Mike’s AC215 Dual 15W Amplifier

Mike’s AC215 Dual 15W Amplifier

So, Mike and I started discussing this amplifier concept late in the evening at the bar in the hotel following a Red Special Meet-up in Theale. Mike was asking if there was a way to make an AC30 lighter and more transportable as generally he needed two amps and didn’t fancy lugging two 30Kg amps everywhere with him. He also wanted to use them at home and in the studio, so space was also a major consideration.

Mike then asked if there was a way that two amps could be mounted into one single cabinet. I stood there and did some basic designs in my head, then replied with one of my classic Nigel Knight lines: No, we can’t fit two AC30’s into one AC30 cab, but we should be able to fit two AC15’s in there! From that point on, a myriad of discussions ensued followed by some 3D CAD sketches and renders of what the amp would look like. I pretty much had to fully design the amplifier before I could apply costs to it, so we spent many emails discussing the audio signal paths, buffers, splitters, tone stacks, master volumes and attenuators before getting onto the cabinet itself. We decided that the front baffle should be split with the individual speakers aiming away from each other in order to increase the acoustic separation when recording. Mike also wanted the AC125, as it was by then named, to still look like an AC30, so I had to try to fit it all in. At the end of the day, the finished cabinet was the same width and height of an AC30. It was just 30mm deeper.

With the concept now approved and costed, the amplifier circuit boards and chassis members were designed and built. Special transformers and chokes were developed just for this one-off amplifier. The up-rated output transformers, rated at close to 23W rather than the stock 15W just about fitted on the pre-sized bobbins and cores (nothing any larger would fit). The mains transformer was configured as a single primary with multiple secondaries that could drive both amplifiers whilst maintaining full channel separation.

The amplifiers signal path was also a little different to normal. For starters, the input signal from a single input jack would first pass into a high impedance buffer circuit before being split two ways and fed, via an isolating transformer to two FX Send sockets on the rear. These were sited next to the two FX Return sockets and hard-wired ‘normalled’ to them such that with nothing plugged-in, the signal would route directly from the sends to the return paths. At this stage we are now working in two-channel stereo mode. The return jacks are effectively the inputs to the amplifiers and can be used as such if required.

Each amplifier channel has its own volume control and switchable (via pull-pot) three-way tone stack. Both amplifier channels are then routed via a switchable stereo Master Volume that acts on both channels simultaneously.

The outputs from the transformers are also fed to a speaker patchbay on the rear such that a pair of Load Star Attenuators can be easily clipped to the back of the amp and patched in with minimal fuss.

So, to conclude, this amplifier can be a BM style or TB Style AC30 just simply by plugging the guitar into the input jack. It can also be two AC15’s with standard ‘Normal’ channels or Top Boost channels by accessing the amps by the return jacks. Or it can be an entire rig whereby its internal splitter can feed a chorus or delay unit and the return Wet and Dry signals can give a wide spatial stereo sound field.