pre amp tubes

Generally, you can use a 12AU7 or 12AT7 in place of a 12AX7 without fear of harming your preamp, as they are electrical similar but the U7 and T7 have lower gain. That doesn't mean it will sound the same; it might be better, it might be worse. Checking with YS Audio first is always the safest bet.

I have a Jolida JD9 that will use all three, and with MM cartridges I prefer the 12AT7 over the stock 12AX7. The 12AU7 just doesn't sound right regardless of cartridge.

The heater current of a 5751 is slightly higher than a 12AX7, be sure that your power supply can take it before making this change willy nilly.

Additionally, the gain is 70 vs. 100 for a 12AX7, so your signal to noise ratio may improve, but the lower gain may be a compromise in certain systems.

12AU7 and 12AT7 have no place being substituted for a 12AX7 as they are electrically dissimilar. However if you are more competent than the designer of your circuit, by all means have at it. You will do no damage.

http://bitsavers.trailing-edge.com/components/rca/1961_RC-21_RCA_Receiving_Tube_Manual.pdf

And if your circuit is a hybrid tube/transistor Chinese designed circuit based on op amps, you may very well be more competent than the designer of the circuit.

I have the Audio note M5, preamp and i changed tho original tubes (sounded very woolly) in Telefunken . What a difference !! I suggest: Telefunken,Siemens,Phillips,RCA

Look at Upscale Audio website. Lots of great info, videos, and actual user reviews of pre amp and power tubes. Bought many from them and very satisfied.

information on Brent Jesse's website (I'm only a customer) on brands , sounds etc., as well as available individual tubes and related assessments

http://www.audiotubes.com/12ax7.htm