12ax7 tubes can be enterchanged with xxx tubes?

the 5751 may or may not sound better to you. You will need to try them to see. 12au7 and 12at7 can be put in the 12ax7 but the circuit is not optimized for them. It shouldn't hurt anything thing if that is what your asking. Tubes are very subjective and it's hard to say what you will like. I would personally stick with 12ax7's or 5751 in that position though. There are a lot of 12ax7's to choose from or 5751's new issues and NOS.

Being that the 12ax7 is used for the gain on your CD player the 12au7 will have considerably less gain than the 12ax7.

Try the 5751 first you may like it. A little less gain but you may like them. If your wanting more clarity and nicer sound think of changing the coupling caps. Other mods can be done as well. Good luck!

u can try create audio 12ax7. it's smooth and grain free.

A 12au7 has 80% less gain then a 12ax7,a 5751 has 30% less gain than a 12ax7. I would not do that change! Just try some different 12ax7 tubes,I like older sylvania or Ge you can get them cheap.

I had been using Jolida CD Player (please see my system page)with RCA Blackplates 5751 and Sylvania Gold Brand 5751 for almost 8 years now and counting. You will only "hear" the change in gain if you are using the CD player direct to your amp without a volume control, otherwise, if you are using a preamp, it does not matter and a few clicks/small turns to open the attenuator of your main preamp is all you need.

The 12AU7's or the 12AT7' similarity to the 12AX7 is the pin out and the glass envelope, it should stop there.

Try 5751(RCA, GE, Sylvania). Once you hear it on your CD player, you will never use a 12AX7 tube in this particular player again. Oh, I measured the output of the CD player using 1kHz test tone before to compare the output to that using 12AX7's, I believe if I recall correctly, I got approximately 0.9 to 1.2 volts for 5751 depending on the input impedance of its load. The 12AX7's was about 2.15V I think.