Marshall JCM 900 mod

[sibo]

Hey supawah, don't leave me hanging!

[tschrama]

Old thread, I know.... but I happen to be busy on a JCM900 4100 DR and I would like to share some tips.

The 4100 sounds great as-is with some power amp distortion. I can do that with my attenuator, and it just sounds marvelous. But you can help the preamp to sound a bit better to:

1] remove the boost channel pre-EQ, by removing C27 (4.7nF). This removes the ice-picky fizz and gives some extra sub-bass as a deep mod
2] Make sure the cap aound the second opamp stage of the boost channel is 100pF. Some came with 220pF and that is just too much. Change it to 100pF you might even try 47pF or 22pF as in the JMP-1
3] The input resistor of the 2nd opamp of the boost channel is 10K. You might try to picky-back a 10K over it so it is effectively 5K. If you do this, keep 2] at least 100pF.
4]Yes, you can easly remove the diode clipping for real tube clipping. I don't believe that sounds much better but it is easy to accomplish:
Remove diode that is next to the diode brigde. This effectively takes out the whole diode bridge. Remove the 220K resistor around the tube of the preamp. Now the tube is running open-loop, with full gain, no more "handcuffs". Now the signal is much larger, so the post-EQ attenuator (470K-470K) must be altered to 470K-47K.

There you have it, no more diode-clipping, genuine tube stage with cathodefollower connected traditional Marshall Tonestack.

Grtz,
Thijs

[tschrama]

Oh, allmost forgot: a simple Boss SD-1 in front of the JCM900 DR goes a long way. Efectively making it a 4 channel amp with clean, crunch, more crunch, and super saturated metal.