The 12AT7 ECC81 is a high-frequency twin triode in a small 9-pin miniature glass package. This tube has a mu factor of 60, as compared to the higher gain 12AX7 with a factor of 100, and the 12AU7, with a mu of only 20. As you can see, it shares similar pin connections with it's cousins, but these tubes are not interchangeable. The 12AT7 is commonly found in Fender amps as the phase invertor and reverb driver tube .Also known as a ECC81.
