Groove Tubes

This tube 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 12AT7's make-up is very similar to the 12AX7 and can be substituted for it. Some players 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 the GT-12AT7 is selected for durability. The GT-12AT7 is tested under extreme listening conditions and guaranteed to be free of excess noise and microphonic ring.