Yeah, the class A thing creeps into the sales talk. It must be the singular "A" letter that sounds better than "AB" :yup:
They also make it seem that autobias came after fixed bias (historically speaking), whereas the reverse is true. Denny basically took a Mullard/Philips circuit and played a guitar through it. The thing is, he was bright enough to listen to the amp as it was, and appreciate the qualities. Fixed bias gives more headroom, but less "tone". Cathode bias is safer though (generally speaking) and does make engineering sense as well as musical sense. Negative feedback was also invented later. All early amps had no NFB, since it sucked the gain and the harmonic distortion away. So the AC15 is really a pre-war amplifier using the modern miniature tubes. This tube arrangement "ef86->12ax7->el84" is louder than any other set of tubes that has gone before. The EL84 is the power tube with the most amount of gain of all. So it sounds louder in the same circuit than other power tubes.
Here's a Mullard circuit that nearly looks like the AC15, and it was published by the tube manufacturer.
So Denny ripped out the NFB; changed the tone controls to early radio type and a star was born?
But luckily for us, the EL84 turned out to be a damn good sounding tube too.
