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.

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.

The tube most likely to give you the warmer tone is an NOS Mullard. The Blackburn Mullards and all of the 50s to 60s longplates will be your best shot. I should forewarn you the market is rife with spent used pulls that have questionable performance. The Jolida does not put a lot of demand on the tubes so you may get away with a couple of weak ones.
In the out put section of the Jolida CD player I was quite pleased with the 80s Mil Spec Mullard which used to be available here on the Gon by the same distributor of the KR tubes. I think it was the CV 4004.
For a wide sound stage I agree that the 5751 is the ticket. I would stay with grey plates as they always seemed a bit warmer to me. For Bass emphasis in particular I would opt for the GE, with the triple mica being preferred. The 5 star, the JG mil spec, and civilian all possess abundant bass. The 5751 Blackplates of all brands are in general a better all around tube IMO. They have taughter bass and cleaner treble.
As you have been told it is always a matter of taste. If you want to save a bundle, and just get a sense of how a 5751 might sound, I would certainly try the current production Sovtek, which the dealer will back to boot. It doesn't hold a candle to my and most everyone else's favorite Triple Mica Blackplate 50s vintage Sylvania, but they are virtually extinct as true NOS. The cost for good pair is prohibitive and may not be appreciated in that application.
FWIW only a 12AX7 or it's known close subs like the 7025 6681 and 5751 functioned in my CD player. I.e. the 12AU7s nor the 12AT7s simply did not work for some reason.
Good luck

5751 should work fine. 12AT7 might work. I wouldn't try 12AU7.

I like the 1950s GE and RCA 3 mica black plate 5751s.

Can anyone provide a good source for the RCA 5751 triple black plates. I ve seen them for sale by upscal audio 50/ea. is this the going rate? do i need to purchase the gold version for xtra 25/ea for my application?

$50 for a more or less gaurunteed tube of that age and scarcity is a very reasonable price. In that application, I really don't think you need to spend the extra money for the quieter tubes. Obviously YMMV etc. try the standard grade and see for yourself.
FWIW The RCA triple mica blackplate is indeed a very good 5751 but not particularily warm.

IMO:
So many really good choices in vintage 12ax7 that I would recommend trying:
Almost any 50's or 60's vintage USA made 12ax7 will sound better than current production ones. Try RCA or GE or Sylvania grey plates also...not quite as refined as the soo sweet black plates of that era, but still very good, and don't command the premium prices either.

Holy grail 12ax7 for me: Telefunken smooth plate, closely followed by the ribbed plate:

Any Mullard or Amperex from that era:

I have 15 12AX7WA Sylvania, USAF stamped mil spec tubes dated 12-64 that are some of the finest tubes I have...absolutely smooth, very quiet, with very fine detail that I paid $5/ea. for....so you can find super tubes for fair prices if you just broaden your search field a bit.
Good Luck....tube rolling is great fun!

Twinbugles would you kindly share where it was that you found $5 12AX7s US made in the 1960s. I would like to expand my search to places offering such bargains. Can you Help me out? If they also have RCA 1950s triple mica blackplate 5751s for a similar price please pass that along as well. Actually prefer Sylvania but any brand of 5751 would be fine Thanks in advance.

i have c.a.l. sigma 2 dac that has a 12ax7a stock. so far i have tried a JJ 12ax7 ( a bigger sound stage, more detail but not as smooth) a phillips 5751(didn't really notice much of a drop considering the gain difference, less detailed and not really as coherent). i bought a psvane 12ax7 from grant fidelity here in calgary, and have about 20 hours on it so far. it seems to have most of the qualities i was looking for-smooth with a nice big soundstage. so far it's my favorite, with the stock tube second. i'm gonna give the psvane a bunch more hours and see what happens, all the tubes so far are relatively cheap-$10.00-$37.00. if i'm still searchin', i may pay some big bucks for some nos mullards or the like for $100.00 plus for a single tube!

Hi, Mechans:
The 12ax7 Sylvania USAF tubes I bought locally, friend-of-a-friend type deal. Almost all of my other tube purchases were made by going to vintage radio shows and an event called a "Hamfest" which is a swap meet for ham radio collectors. These are where I have found my best deals, when some elderly gentleman has a box of unsorted tubes he is selling for $20. I buy the box, plus any others I can find, and go home and sort the tubes and test the ones I decide to keep (I have a tube tester also). The majirity are tubes I cant use or have no need for, but I always end up with some I am looking for.
Just check your local paper, or go to a local electronic shop and ask around.
Good luck!
PS:
If you are interested in contacting me regarding tubes, my email is
Twinbugles52@comcast.net

Thank you Twinbugles-
I have heard of Hamfests but I am afraid they will peg me as an audio guy not a radio guy and jack up the prices. What do you do with all the odd tubes. I have bought lots like that in the past and don't have the heart or mindset to throw them out. I am in southwest Ct. near Danbury and the NY border for the time being. You can use the email feature they have to look me up . If you know a hamfest is happening in my area or a couple of hours by car please let me know Thanks again.

I know this is stating the obvious - but any ECC-83 is a direct sub for a 12AX7.

The most thorough discussion of tube equivalents can be found at Joe's Tube Lore on audio asylum.

I have not done substitutions in a Jolida, so my comments are just a general impression of various 12AX7 variants. For a warmer sound and big soundstage the Mullards and the Amperex are certainly good candidates. I have found that in a some applications, the Mullards sounded a bit phasey and the center image was not as well defined. In almost all applications, Amperex Bugle Boys sound great to me, though they can be somewhat pricey.

The blackplate 5751s are also a nice, reasonably smooth, yet dynamic sounding tube. They deliver considerably less gain than a 121AX7, so you need to experiment to see if they will work (in most applications that would not be an issue, it may be more of an issue in a phonostage).

I like the Telefunken ECC83, but, that tube is more on the lean, clear and dynamic side than on the warmer side. I run ECC803S in my phonostage, but, these days, they probably cost more than the Jolida (and they are VERY lean sounding).

The 1960's RCA 12AX7 that came with my phonostage sound quite dynamic and punchy, but, they are leaner sounding than the Mullards or Amperex tubes.

If one of the above posters is correct about the Jolida not pushing the small signal tubes very hard, then a purchase of new or relatively new premium tubes would be a decent investment because these older tube have PLENTY of built in life.

Good luck on your search.

Anybody tried mullard CV4004? Your opinion?

I have been using the 12BZ7s (gray plates sound smoother than black plates) in my Jolida for more than 2 years. Significant improvement in my opinion; smoother analog-type sound with more presence with my MG 3.6 Maggies. The 12BZ7s draw twice the filament current (300ma per tube) as the 12AX7 so I had to mod the 12 volt power supply to protect the 12V regulator chip that is rated at only 1 amp. The 12BZ7s worked fine without the mod but pushed the regulator to its limit. (No issues with plate voltage or current) The 12BZ7 is taller so you can't use the tube shields. The tip of the tubes just clear the top cover and protrude slightly through the vent holes.