Inherent Difference - 12AU7 and a 12AT7

Because these are different tubes and not really interchangeable it's hard to give you an answer. The 12au7 has a gain of about 19 and the 12at7 is 70 I believe. You would need to change the biasing of the tube to get it to perform to it's best. You could plug an au7 into at7 position and it should operate but not at it's best because of what I mentioned. 12at7 are often found as phase inverter/splitters. I commonly see 12au7 or 12ax7 as the preamp tubes in the first stage of amplification. Can't give you an exact reason why. I have used amps and preamps with both and it came down more to the brand of tube more than what kind of tube for me. "referring to these two tubes" Some people like the 6SN7/6SL7 family better and don't like either. I like all mentioned above. Don't know if I have helped you?

Well here is the issue; I have a Hovland HP-100 preamp and initially they used a 12AU7 in position 3 of the line stage. (Position one and two are both 12AX7's). In subsequent issues of the 100 they went to a 12AT7 in position 3. I can't even remember now which I am currently using but I need to replace them and am debating which way to go.

I love the Hovland but the tubes always get noisy fairly fast; after a year or so... I guess that while I am replacing the AX7's I could buy one AU7 and one AT7 and see what difference I could detect. I am just wondering if one or the other would help cut down on the quick degradation to noisy status...

Some info on the gain characteristics of the "12" family of 9 pin miniature tubes:(http://thetubestore.com/gainfactor.html) I would suggest some of the better/quieter, and longer lasting, European varieties(NOS- Siemens, Teles, Valvo, Amperex, Mullard). I've listed them, in order, from most transparent, to warmer/tubier. YES- Each manufacturer has a characteristic sound, in MOST gain circuits, regardless of the actual iteration.

Just because the company switched from an au to an at doesn't mean that you can. I'd bet they changed something or determined that more gain was needed and made the modifications. As Hovland before you even consider it.
As far as lasting 1 year, these tubes should last 3000 to 5000 hours. Unless you are leaving it on 24/7, you should get many years out of these. Even if they are wearing out, they shouldn't get noisy so easily. Usually your dynamics will just flatten out?

I'm sure that it is okay to swap between the AU7 and AT7; Hovland confirmed that for me several years ago; when I asked them what the differences were they said 'try it'and let them know what I heard and liked...

I think that I will just buy one of each and find out for myself. I have used the NOS Mullards from Upscale Audio but again these seem to get noisy way to fast. I may contact the Tube Store and get theirs and try something different.

Have anyone tried the outrageously expensive EAT 12AX7's? I am tempted to try them once; wonder if Music Direct would absolutely stand behind them and take them back if I was unhappy with the performance?

And, wonder why EAT does not make an AX7 or AU7?

Stickman as I mentioned they should work without issue but if you go to the web and look at the tube data you will notice significant differences in the voltage and current characteristics of these two tubes.

Kind of like driving a Porsche with a Chevy engine in it. It may work but it's not how it was designed to run it's best. I wouldn't spend a lot of money on nice tubes just to try it. But that's me.

Maybe just get some nice 12au7's like Rodman mention all he said are nice tubes. I have a friend that uses the EAT tubes and really likes them but I am not willing to fork up the money to try them.

The do make a AX7 Music Direct carries them. $225 a pair. That is what my friend owns.

Sorry that may be a piece. http://www.musicdirect.com/product/73403

EAT DOES make the 12AX7 tube, but it is referred to by the European moniker: ECC803S, which was the best of the best, for that tube family(http://www.gcaudio.com/cgi-bin/store/showProduct.cgi?id=501). The 12AX7, and the 6DJ8/6922(ECC88), are the two most popular 12 volt, 9 pin miniatures(most commonly used), in the industry right now. I'm certain EAT took that into consideration, when deciding what to manufacture.

Sorry,I meant, why don't they make a 12AU7 or a 12AT7...

Same pinout, but the gain is different. A 12AU7 only has a gain of 17 or so, a 12AT7 has a gain of 60. The 12AT7 was originally designed as a VHF oscillator or mixer tube, but it can be used in audio circuits.

The 12AT7 has more gain and will sound louder. Great if you want to beef up output from a DAC or preamp that normally uses 12AU7 tubes.

Because the current draw of a 12AT7 is similar to the 12AU7, in most applications a 12AT7 it can be substituted for a 12AU7 and the primary considerations are more gain (louder sound) and the perceived quality of the sound.

I used a 12AT7 in my Audio Note CD player in place of a 12AU7 and got wonderful sound from a particular Mullard that exceeded other 12AU7s that I had tried.

However, you cannot substitute a 12AX7 for either because of a huge difference in current.