"Powell" said:

In any case, when the design calls for AX7, try 7025 RCA.

And, I understand it ECC83, which is very pricey:
http://store.yahoo.com/thetubestore/12ax70ectyp.html

ECC83 is just the European nomenclature for 12 AX7.
There are as many different ECCs as there are AX7s.
I use the Mullard CV4004 for a phono preamp, which has to deal with
similar small voltages as a mic preamp.
This is a very expensive tube if you have to buy them today.
I'm glad I bought some stock 15 years ago, when prices weren't
skyhigh.
The RCA7025 is a very silent substitute, and would be my second choice
in a RIAA or mic preamp.
Forget about Telefunken E83CC, they're way overhyped.
Strange enough, their ECC803S is a far better tube for these purposes,
it's just damned hard to find.

For a microphone amp, a better choice would be EF86 in
triode, IMHO.

I didn't see where the EF86 was a 12AX7 substitute... are
you sure it is? And it appears appears to be a pentode
design.

Correct, I'm talking about diy designs.
This won't work in an existing amp, obviously.
BTW, any pentode can be converted into a triode just by tying the
screen grid to the anode.

I'm leaning toward a highly sorted RAM tube because
of the noise factor. Or maybe purchase both the
Sovtek 12AX7LPS and Ei12AX7 types... can't justify
all three purchases, unfortunately.
http://www.tubeaudiostore.com/tubes.html

Hmmmm.......I'm not very fond of the Sovtek 12 AX7LPS.
It probably would be better to use this tube in a higher level stage
such as a phase splitter in a poweramp, where its sonics are not that
critical.
Sadly, I don't have experience with the EI.

But be aware that between the same brand and type, differences may
exost that are larger than between different brands altogether.
As the famous Alan Marcy (Tubegarden) already pointed out, just
plug'em in and listen :-)

--
Sander deWaal
Vacuum Audio Consultancy