what he has (and only has cryo-treated left), even if a few Head-Fiers called them their end game tubes. Also, it's fun to collect and try various types from "Oskari's List"