Is a 12au7 a safe replacement for a 12ax7

In a phono stage the power dissipation difference between these two tubes does not mean anything. The main difference is voltage gain. If you have enough gain to listen at your normal levels it is just a matter if you like the sound of one tube versus the other
Alan

Have a you tried a 5751? This is a direct drop in replacement for a 12AX7 and has 30% less gain.

There's no problem from the standpoint of the maximum ratings to swap a 12AU7 for a 12AX7. But I've yet to see a phono-preamp circuit where this swap won't have a significant effect on the RIAA accuracy. If the circuit uses passive EQ, the 12AU7's lower plate resistance will affect the drive impedance to the EQ network, and alter its response. If the stage uses active (feedback) EQ, then the tube's lower transconductance will affect the loop gain . . . and the response of the active filter.

If you enjoy swapping tubes around, then certainly go ahead and give it a try - you won't hurt anything. But it's a safe bet that the sonic differences you'll hear are overwhelmingly caused by differences in frequency response, in the same manner as a tone control.

Thank you to the above posters
in the past,when rolling tubes it's usually different nos brands of the same tube.
I have also tried 5751 tubes to lower the gain.This helped somewhat.
Even tried 12ay7 's.This worked for the gain but I couldn't find a brand that pleased my ears.
I had some 1960 12au7 so I tried them. This not only allowed better use of the volume control but a clarity ,body and space in instruments that I had never experienced in my listening room.
Trying to find information on this tube I noticed the difference of dissipation and that brought about my question.
Again thank you for your responses

I had the same problem with my phono preamp. Too much gain
with the 12AX7 that should be used.
I tried a 12AU7 and that was not enough,so I tried a 12AT7 and
it is perfect with the output of the cartridge I use.
The gain factor of a 12AU7 is 19 and the gain factor of a
12AT7 is 60. BTW I never had a problem doing this in eight
years. So if the 12AU7 works for you don't worry about it.