12ax7 tubes can be enterchanged with xxx tubes?

You may want to try the RCA 5751 3 mica Black plate...Sweet with good bass...Its all a crapshoot,but the RCA's are a nice sweet 5751 tube.........

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.

It's my understanding that the 12bz7 might be compatible with the 12ax7 as well. You certainly want to check and see if they will work in your piece though.