Colin’s KAT BM Plex-30 Amplifier

Colin’s KAT BM Plex-30 Amplifier

Colin originally asked me to have a look at his Dave Petersen-built AC30 with a view to carrying out some repairs and modifications to it. It was subsequently decided to leave the original amp as was and design and build a new amplifier from scratch. It just gets to a point when you are looking into making sizable modifications to an already heavily modified amp, that it becomes far more economical to start from square one rather than rip out most of what’s there and try to modify the boards, transformers, chassis etc. So, that’s exactly what we did.

We started with a blank piece of paper and Colin outlined what he was looking for. Basically, he wanted a full-blown Normal channel for BM-style tones, a Plexi-style channel for a little grit and an EF86 channel for that lovely vintage Vox jangle. The first two were relatively easy, the EF86 channel required a bit of lateral thinking as EF86 valves are notoriously microphonic and paired with a loud AC30 power amplifier can quite easily burst into unwanted feedback. My solution was to mount the valve inside the chassis to protect it from the sonic wavefronts emanating from the rear of the speakers. The valve was mounted on a weighted plinth (to give the assembly some mass), along with its circuit board and components, that was then suspended in vibration isolating shock mounts from an aluminium sub chassis. For a belts and brace’s approach, the valve was also encased within a thick heat-shrink sleeve to reduce envelope ring.

So, the amplifier was configured as three separate input channels, each with a high and low input jack and channel volume control. These could then be routed via a three-way tone stack at the pull of the ‘Middle’ tone knob. The tone stack was also configured to add a touch of gain on engagement.

Post tone stack, a Master Volume control could also be brought into the signal chain at the pull of the MV knob. A standard VOX ‘Cut’ control was also included and was in circuit full-time.

Output-wise, Colin wanted the amplifier and cabinet to be as flexible as possible, so an amplifier output and speaker patch panel was added to provide access to the amplifiers’ 8 and 16 Ohm transformer taps as well as the individual speaker units and cabinet as a whole. The patch panel was fully ‘normalled’ which meant that all the correct routing was done with no leads inserted.

The chassis was constructed from folded and welded airframe-grade aluminium and finished with a smart Alu-printed facia panel. The complete amplifier assembly was bolted into a new finger-jointed Baltic-ply cabinet that also housed a Celestion Blue and Anniversary speaker.

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