Yng- I reread your post this morning and it's even more absurd than I initially thought! Are you actually suggesting that all 12a-7 series tubes are manufactured as 12ax7s,then tested, and the ones with lower amplification factor are then labeled Au or AT etc? This is completely nuts! Buy yourself a copy of the RCA receiving tube manual(Op Amp Technical Books in Hollywood and numerous other places on the web) and read some of it before you offer any further advice to anyone concerning vacuum tubes. While a 12AT7,12AU7,12AV7,12AX7,12AY7and 12AZ7 are all dual triodes, they are all uniquely different designs with different Mu, plate resistances , aplification factors etc. If your theory were true how would you explain a 12DW77?(do some research if you don't know what it is). Nothing personal, but lets not spread complete disinformation on this forum just for the purpose of impressing people wiht your knowledge about a subject that you aren't that knowledgeable about.