Replacing 12AX7 to 5751 for pre-amp

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.