Since all the "good" 5751s are gone or priced astronomically high, I decided to roll some current production tubes, and maybe sift through everyone else's NOS trash.
So to have a baseline for some of my testing, I measured the frequency response of the amp with the stock tubes: JJ ECC83 with unknown driver tubes. The driver tubes were in white boxes labeled 6P14 with no markings on the tubes themselves.
The frequency testing was done driving a pair of HD600s, with the switch on the "high" impedance setting.
I got one of these 5751's last week and it sounds very good. It's 1959. There's a (?) third mica or thin clear disk above the other stuff. The other 2 micas are real thick. It's the best sounding tube I've found in so far.
5751 JAN-JG General Electric (USA)
JAN-JG 5751, 1959 date code. Grey plates. Limit 4 per customer.