Nathan Mathers AC30

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 amplfier as well as increased reliability. A by-product of this research and design is the better understanding of the standard TBX amplfier 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 Nathan, who's amp was quite i'll due to a number of issues, it seemed the sensible thing to have them dutifully modified and adorned with the golden KAT badge of honour!

Here's a quick rundown of the works!

We started on Nathans amp 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.

Nathans amp had been heavily and wrongly modified such that at full undistorted power, it was only pushing out 6 Watts! Addtional resistors had been added to the cathode line of the EL84's but after the decoupling cap, so were simply adding lots of negative feedback to the output stage. No idea why they were there, but they had to go.

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. 

The chassis stripped and drilled ready for the new pot and jack socket.

The finished amplifier. This time pushing out 31 Watts undistroted rather than 6 Watts!

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