pre amp tubes

I would strongly suggest not subbing those tubes ....   if the circuit calls for a 12ax7 use one, or an accepted equivalent.   Ecc83, ecc803, cv4004...   a 5751 or 7025 are both lower gain ax7 subs.  

I have a pair of early 60's RCA 12ax7 in the input stage of my amps and they are one of the most well rounded , balanced sounding tubes I've ever owned.   

I bought the amps with those tubes and they probably have some serious hours on them.  That said , I think I am going to try the Gold Lion 12ax7 as I have heard they are the best current production 12ax7.

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.