NOS Tube Recommendation for Quicksilver Phono Pre Amp

If I'm wrong please correct me.  The 5751 has lower gain than a 12ax7, but the are otherwise interchangeable.  As such either one may or may not be an optimal choice for a particular circuit design, but they will work.

Yes, it has only 70% of the gain of a 12AX7.  Aside from reducing gain, if it is used as part of the equalizer circuit, that lower gain could screw up the equalization curve and mess up the sound that way.

@larryi @cey , this is great stuff (insights like this are what I was hoping to find when I posted this question). Quick follow-up: if I decided to go with 5751s (several people have reported these clean up the sound in their phono stages), is there a particular position I should avoid so that the RIAA equalization curve isn't disrupted (I will ask the same question when I call Mike over at Quicksilver today and report back)? My current thinking is to put the 5751 in the gain position, and just roll the input and cathode follower tubes with NOS tubes (same tube type).

I'm thinking GE triple mica 5751 for gain, Rayethon/RCA 7025 12AX7 for input, and Telefunken/Amperex Bugle Boy 12AT7 for cathode follower... Also, once I go with a 5751 in the signal path, do I need to make all of the tubes 5751 because of the lower gain?

thanks again

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?