NOS Tube Recommendation for Quicksilver Phono Pre Amp

GE 5751 TM BP are very nice tubes. Looks for the "silver clips" variant that was produced only from 1952 - 1954. It has bracing clips over the plate "ears" that poke out past the micas. Way over-the-top construction. Then the penny pinchers took hold of production, and "quality" features like this dropped off. The "silver clips" tubes sound sweeter (better) than other GE TM BP.

Every 12AX7 / 5751 has its own sound, and nobody can really predict you what you might end up preferring. For me - 5751 generally projected a tighter, more precise image with smooth clean sound. 12AX7 project a larger, slightly less precise image with a ballsy, dynamic sound.

Mazda silver plate 12AX7 are kind of amazing, combining the strength of both types. They have 5751 style "T" plates, which is unusual for a 12AX7 (and also the silver / nickel color). They carry a little extra energy / zing up top, so if your system leans bright they might not work, unless you can balance them out (e.g. with Mullards). But wow, what a tube! DON’T make a mistake of getting Mazda / Cifte silver plate 5751 - those are ridiculously bright, just about unlistenable.

Teles are a nice, good all around 12AX7. Ribbed plates a little more dynamic and details, smooth a little warmer and well...smooth. I prefer ribbed. Don’t way overpay a tube dealer for these. There are tons of them still out there. Buy them all day used (tests strong) and throw out the duds.

Mullards are warm. So warm. Usually too warm.

5751 are close enough to 12AX7, it’s always a safe swap. Most tube circuits are pretty robust and flexible. I’ve accidentally swapped a 12AU7 (5814) into a 12AX7 slot and it sounded better that I would’ve imagined. The measured differences between vintage and new 12AX7 is probably on par with that between vintage 12AX7 and vintage 5751. In a phono stage you might change the EQ a bit, but that's more subtle than you might think. 

Don’t drive yourself crazy with tube rolling, because you can easy get into a habit of liking every new sonic perspective you hear and thinking "this is the one!" every time, and it never ends.

@larryi  thanks for the explanation.

To the OP, I agree with DeKay's first post.  Let the preamp settle in before making changes.  How many albums have you actually listened to for your sonic conclusions?

@larryi 5751 and 12AX7 specs differently in a few significant ways. gain is one of them. heater current, plate resistance are 2 others. 

@crawfishdaddi 5751 is not a drop-in substitute for 12AX7, especially not in phono circuit. any such substitutions are circuit-dependent. ask quicksilver if such a substitution is ok in their circuit.

@onhwy61...I hear you (you and @dekay are not wrong). I've played about 15-20 records on the new phono stage and I am committed to waiting at least the minimum 30 hours of recommended break-in time before making any changes. 

However, I am getting my ducks in row and trying to learn as much as I can in the event that this coloration issue doesn't go away on its own. I also struggle with the idea that I should just keep waiting for months on end hoping for it to improve (I think a lot of time, people just get used to how it sounds whether it actually improves or not). 

I listen to records 3-4 times a week for a couple hours at a time. At that rate, I feel like 2 months is enough time to know if it's time to roll the tubes. I know that's a little on the impatient side, but what can I say.... I'm obsessed . 

@crawfishdaddi 

OK, now I see you have stated the result seems a bit too warm. So you want something with a bit more "zing". In that case, you should check out the following:

• Mazda silver plate 12AX7 (expensive)
• Telefunken ribbed plate 12AX7
• (Russia) new Tung-Sol 12AX7

Electro Harmonix 12AX7 are fairly bright. I usually don't like them, but they can certainly counter warmth. Later Sylvania 5751 were also pretty bright, and I didn't like them - but who knows, here. 

RCA are weird - most are warm but some of them have been unbearably bright (and those tubes have been from Andy @ VTS with the cleaned pins; wonder if that's related).

If a phono stage has a strong "voicing" on its own, you are not going to fully counteract it with tube rolling.