Fender Groove Tube Valve 12AT7/ecc81 is used widely in most of the US-made amps and, in particular, the Fender amps. Fender amps with reverb use two 12AT7s - one for the reverb drive stage, and one used as a phase inverter to drive the push/pull output stage.
The 12AT7s make-up is very similar to the 12AX7 and can be substituted for a 12AX7 in a pinch, although it won't sound very good. Some players do prefer to use the 12AT7 in the driver stage in a Marshall for a cleaner, brighter tone. (The driver or phase inverter is the 12AX7 located nearest the power tubes.)
The 12AT7 is rarely used in a tone-generating stage, but rather in function stages, so our GT-12AT7 is selected for durability. The GT 12AT7 tube will some times bear the number 6201 as well as 12AT7. This is a special (and more expensive) production run with a rugged construction. The GT-12AT7 is tested under extreme listening conditions and guaranteed to be free of excess noise and microphonic ring.