Originally Posted by
RothwellAudio
Without seeing a circuit diagram it's difficult to say exactly what will happen when using 12AX7s instead of 12AU7s but generally valve amps are pretty robust and there's a good chance that no harm will come of it. Worst case scenario is that there will be more power dissipation in a valve or a resistor than they're designed for and they'll either slowly cook or burn out pretty rapidly. Also, a voltage across a capacitor could rise too high and cause failure. However, more likely than not, no harm will be done.
Bear in mind that the operating points for the amplifier will be different and distortion might be worse, maximum power output might be reduced etc. Best to just stick with a 12AU7 where the circuit is designed for a 12AU7 imho.
^ Good advice. +1
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