12ax7 tubes can be enterchanged with xxx tubes?

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!