Marshall JCM2000 mods

Written By Mathew Taylor

I recently commented on a Facebook thread discussing the JCM2000 (was it good, what issues etc), and gave my usual verdict - that is was a great amp, once some (simple-ish) mods had been carried out.

When asked what mods I had done, I realised that apart from a large collection of badly named Spice files, I hadn’t properly written down what changes I had made, why, and the results. So this is it…

Please note this is a work in progress so it will improve over time. If this message has gone then you’ll know it’s finished.

Why mod the JCM2000?

The JCM2000 has/had a pretty terrible reputation. Not as vintage-mojo-infused as a plexi, as classic as the JCM800, or as hard core as the JCM900, the JCM2000 was a bit meh. Add the well known bias issues to the mix, and it doesn't make a pretty picture.

However, like the Fender HRD series, the JCM2000 was the main Marshall used for backlines. And like the HRD, I think this was it's problem. It didn’t have it’s own individual sound & in trying to make it a jack-of-all-trades, the designers had traded it having an identifiable character for versatility.

So, in my wisdom I thought I would have a crack at fixing this. Or at least copying the work of more qualified people (Joey, Jason Tong, Fortin).

Aim of the mods

  • Green channel 1 (Clean) - Keep mostly as is, remove some of the bass whilst still maintaining the full bluesy tones.

  • Green channel 2 (Crunch) - Move the tone closer to the JCM800

  • Red channel 1 (Lead 1) - Make this channel closer to channel 1/Plexi tones. Alter gain to be more like a lead boost/step up from Green channel 1.

  • Red channel 2 (Lead 2) - Remove mud, tighten bass. Make this channel more of a definite modern metal sound.

Misc

  • Install Dr Tube stable bias kit

  • Add choke

  • Adjustable/switchable deep circuit

  • Adjustable/switchable mid circuit

What was modded

The starting point for these mods was the Joey Mods page on the Marshall forum (link to follow), then Jason Tongs Headfirst mods (link), then the Fortin mods (link).

The brackets indicate where I took took the final value from, though a lot of the same components are changed by each ‘modder’ just the values differ. When I get the time I’ll see if this is better in a table showing the effect of the different mods/values.

Main board #60

  • C12 - 4.7uf -> 1uF (T&T)

  • R14 - 1.8k -> 2.7k (T&T)

  • R15 - 1k -> 2.7k (Headfirst)

  • C13 - 1uF -> 680nF (Headfirst)

  • C17 - 470pF -> 680nF (T&T) - this seems suspicious, so be warned.

  • C16 - 4.7nF -> 2.2nF (Fortin)

  • C18 - 470pF -> 680pF (T&T)

  • R23 - Add 220pF in parallel (Fortin)

  • R31 - 1k -> 830R (SIR 36/Slash mod)

Board #61 - Channel sculpting

Channel #1

  • C8 - 4.7nF -> 0.68nF

  • C7 - 470pF ->2.2nF

  • C6 - -> 2x 2.2nF in series

Channel #2

  • C10 - 4.7nF -> 2.2nF (Joey)

  • R20 - 47k -> 1nF (T&T)

  • R21 - 150k -> 220k (Headfirst)

  • C12 - 470pF -> 220pF (Headfirst)

  • C13 - 470pF -> 680pF

Resonance Circuit

  • R43 - 10k -> 10k Audio Pot or 3 way switch (switch looks cleaner)

EQ circuit

Mid Shift - Ignore the below as I need to check the exact connections. However the idea is there.

  • Swap push button for a 3 way switch

  • C20a No connection - Mid cut

  • C20b 470pF -> 220pF - Mid normal

  • C20c 1nF - Mid boost

Power board

  • R??? - 270R -> Hammond 159R Choke

As ever, the filtering and signal path components were upgraded. Particular recommendations are:

  • Filter capacitors - Nichicon

  • Cathode bypass capacitors - Nichicon ‘Muse’ bi-polar electrolytics.

Mathew Taylor
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T&T x Vox AC15