The Ultimate JCM800

Modding your Marshall

I am really trying to make this very section as informative as possible. However, some people may feel these mods are not to be taken seriously. After all, why would ‘the good stuff’ be free on the Internet? These are excellent mods for those who want to just push their JCM800 over the edge without adding tubes. They are tried and tested many times over. I completely admit they are nothing new or original, and there are many areas in the JCM800 circuit ignored. Some aspiring gurus may feel they have ‘better’ or more imaginative mods. For those of you that fall into this category, and those who think these mods are just ‘a joke’, I say ‘Start your own amplifier company, or your own mod-shop, and put these mods to shame.’ For the rest of us, we’ll just have to wallow through all of this mediocrity.

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 Plate^TM, 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.

If this is your idea of awesome tone, you are visiting the wrong website.

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.

Looking at the preamplifier and tone control section, we can ‘beef up’ this JCM800.

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.

Typical JCM800 input section.
Vast tonal improvement can be obtained without the removal of the circuit board.

We will have to remove the circuit board to get at the cathode resistor seen in point #4 above, and we may want to do point #3 at the same time. This is a second treble peaking circuit, and we have a few options. First is just to simply remove the 470pF capacitor. Decide if this is enough, or continue on thusly. If the tone isn’t to your liking, you can always put it back. Or, if we want a little more ‘push’, we can lower the 470K resistor. Try 100K (Brown-Black-Yellow) or 220K (Red-Red-Yellow). You will also see (not notated above) an 820-ohm (Gray-Red-Brown) cathode resistor on this stage. We can bypass this resistor, with anything 0.68uF and higher. You should know what the different capacitor values will do to the frequency response, so please make an informed decision. Remember; too much gain always results in a Marshall that is noisy, and squeals at high volume settings. Plus, this Marshall tends to be thin and bright-sounding at low volume settings, so the goal was to ‘correct’ this. The ‘modest’ extra gain thrown in is nice, but not always necessary. To repeat what I mentioned earlier (because I can’t over emphasize this point); 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. Below is a photograph of the area we are interested in, with mod ideas #3 and #4 highlighted for hopefully easier understanding.

Some modification ideas necessitate board removal for professional results.

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.

‘Stock’ Marshall tone circuit has a surprising amount of tweaking possibilities.

You can start by changing the value of the ‘slope’ resistor (seen above circled in blue), and move the ‘range’ of the tone controls to suit your liking. However, I tend to leave it as is; typically 56K (Green-Blue-Orange). I do like to alter the range in the bottom end a little by increasing the value of the capacitor seen above circled in red. This definitely increases the effect of the Bass control. Try a .047uF, and if that isn’t enough, a .1uF is as large as I would recommend. This definitely means gaining access to the trace side of the circuit board, so be sure this is something you are interested in. If you are getting to the circuit board anyway for other preamp mods, this is the time to tweak the tone circuit. The last modification I’ll discuss here in the preamp is at the Master Volume control. If the control seems to make your tone thin out at low settings, even after all of the previous modifications, we can do the following. Insert a 100K (Brown-Black-Yellow), 1/2 watt resistor between the middle wiper and the wire going out to the phase inverter circuit. Cover the resistor with a piece of heat shrink tubing. I have also been known to dab a spot of hot glue or silicone on the back of the Master Volume control, and affix this extra resistor firmly in place. This may seem like overkill, and it might be. But I do it anyway.

• I am completely aware that you can drive the tone controls without resorting to the cathode-follower circuit as used by Marshall. By Plate-loading the tone circuit, you will get an awful lot of extra gain. But the key word here is ‘awful’. If you wish to experiment with this idea yourself, please feel free to do so. Any semi-competent technician can figure the procedure out, so no drawings or instructions are given here. However, I will warn you now that this mod requires extensive rewiring of the tube socket. Also, the previous DC coupled stage will now have to be rewired as well. This is a lot of work for dubious results.
• You can also use the Internet favorite post phase-inverter Master Volume control. Those ‘in-the-know’ refer to this circuit as ‘PPIV’ or ‘PPIMV’, and it has been discussed ad nauseam. If you wish to utilize this circuit, the drawings and instructions are easily available. MIKE’S TUBE AMP PAGES has an excellent overview (probably the best), as do books from Groove Tubes and Kendrick. These are typically in ‘The Trainwreck Pages’.
• The phase-inverter can also be modded for more gain. Rather than the 82K/100K plate-load pairing, try 100K/120K or 120K/150K. Too much gain here will result in a tone that is rather unpleasant. Be forewarned.

I have completely avoided these areas until now because I feel these ideas can be abused quite easily. I have said this many times; we do not want excessive amounts of preamp gain. This just takes away from the Marshall tone, and turns a fine amplifier into a unstable power tool.

Output section has surprisingly few areas that need ‘improvement’.

Here we see the output section of a typical JCM800. There isn’t a lot to do tone-wise, but we can ‘improve’ on the reliability of the amplifier as a whole. At point #1 we see the bias-feed resistors. Usually these are 220K for EL34 output tubes, and 150K for 6550 output tubes. I lower both resistors, regardless of tube type, to 100K. This keeps the tubes on a ‘shorter leash’, and does promote long life. The theory behind this move is that the chance for the output tubes to draw grid current at high volumes is reduced. If you wish to insert your PPI Master Volume control here, I suggest 100K rather than 250K. We can further promote tube reliability by connecting the Suppressor Grid to the bias supply, as seen at point #2. This was seen already back at MARSHALL MYTHS, but it is worth repeating. By combining these two ideas, we do not have to resort to increasing the Screen Grid resistor values, something many aspiring gurus advocate. Lastly, here are a few tips for the output jacks themselves (not seen in the above schematic). I like to remove the ‘open circuit’ jacks and install ‘closed circuit’ jacks, as seen at the input. Another option is to wire in a resistor across the output secondary. I like to use 180-ohms/10-watts, but anything in this range will work well. This is helpful in case you have a habit of turning the amplifier on with no speaker load connected.

Well, there you have it. If you follow this recipe, you should have the ultimate JCM800. Next on the ‘To Do’ list is auditioning different pickups in your guitar! When you are ready for a new set of tubes, learn the truth about biasing by CLICKING HERE. The idea of the Screen Grid supply seen at MARSHALL MYTHS can help tubes last longer as well, so do read up before altering your Marshall. The best part of all this is the amplifier will look completely stock, and we haven’t drilled a single hole! I gave you very few ideas that ventured past the preamplifier section, but the few I do give seem radically different compared to what I see of other technician’s work. Good luck, and remember to have some fun while you’re at it. Your JCM800 is definitely ‘improved’, and will have your friends wondering how yours seems to sound so much better than theirs. This will be our little secret.