Repair work on a NARB amplifier

This very rare NARB amplifier came in for some repair work.
NARB were a very limited run of Marshall's to avoid legal issues.
This amp is number 14 of the 14 ever made. Narb is Bran spelt backwards . Ken Bran and Dudley Craven were the original designers of the JTM45. The original 45s were hand made in Garden sheds.
www.dudleycraven.com/
The amp was only producing 25 watts with severe crossover distortion and clipping. The amp has 4 out put valves of which only two are similar in make and matching. The preamp valves are worn 7025 not 12ax7 /ECC83 . This accounts for some of the low level drive and the output valves are mismatched. On inspection the negative bias voltage is too high and could not be brought down by the rotary preset adjuster. The valves were only drawing 1.5ma when the amp arrived almost completely shut off and in class C operation. Changing of a faulty (high in value) network resistor has allowed 35ma current draw from the valves and cured most of the distortion albeit a new set of matched valves are required. The amp is now producing nearly 100 watts rms but with some crossover distortion because of the mismatched valves.
Many components have been changed from original factory fitted. Two resistors feeding the output valve with grid bias were wrongly fitted 68k not the usual 220k. This would almost certainly load the phase splitter and attenuate HF response by forming a low pass filter. This would also reduce drive to the output valves. These have been swapped out . The 100 k anode load preamp resistors have also been replaced . The originals had drifted very high in value.
Several resistors and electrolytic caps have been poorly substituted with components with inadequate lead lengths . These have been mounted piggy back style onto existing leads or associated components.
The amp had wrongly fitted mains and HT fuses. These have been replaced for the safety of the amplifier. A 5 amp car fuse was in the HT holder! Not the required 1 amp protection fuse.
Test amplifier for 1 hour with pulsed tone (10% duty cycle )at full power .
With a complete re-valve the amp should perform reasonably well. Some minor cosmetic component changes for age related parts may make minor audible differences. The controls are aged and may need replacement in due course. With a new set of tungsol valves I recorded 120 watts rms mid band @ 1 Khz .

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