1947 was the year the 12AX7 first appeared, since then it's been VASTLY improved upon, I've always replace bog standard 12AX7 with a 5751, Siemens if I can find one, but any brand will be MUCH quieter and more long lasting.
The PHILIPS one is a superbly warm and rich sounding one and is a great valve to use in Fender Hot rod series of amplifiers as it tames the fuzzy more drive setting.It also works well in the front end of any amplifier if gain reduction and clarity are your aims. It has more midrange and less top end than The G.E 5751 which also makes it the ideal choice for blues and harmonica players.
The 5751 has a bit less gain than a 12AX7, (30% less) and a more linear tone, Smoother, with outstanding clarity and "chime." Overdrives in a different manner when compared to other tubes (not as harsh) Very cool.
Although I must say I prefer the sound of a 7025, it's Fabulous in Blackface Fender amps. It's been tried in a 66 Vibrolux reverb 2x10 combo and the tones were all there. "Light and shimmering or really aggressive, it's all good."
BTW the ECC83 (B339, B759, CV492, CV4004, CV8156, CV8222, ECC803, ECC803S, E2164, and M8137; and the lower-gain low-noise versions 5751 and 6851, intended for avionics equipment.)European designation is only a horse of a different colour but running in the same race, in this case 6.3 volt heater as opposed to the 12 series valves that are of course 12.6v. So you can see, NOT interchangeable easily.
My God isn't Wikipedia informative.. I HAVE paraphrased some of the info..as is my
wont..