Here are some bits of information that have proved useful during my build of the “MOD 800 2204 Small Box kit – 50w Output – Master Volume British Amp Kit” from Modulus Amplification, a UK-based amplifier kit supplier. You can find the kit here.
Sadly, I’ve been having issues with this kit. It functions as an amplifier but has none of the mojo of a JCM 800. It is sadly lacking in gain; my Plexi (also a Modulus build) has much more of that on tap and sounds much better.
Anyway, I’m adding a few things here that I’ve found useful as I try and diagnose the issues with my build.
There may well be errors in my diagrams! I strongly advise you verify everything yourself and treat this resource as a guide only. If you spot any mistakes please leave a comment and I’ll make the necessary corrections.
Mapping the Modulus layout to a real schematic
I worked to the following schematic:
Given that schematic, I tried to identify the component numbers on the Modulus layout as follows:
There is definitely one correction that should be made to the Modulus layout. R15 is given as 56k. That’s wrong as it should be 33K. This is the slope resistor and it affects midrange. I believe Plexis used 56k but the JCM 800 used 33k. I also believe a possible mod is to use 47k for a beefier midrange (Soldanos reportedly use 47k).
A quick note here about C2. You’ll notice it’s missing from the Modulus kit. C2 should be a 100pf disk capacitor that sits between the plate and the cathode (pins 1 and 3 I believe) of the first preamp tube. This cap was actually missing on older circuits and added later so I guess that’s why it’s missing here. My understanding is that if you intend to mod the amp to add more gain, you might want to consider adding a 100pf 600v+ ceramic disk cap.
Marshall JCM 800 Service Manual and the Modulus layout
I found a PDF of the old “Marshall JCM 800 Service Manual” which included some example voltages at various test points in the preamp circuit. Here’s the schematic with the points labelled 1 to 17:
And here’s the voltage data associated with those test points as found in the service manual:
And then for good measure, here’s the test points on the Modulus layout (as well as I could identify them):
Rob Robinette’s useful information
Rob Robinette has a great page explaining how a JCM 800 works. I thought it would be interesting to map some of Rob’s information about component names and voltages to the Modulus layout. This is what I ended up with:
In general, the 0.022uf capacitors are going to be coupling caps. I didn’t bother to label most of those.
NB: Usual caveat here. I did my best to get the voltages in the right places errors are likely. Double-check yourself!