June 2005:
This circuit came to be after I built the Fetzer Valve from Runoffgroove. The Fetzer Valve is a booster that is a FET-based version of the classic Fender 12AX7 input stage. The Marsha Valve follows the same idea, only it is a FET-based version of the Marshall JCM800 12AX7 input stage. The sound of the Marsha Valve is very tube-like and it adds just the right amount of Marshall character to the sound without being overpowering. It also reacts very well to pick attack and cleans up nicely when rolling back the guitar volume.

The original JCM800 preamp has a 470p cap in parallel with a 470k resistor between the 22n cap and gain pot. I omitted both of them because it made the sound too bright. Removing both components warmed it up and smoothed out the highs. It was actually a common mod to have the 470p cap changed in the preamp or have it removed altogether. There is also a 1n cap on the gain pot in the original JCM800 preamp schematic but that was also omitted.

The JFET used in prototyping and testing is the NTE458 but a J201, MPF102 or even a 2N5457 will work too, each with different amounts of gain. Just watch the pinout of the NTE458 as it is different (DGS) from the other three mentioned (DSG). There is a DSG pinout version of the NTE458 so look at what the package says. The NTE458 also has more gain than the others.

Use a transistor socket for easy changing of different FETS. Make sure to re-bias to around 4.5 volts. Although the schematic calls for a 1M pot, a 100K or even 500K pot will work fine.

Enjoy! And if there are any questions, please post them at Aron’s Stompbox Forum. This has been shared with everyone there.

Schematics

• Version 1.1

Layouts / Parts List
Vero and PCB Drawn by Richard Boop

• Vero and PCB Layouts
• Perf Layout
• Parts List