The famed D-upgraded circuit of the famous 5e3 Deluxe built by ERSO Amplification. This amp keeps all of the beauty and mojo of the original Deluxe while unleashing that circuit's full potential. This amp is vicious and makes some of the 50 and 100 watt Fenders that I have owned, sound puny. And this is all coming from 15 watts. It is the nastiest 15 watts I have ever played.