Lord Luke's AC30 BM Upgrades

A couple of years ago, I began working on a new Amp for Brian. This was a new design that was optimised around everything he needs and wants from an amplifier as well as increased reliability. A by-product of this research and design is the better understanding of the standard TBX amplifier and how it can be made to really 'sing' in BM mode whilst still being reliable.

These modification works have become increasingly popular, but one of the earliest adopters was Lord Luke of Dudley, who decided to throw all four of his AC30's at me in quick succession in order to have them dutifully modified and adorned with the golden KAT badge of honour!

Here's a quick rundown of the works!

So, with the TBX, we start by stripping out all of the pots and switches from the PCB. We also remove all aspects of the circuit that are not required in BM mode of operation. No sections are left open circuit and unused inputs are pulled to ground. I also remove all the un-used twisted heater cables to help reduce hum. The old choke is removed and if the unit is fitted with a poor quality output transformer, I'm afraid that has to go as well. The GZ34 is removed and replaced with a KAT solid state rectifier block.

The chassis is then drilled for the new choke, O/P TX if required, new power block and new volume pot location. Four of the valve base pop-rivets are drilled and replaced with screws to hold the insulating stand-offs for the new grid resistor board.

The new power block, choke and TX now fitted, wiring can commence.

I add a grid resistor support board as I personally think the original design is quite appalling and this method ensures the resistors will not break at the valve base joint. 

A new powder coated and silk screened steel panel is fitted along with new jack socket and CTS pot. A new ivory knob and jack nut is fitted to finish off the front nicely.

A brand new set of Mike Hill Valves are installed and the amp run up to speed. The unit is tested at full BM levels into a force-cooled load box and everything is checked to make sure all is running well within it's limits. Even at full levels, there is minimal screen grid dissipation.

The fully tested unit is then re-installed into the cabinet with new hex-drive M6 screws. 

Centre done.. Left, Right and Spare were next!