[quote author="kevinnyc"]what are some of the advantages and disadvantages of using the 12AX7 tube as the first gain stage in a mic preamp? Also, through my research of tube preamp schematics I noticed that the German designs used pentodes a lot and the American designs orften used triodes (on average, especially documented in Radiotron Designers Handbook). Does anyone know why the Germans favored pentodes over triode design? Can pentodes opperate with comparitively lower noise than triodes in preamp applications?[/quote]
As a general rule, no, since pentodes have an additional noise source called "partition noise". It's still possible to make quiet pentode-based preamps, though, especially with high-ratio transformers -- which have their own set of issues.
It's also possible to use pentodes in triode mode with good results. The EF86 and 5879 often get used this way.
As to 12AX7s...under certain circumstances they can be very quiet, although there are other tubes that, if properly operated, are even quieter. The distortion produced by 12AX7s depends very much on how it's biased and what its load is, but that's really true of any tube.
Finally, some folks strongly dislike the "sound" of high-mu triodes like the 12AX7, although I've never read a description of what exactly they don't like. (I put "sound" in quotes because a triode's sound can vary all over the place, as described above.) In general, medium- and low-mu triodes can be set up for lower and more benign distortion than high-mu triodes, but again there are plenty of exceptions.
Peace,
Paul