What component is it being used in? Preamp, phonostage, amp input stage?
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regardless the device, tubes often influence the downstream signal.
12AT7s are sometimes on the input side of the amplifier and as such will flavor the overall sound one way or some other.
RCAs from the 60s for example which I own, soften up the audio a tad, adding a bit more relaxed sense of being.
from the same Era, Amperex Bugle boys step up the notes leading edges. and add more bottom end and insightfulness to the music.
I've mixed the rCACs AT7 and Bugle Boys AX7s on the output side, achieving solid dynamics while still presenting a sense of ease to the music.
with RCAs on either end, things became a bit too soft for my tastes. with Bugle boys on either side things greatly perked up.
its like baking a cake some what... its always a result of the types of tubes being used in conjunction with each other.
its a tad harrowing from time to time trying for a particular 'flavor' and the safest bet IMO is to begin by using the same brand, model and ear of NOS and if not happy wash, rinse, repeat. or try other combinations.
there used to be a website which gave pretty close or accurate accounts on various tube type and era sonic attributes called joe's tube lore.
Depends on how you like your digital to sound. In both my Manley monoblocs and tubed CD player smooth plate Telefunkens can’t be touched for overall naturalness, soundstaging and bass extension. Phillips “miniwatt” are also very good, but tip the balance upwards. Siemens triple mica’s even more so. Raytheon also made a good 12at7 that I like very much in my applications offering good clarity and which can be found relatively inexpensively. Many 12at7 variants out there also that are worth considering, but don’t fall for the often stated claim that a particular variant is “better” or that a particular variant sounds a particular way. Depends on the particular manufacturer of the variant; not all examples of that variant. Good luck.
as was said, even the era or vintage of the tube, and not merely its brand name makes a diff!
another take on Telefunken tubes is IMO they render the top ends as sweet and clear as one could ever hope for, but do little to keep the lower end as adept. I felt Telefunken was not the choice if more dynamics and transparency and improving the Botton end was the desire... so there's another reason why one definitely needs to dip some toes into these waters and find out for themselves.
one last note. Amperex on par in the 60s era at least produced as transparent and strong a bottom end tube as one could hope for, and this was across several models.... 6922; 7308; and 12AT7; 12AX7.
each of these tubes although aimed at differing applications produced similar sonics. all of these tubes I've used in various preamps. some amps use the 6DJ8/6922;7308, to split the incoming signal as they are all dual triodes. often this occurs in hybrid amps. PSA for example uses this approach in their current upper end amps.
its important as well to indicate the application to the tube seller. phono preamps for eX. are best suited to have as quiet a tube as possible. not every tube seller can address this area as well as some other tube sellers but its always something one should say. the better tube vendors will keep phono preamp tubes separated from standard line stage and charge a bit more accordingly.
that said, I'll opt for the phono stage preamp quieter tubes every time. even in std pre amps. why not? pay a tad more for less noise