12at7 with 12ax7 tubes

Actually, there are some semantics involved, as the 12AX7 can be replaced with a 5751, 12AT7 or 12AY7, but the 12AT7 can only be replaced with a 5751 or a 12AY7. ERGO, "interchangeable" may not be the correct word.

Rodman or anyone else, have you actually substituted 12AT7s for 12AX7s or other swaps mentioned in the links?

It would be great if we could swap tube types but the Tube Store writeup says:

"These substitutions can also work in home audio amps, but we don't recommend trying this unless you are having trouble finding the correct tube type for your amp."

Yes- and for decades. Guitar, microphone pre, and home audio amps. The AT's are usually slightly less microphonic/noisy, and sometimes it's nice to have less gain in a driver stage. The 6201 tubes are especially nice in that(quietness) regard. I don't have anything presently, that uses the AX family, but not much has changed in tube circuit topography(input/phase splitter/driver stages) in the past 40 years(how long I've been paying attention), outside of the quality of the parts used in construction. I do have a pair of gold pinned, triple mica 6201's, left over from an amp that I was building, if anyone would like to try them(free).

Hi Rodman99999,

Thanks for your added comments. As stated I have replaced a pair of AX for a pair of AT's with no ill effects other than the lower gain. Tell me more about the 6201 tubes as I don't have knowledge about them. Will they work in place of the AX family?

Hi South43- A quote from a tube site: "6201- Usually appears along with 12AT7WA designation (military version) on the same vacuum tube. However, some tube makers made commercial versions of a 6201 as an "industrial" 12AT7. Sylvania made a 6201 vacuum tube in their Gold Brand line, and it came in gold boxes, gold silk screen on the glass, and most had gold plated pins. Amperex's PQ version of the 6201, also known as a E81CC with gold pins usually made in Germany or Holland.. Earlier USA made 6201 as well as the european made 6201 vacuum tubes, often have an extra mica spacer on top. These are also known as "triple mica", and are usually very quiet and well suited for hifi audio applications. Overall, the 6201 is a huge upgrade over a standard 12AT7, and were designed to meet military specs with critical applications." This site has some pics: (https://www.tubeworld.com/12at7.htm)