Marshall Jackhammer JH-1 Mods

A few weeks ago(as of 5 July 2010), I bought a Marshall Jackhammer JH-1. As is, I could almost get a sound I liked, but it seemed like there was some tone sucking going on somewhere. The distortion setting was almost junk. I replaced the two 1N4148 diodes with two j201 JFET's wired as current limiting diodes(the gates tied to the sources). The 1N4148 diodes are the ones that clip with the distortion setting. Because their threshold voltage is lower than the two LED's used for the overdrive setting, the LED's never turn on in distortion mode. While I do not have a on voltage measurement for the j201 "current limiting diodes", it is obvious from switching between the two settings that their on voltage is lower than the LED's. 

This actually seemed to make the tone problem a little worse. I figured there might be something cutting high frequencies for the distortion mode to reduce the harshness of the clipping of the original 1N4148 diodes. Because the j201's do not clip as harshly, it makes sense that using them would cause a greater loss of high frequencies. The feedback path for IC2A contains a network of capacitors and resistors that provide more feedback for high frequencies than for lower ones. (I am cowardly omitting the schematic for fear of copyright violation.) When switched to distortion mode, an additional capacitor, C27, and resistor, R30, are added to the feedback path to further cut high frequencies. I yanked the two wires for connector 8 to prevent C27 and R30 from being switched in. This helped a lot.

Yet, the pedal still seemed to lack clarity. It worked great as a boost for my little Valveking Royal 8, but when running the Royal 8 clean and creating all the distortion with the pedal, it just didn't seem quite right. I read where using a lower value for C22 helps quite a bit. I simply removed C22, and now this thing kicks ass!!! Of course the opinion of others may differ. With the overdrive setting, I can see why a low value C22 might be needed. With C22 gone, it seems to be much more prone to feedback or possibly internal oscillation. I may solder in the recommended 470pf capacitor for C22, and see if this helps. But then again, there is little difference in sound now between the distortion and overdrive settings, so I might just use the distortion setting. I might also replace the LED's with something else with a similar threshold voltage that clips a little differently.