I'd really need to dig out my copy of Aspen Pittman's book, but there's two possibilities. It's either a better build of tube, such as a JJ vs Sovtek or similar, or it's a tube that passed a screening process for gain and microphonics to weed out less desirable tubes. Are there any other markings, and could you possibly post a pic or two of them for comparison's sake?
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"You don't have to be great to start, but you have to start to be great." -Zig Ziglar
Acoustics
2013 Guild F30 Standard
2012 Yamaha LL16
2007 Seagull S12
1991 Yairi DY 50
Electrics
Epiphone Les Paul Standard
Fender Am. Standard Telecaster
Gibson ES-335
Gibson Firebird
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