Marshall JCM900 Dual Reverb mod

The other day I was reminded of a High Gain Mod I designed for the Marshall JCM900 Dual Reverb (Models 4100 and 4500) in 2015.  These heads don’t have a very good reputation, so can be bought for relative little money. I can make these amps sound #spectaculair#. With this mod, these amps hang in there with modern German and American amps, without loosing its marshall pedigree.

If you feel confident working in amps, you can do this in a couple of hours.

post #24 for details

If not, I am happy to do the mod for you for € 99,-.

What this mod does:
This mod effectively takes out the pre tube diode clipping. The tube is running open-loop, with full gain, no more “handcuffs”. There you have it, no more diode-clipping, genuine tube stage with cathodefollower connected traditional Marshall Tonestack.

Here are some quotes about the mod:

“First and most importantly, the mud is gone! You can tell what I’m playing even in the high gain clips, which before I always struggled with. ……. The tone stack is now very different ……. No matter the channel or gain setting you can dial in exactly what your ears are wanting…… The amp now FEELS very different. It feels alive like when you have the stock amp cranked…. The lows are super tight and yet there is more of it now, the low mids now dropkick you in the chest, and you can dial the treble knob up to 7 or 8 before you ever think it’s ice picky, but if you need more, its all there. “

“At lower gain settings (6/20) on ChB, it sounds very plexi-like to me (or at least compared to all the clips I can find).. At Pres 10, Bass10, Mid 8, treb4 it’s got the KeRRRANG and extremely high note definition. Bring the gain up to half way and it is more like the 800 clips I hear. Throughout both of those settings, the sustain is ridiculous.. strike a chord and it just keeps going and turns into harmonics”

“Now ChB with the gain cranked is just bone crushing yet crystal clear.”

“He NAILED it with these mods.”

“I can get some absolutely mind-bendingly thick and clear metal tones …the mods removes fizz and it makes the high gain stuff SO FRIGGIN SMOOTH.… metal tones.. you know the high gain modern metal sound that is ultra clear, wide, thumps, has soaring highs and yet somehow really smooth because it is crazy loud and the amp is working hard? Yeah, sounds like that. At any volume.”

“I just heard the modded 900, great work! That last take is incredible, sounds perfect and ready to be put on record.”

“Still loving this amp every time I plug in. If I get channel A up to 8 and set the gain to mild crunch, it is Mind blowing for a clean/dirty channel via guitar volume”

“Wow, it is ridiculous how pinch harmonics want to jump off the fretboard now..”

This is a describtion of the mod in a nutshell:

1] C27, (4n7)snipe one leg off
2] C35, (220pF) to 100pF
3] R42, (10k) piggy back another 10K over it. But i Like it as is.
4] D10, (diode) snipe one leg off
5] R60, (470k) Pickybag 47K over it
6] R13, (220k) snipe one leg off