Circuit JCm800 Amplifier
In the view of many guitar players, Marshall has not made a good amplifier since the JCM800. Yet, the JCM800 is not without a few areas in which we can 'improve' on an already good thing.
Below are a few select ideas and observations that resulted from the requests of many guitar players in search of the 'ultimate' Marshall. Of course, tone is in the ear of the beholder, but these ideas do make for an amplifier with a little more 'oomph', and address the issues of 'thin tone' at less than ear-splitting levels. If you are looking for some blazing Marshall tone without resorting to fuzz pedals or using a Hot PlateTM, you are in the right place. None of these mods will make your JCM800 sound like a chain-saw, and many Internet 'critics' complain about this point. If you crave this much gain, feel free to wire up a humbucker to your Husqvarna. The key here is I do not drill or add tubes to the chassis. The amplifier can always be put back to stock if so desired.
First up, here is very bad sample of the official Marshall schematic for the 2203/2204 preamplifier section. I have circled a few areas we will look at. The object here is to decide where you need the 'improvement', and to try a few of the remedies. Please try them 'one-at-a-time', and decide when to say 'when'. I may even make a few suggestions not noted in the schematic below. The bulk of any mod-merchant's work seem to be in the preamplifier section, whilst completely ignoring the phase inverter and power section. Well, I can't argue that point, so these ideas won't seem very different. However, we are not adding an extra tube, so any vintage value can easily be restored if the results are not what you expected.
Layout JCm800 Amplifier
Let us start right at the input jacks. What you should note is the 'High' input is a single gain stage feeding the closed circuit jack labeled 'Low'. The 'Low' input is attenuated by a parallel combination of a 470K resistor (Yellow-Violet-Yellow) and a 470pF capacitor (the occasional JCM800 I see has a 510K resistor (Green-Brown-Yellow) with a 470pF capacitor). This is seen above at point #1. The capacitor/resistor pairing is referred to as a 'treble peaking' circuit, and accounts for some of the thin tone at low volumes. Also on the Volume control, at point #2, is a 'brite' capacitor. This is really not conducive for a fat tone at anything less than full volumes, and many guitar players complain about the situation. In a split-second, I always 'snip out' the .001uF capacitor across the Volume control. This is sometimes enough for certain guitar players! Between the Volume control and the 'Low' input jack you'll see the treble peaking circuit. So, no board removal is necessary for altering this area as well. I usually remove the pair, and use a 68K resistor (Blue-Grey-Orange) in their place. Another thought is to insert the 68K resistor at the wiper of the 1Meg Volume control. This accomplishes two things; number one is the treble peaking is 'removed', so the full tone returns. Secondly, we get just a little extra 'garlic' to the preamplifier gain. Now, how much 'gain' is enough is very subjective, and problems of oscillations and instability can arise. Therefore, I recommend only doing a little boosting to the preamplifier. This especially applies to the cathode resistor seen at point #4 above. The stock value is 10K (Brown-Black-Orange), and I do not usually go lower than 4K7 (Yellow-Violet-Red) for noise and stability reasons. I have seen many modified amplifiers where this has been changed to the 1K5/22uF seen in most Fender type amplifiers, but this can get to be too much. Remember, we don't want to radically alter the tone (do we?); we want a little more gain, and to 'fatten up' the overall response. Ditto for that 'unused' capacitor spot beside the 10K resistor; the temptation is to utilize one, but hold off for now. The nice part is this amplifier will still 'clean up' by backing off the Volume control of the guitar, unless we go 'ape shit' by adding far too much gain in one single stage. Adding small amounts of gain in multiple stages is far superior, and something most gurus miss. Below is a photograph of a typical JCM800 input jack section, with a trio of the suggested mod areas pointed out.
The final preamp modifications involve the tone circuit, and are as follows. We may want to fatten up the tone even a little more, and here's how we do that. First, the 'Middle' potentiometer is a 25K unit in stock Marshall amplifiers. We can either switch to a 50K potentiometer, or solder a resistor on the original potentiometer. This was seen on many Fender amplifiers that did not have a 'Middle' pot, and I thought the idea carried over to a Marshall quite well. I use something like a 6.8K (Blue-Gray-Red) or a 10K (Brown-Back-Orange) resistor. You will 'lift' the grounded tab, and solder your 'new' resistor right here. This way, even with the Middle control on 'zero', we have at least some midrange response happening. With the control on '10', we have a little modest boost. Ahead at MODS and ODDS I mention the early Mesa Boogie amplifiers completely lifted the Middle control ground connection for a 'Lead' boost. This is all along the same approach, and does work quite well. Next, we'll study the actual tone circuit itself. Check out the diagram below.