Groove Tubes 12AT7 Pre Amp Tube
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 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.)