Can I use 7025 tubes instead of 12ax7

You can substitute either. The 7025 was a low noise 12ax7 primarily used in Fender guitar amps. The 5751 has 30% less gain than a 12ax7, but it will work.

Yes, you can use both, but the 7025 can use 2 different grid voltages (-1 or -2). The -1 grid voltage has a plate voltage of 100, while the -2 grid voltage has a plate voltage of 200 which pretty much equals all characteristics of the 12ax7 tube. Hope this helps.

The 7025 is a direct 12AX7 substitute, the 5751 is not. In many cases the 5751 can be used in place of a 12AX7, but in some they sound positively horrible, as in a CAT preamp line stage I had.

Why not use the finest NOS ECC83 such as Telefunken or Siemens? Personally, I prefer the TFK ECC83 "Ribbed" plates. The rare & expensive TFK ECC803 is extraordinary in sonics and price but the ribbed ECC83 is exceptional in its' own right. Siemens E83CC is another winner.

Jim Ricketts/tmh audio