JCM800 Emulation

December 2004:
As a long-time rocker, one of my favorite amps of all time is the JCM800 series, specifically the 2203/2204 models. These amps were the staple of what defined the sound of the 1980’s metal genre. This circuit is an emulation of these famous amps in a stompbox form.

Both myself and electrictabs (a regular contributer at Aron’s Stompbox Forum) have taken the time to make this circuit. Although each of the schematics differ, they were born on the same principle, to emulate the JCM800. This was done in much the same way as Runoffgroove’s Thunderchief, which is an emulation of the classic 100 Watt Super Lead. The JCM800 and Super Lead are very similar designs. Also note, we did not collaborate when each of us drew our schematics. It just so happened that we were each thinking the same thing at the time.

This circuit was born by replacing each of the preamp tubes with a JFET just like the Thunderchief. Rather than omitting the tone stack, we both decided to include it. The tone stack can be omitted, however, to get that “all on 10″ sound. A JFET was also used in the phase inverter and power amp stages.

A big thanks goes out to the guys at Runoffgroove and Aron’s Stompbox Forum for their vast knowledge and input, Schematic Heaven for the JCM800 schematic, Buck Sears for the layout, PCB and parts list, and Electric for his schematic and taking the time to design this as well.

Possible Mods
These mods apply to Electric’s schematic as it has been proven to work. Some of these mods are for the JCM800 amp specifically but may work in this circuit.

  • Add dual low pass filters. See Runoffgroove’s Thunderchief.
  • Remove the entire tone stack and replace it with Fender values – 220pF treble, .1uF bass, and .047uF mid caps.
  • Remove the tone stack entirely. The circuit really opens up.
  • Change the slope resistor to 56k. This is the 33k resistor between the 470p and 22n caps in the tone stack. This will make your tone controls much more responsive.
  • Change the slope resistor to 10k. This will make it sound more open yet still allow the tone stack to be used.
  • Adjust the 470k/470pF network on the front end to your personal taste. Some people prefer the tone with the cap completely removed.
  • Try a 500k pot for the treble and 50k for the mid. Or try 100k for treble.
  • See Runoffgroove’s Thunderchief for other mods that may work.

Schematics

Layout/PCB/Parts List
Drawn by Buck Sears