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Make sure you are not looking at new versions of these.
Telefunken for clear, cooler, neutral, open and detailed sound.
Mullard for warmer, smoother sound.
Amperex Bugle Boy for warm, open and detailed sound.
Sylvania and GE avoid grey plates.
RCA black plates clear and detailed but can sound pinched.
Talk to Kevin at upscaleaudio.com
Owned a Trumpet a few years ago; very nice amp. It’s really hard to beat Telefunken 12ax7. I mean you can, but the Teles are far more plentiful and not too expensive. I believe you can also use 12at7 (or 12ay7) in the 12au7 slots (in fact mine didn't ship with 12au7). Not really a fan of any 12au7, though I haven’t tried Teles there. I’ve always wanted to like Mullards, but in the end I just have to move on to something else. Amperex are good but I prefer Teles. I sometimes like the 5751 sub for phase splitters in amps, but have found it best to stick with the 12ax7 in phono stages.
I totally agree with avoiding Sylvania small-signal gray plates of any kind! Yuck! The earlier black plates can be good, though.
Congratulations on your purchase. I looked around for one of those years ago but was unsuccessful. For 12AX7, TFK would be my choice if you can be certain they’re genuine. For 12AU7, my first choice would be to ditch them if they’re in a gain stage, but they’re ok for a cathode follower. If replacing AU7s, I’d recommend either 6FQ7 or 12FQ7, depending on the filament voltage. 6CG7 is superb but you’d need to re-wire the filament. DO NOT use 12AY or 12AT, unless Hagerman says ok. Those types are electrically different from 12AU.
Electrically, 12AU = 6FQ = 12FQ = 6CG, for practical purposes.
Note that when I say depending upon the filament voltage, I mean that you will need to ask Hagaman whether the unit provides six or 12 V for the filaments. Then choose the tube accordingly as regards 6FQ7 or 12 FQ7. When I say the 6CG7 would require re-wire, I am essentially saying don’t use it unless you are capable of doing the wiring or you have a tech who can do it.
I also forgot to mention that if you stick with the 12 U7, I would recommend Amperex bugle boy, again assuming you are certain that they are what they say they are.