2003 Marshall JCM 800

This amp is a Marshall Re-issue of the 800 series made in England to similar standards of the originals. This amp needed a new LED illuminated Rocker switch for ON/OFF, and several solder joints retouched - some of which were missed by the factory all together. The bias C- supply voltage was modified to provide a more robust Plate Dissipation. The original value resistor in the bias voltage divider position was different from the schematic, as this particular amp seems to be an early revision of the circuit. I added a 270k 1/2 watt resistor in parallel with the 56k resistor on the PCB to bring the total value down to approx. 47k - the revised value. This allows the bias to go to %70 with a wide variety of tubes/brands. A whole new tube set was installed.

The preamp tubes were all 12AX7’s; 1 Genalex Gold Lion, 1 Tungsol, and 1 EHX, all in positions picked for best quality distortion. The power tubes were changed from the original EL34’s to a matched quad of JJ 6CA7’s. The 6CA7 is the American cousin to the EL34 and has the same ratings but with a sound closer to the 6L6GC. This helps tame some of the fizzy high end this amp had while providing better bass response. The Fixed Bias was set to %70 Class AB Plate Dissipation with a B+ of 434VDC, perfect for classic rock - which was what this customer played. The amp was also cleaned and the hardware tightened.