Ok to substitute 12AX7 for 5751 tubes in CJ Amp?

12AU7 = 5751,

No. I think you mean 12AT7 with an amp factor of 60. The 12AU7 is much lower at around 17 at 250Vdc. 5751 is 70. 12AX7 is 100.

If it helps, here is a post from my Virtual System page:

Swapped out the (phone tubes) GE 5751 BPTM for NOS Sylvania 5751 Gold GPTM.
Firmer and tighter bass, a little dryer than the GE which are very liquid. Definitely a bigger presentation with the music swelling more appreciably. The sound is a like the Telefunken sound. I always feel that grey plates have a slightly grey coloration whereas black plates seem to have a slightly darker presentation... like there are are blacker silences. There is a pretty stark difference between these tubes.

Sylvanias:
speakers disappear, I can't even imagine the sound is coming from the speaker.
fuller, more beguiling bass, upper bass and lower mids.
spittier dryer treble.
more pluck and snap on strings.
piano sounds more expansive.
treble not as sparkly as GE, less air, seems busy.
color, timbre not as delicate as on the GE tubes.

Switching back to GE:
everything seems a little smaller, quieter and way blacker. More musical,less hi-fi.
definitely more detail and air. I can hear the acoustic here much better.
Crystal Gayle on Waits - Is There Any Way Out of This Dream is simply more musical like a 300B SET. Wispy, breathy trumpet. Recording hiss is more audible.
Looks like the GEs are staying. They make me smile, the Golds would impress the those who like a bigger, rockier sound.

I found the GE black plate triple micas to be the best. I tried the Sylvania Gold too. They both have completely different presentations.