Brian May's KAT BM30 Specials
Brian’s AC30 BM Specials
Whilst the AC30 amplifier has provided the musical backdrop for so many of us with its lovely jangly tones that even when clean, have some degree of harmonic distortion added due to Dick Denny’s design of not utilizing negative feedback when pretty much everyone else did, it has never really been optimized for Brian’s style of playing. Sure, in the past, it’s had its unused channel pre-amp valves removed, and its GZ34 rectifier valve swapped for solid-state, but not much else. So, when Pete asked me to build him three back-up amps for what was then the forthcoming tour, I jumped at the chance to design an amplifier for Brian that was designed from the bottom-up to optimize his style of playing and set-up.
The design work took around three months to complete, then it was time to have the airframe aluminium chassis sections built, the bespoke transformers and chokes developed and wound, and the cabinets modified.
The actual design was quite unique in many ways, especially for a guitar combo amplifier. For starters, the chassis was split into two units. The power supply was built into one unit and then mounted in the bottom of the cabinet, the pre and power amplifiers were built into the main chassis that mounted at the top of the cabinet as usual. This time though, the valves were effectively mounted in a cage on the outside of the amp. This was done for two main reasons: firstly, to reduce the sonic pounding they were getting by being mounted within inches of the speaker cones and secondly, to allow them to work at much lower temperatures than they previously did inside the cabinet. Both these factors contributed to increased valve reliability from the get-go.
The new up-rated 45 Watt output transformer with just a single 16 Ohm secondary winding was mounted central to and just behind the quad of EL84 output valves. This allowed for very short and equal length anode wires which reduced any stray noise and increased efficiency. The two speaker feeds were dropped through the chassis and connected directly to their own speaker unit thus, negating the need to have a jack connector (they nearly always fell out on tour), and again, increasing efficiency.
The new mains transformer was developed to give just the same amount of sag when paired with a solid-state rectifier as the old transformers did when run with a GZ34 valve rectifier. This was further made possible as we can set the exact mains voltage the amps run at using the power conditioner in the rig, so I could design the amplifiers to run at a specific HT voltage and be stable anywhere in the world.
The power supply unit was mounted in the base of the cabinet. This kept all the inherently noisy components away from the audio sections of the amp. The uprated choke and increased filtering were situated fully up-stream which gave a very smooth HT for all areas of the audio path. Additional drive and filtering circuits were added to control the EL84 screen voltages. This allowed us to control the screen current to a higher degree which, again, is reducing the torture inflicted on the output valves.
The final amps were delivered to Allerton two days before the gear was collected for the tour. The rear panels were still just black painted plywood as I hadn’t had time to finish them in Tolex! We connected them up to the rig and powered it all up. They sounded immense and loud. Far louder than any other AC30 in the rig. Pete decided there and then that he would run these as the main amps rather than the back-ups they were intended for. We both thought it was a brave thing to do as they were at best, fully un-tested in a gigging environment, but they just sounded so good, he figured it was a chance worth taking. So that was that. They carried the ‘main amp’ duties for that entire tour and on their return, I had the chance to finish the backs with Tolex. They have since been on ‘front-row’ duties on every tour and have been ‘hyper reliable’. On the first tour we lost one valve and I think we blew one speaker. Since then and on this last American leg of the tour, we have not had a single failure.