Actually the tube that was in there was an Amperex 6DJ8 A frame...not my cup of tea LOL!!
03-28-10: Sideways
Though the pin-out may be the same for a 6DJ8 and a 12AT7 the two tubes are not interchangeable.......
12AT7
6DJ8
Are you sure the tube was a 6DJ8? The heater voltage on the 6DJ8 is 6.3V and the 12AT7 heater voltage can be configured for 6.3V (heaters paralleled) or 12V (heaters in series).
The tubes application characteristics are different as well.
Jim