Jon Underhill's AC30 Modifications

Jon Underhill's AC30 Modifications

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 and for Jon Underhill, having witnessed one of these beasts in the flesh at the Red Special Meet-up earlier in the year, it seemed the sensible thing to have his two amps dutifully modified and adorned with the golden KAT badges of honour!

Here's a quick rundown of the works!

We started on Jon's amps 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 HD 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.

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.

These two shots were taken with the amp running at full BM distortion levels. You can clearly see that the screen grids are cool and not under any stress..

 The finished amplifier No. 1. No.2 followed shortly..

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