Replacing 12AX7 to 5751 for pre-amp

Does anyone try NOS Sylvania 5751 replacing 12AX7 for preamp? My friend convinced me trying 5751. I did try them for Dynaco PAS 3 and woo sound is warmer, treble is softer special with vocal. Bass is more tighten than before. I want to try more AU7 or AT7. Do they ok to try? I know that we can replace any 12AX7 with other lower current.
Audio guy the gain is 70 on the 5751 versus the 100% gain of the 12ax7. There are other minor differences in operation but will generally work without issue in place of a 12ax7. I have used 3 mica black plate Sylvania's and GE's and they were both nice. Personally liking the Sylvania better.

I wouldn't recommend the AU7 or AT7 unless your amp is set-up to use them. The circuit must be optimized for their use.

I personally now use the TJ Full Music 12AX7 and really love them. They come new and matched. But just like everything in audio. It's personal taste and trial and error. Good luck
I just went from a 12ax7 Tungsram ( and many other 12ax7's ) to a Sylvania 5751 3 mica black plate ( Yellow Label ) in my newly Upgraded Thor Audio Linestage and must say its the best 12ax7/5751 I have ever heard ( in my unit ).Sweet,a touch Forward,Dynamic and incredible inner detail.They are not cheap and are hard to find ,but once found well worth the effort and cost...
Try a set of Full Music 12AX7's. I find them better than the Sylvania black plate trip micas. Sold the sylvanias.
You can't substitute 12AU7 or 12AT7 for 12AX7. There are many 12AX7, 5751 or 7025 that you can try. Good Luck.
I have tried all the 5751 tubes in my 834p phono stage and found the Eat 12ax7's are superior. Not cheap,but the best.

Be aware that you could raise distortion issues with certain circuits by this 12AX7 / 5751 substution. While electrically it may be benign, it is no assurance of optimum sonics. This info courtesy of circuit designer Ron Cornelius via Audio Classics concerning research on my Mac C-22. Good Luck
I'm not sure but, I believe that if you swap 12ax7's out and put in 5751's in a "phono" preamp that it will change the RIAA curve. If someone knows for sure I would like to know.
Thanks for your information. From my taste, 5751 is my choice than 12AX7. For AU7 or AT7, I want to try but I need more advice on my PAS 3. I do not want to get risky on my preamp. Anyone already tried AU7 or AT7 for 12AX7, let me know the sound
Like everyone else on this string;I too have been to "Joe's Tube Lore" on asylum. That said,
I replaced the line stage Telefunken 12ax7s in my Dynaco Pas preamp with Sylvania grey plates and it was a revelation.The sound was much more focused and nuanced.Lower sound floor and much greater mid to high definition.
There is quite of bit of Dynaco Pas info about using 5751s.
One of the things that some have said is that you don't want to use them in the phono stage,but I have to try it too! The amazing focus and definition alone make it worth trying. any body have more info on this?

great thread