Marshall MS-2 , MS-4 Ultra Gain Mod

 Hello! I modified my little Marshall Ms-4 amp a few years back. I still wanted to get a bit more gain out of it. I also noticed it was pretty decent sounding through a 1X12" or 2X12" cab. Therefore I decided to modify the headphone jack and turn it into an "external cabinet" output!

Here's my old article:
http://guitar-dreamer.blogspot.gr/2010/06/modding-marshall-ms-4ms-2-micro.html

 The circuit of the Ms-4 amp is the same as the Ms-2 and it's pretty basic:  a "class A" transistor booster overdrives an IC opamp (KIA6213S). In order to increase the amp's overall gain you need to maximize the boost level of the transistor. The IC is already pushed hard to produce about ~1W of audio power.
 The maximum gain is only available at high volumes. Therefore, you need an attenuator (basically a 1W pot or resistor) to keep the IC power amp volume down. Playing through an external cab will usually sound louder due to the higher deciBel efficiency of  larger speaker cabinets. Hi-fi and automotive speakers and cabs won't do the trick. Use speaker(s) rated at 8ohms or more! 4-Ohm cabs could damage this little amp.

Tools you'll need:
A reliable digital multimeter (DMM), a soldering iron with a fine and clean tip, high grade solder, flux and a pair of pliers. Use a flashlight and a magnifying glass to check the pcb traces and solder joints.

Parts you'll need:
A 20 or 22kOhms , 1/4W resistor
A 4,7kOhms 1/2W-1W linear pot
A 470pF ceramic capacitor
A 20ohm/5W power resistor
A 47ohm/2W resistor
A 250kOhms linear pot
A 2-pin switch
A 1nF up to 100nF capacitor (you can use various capacitance values to find what's best for you).

You don't have to buy all these parts unless you want to experiment with all the modifications referred in this article. Some mods don't even require new parts.

Here's the original ms-2/ms-4 schematic:

http://i31.tinypic.com/rcq89j.png

And here is the new modified schematic for the "ultra gain" mod.

The datasheet for the IC power amp chip:
http://pdf1.alldatasheet.com/datasheet-pdf/view/69377/KEC/KIA6213S.html

First of all, let me just say that you don't have to do all these mods. Nor should you do them all at once. It's far better doing one modification at a time to see what's the effect on the tone of your amp.


  The mod:
All you have to do is to bypass some resistors(R1,R4,R8) and remove or replace a few other components.

 First steps:  The resistors you have to bypass with a jumper (or replace with a jumper) are marked on the PCB board as R1,R4 and R8. This first mod gives you more gain and bypasses the gain pot of the MS-4 (it's always set to max.). You can stop here and  hear the results. If you need more gain. proceed to the next step:

  More gain!: Replace the R3 10K resistor with a ~22k resistor to increase booster gain.  The 10K resistor is the one connected to the C3198 transistor collector and the positive side of the 10uF capacitor. This resistor read about 5,4K on my multimeter...
Then, remove the 180K "tone" resistors as they decrease the voltage gain of the booster. The 180K resistors that should be removed are the ones placed near the Tone 50K Pot. Use a multimeter to trace and measure them. You could also replace R2 with a 1 or 2 Mohm resistor.

Treble boost or maybe not?
-Disconnect the C2 47pF feedback loop capacitor of the C3198 transistor for extra treble boost!
-Replace the C10 470pF capacitor that's connected in pins 1 and 4 of the IC opamp with a 1nF ceramic cap. This will reduce some nasty high frequencies of the power amp.

 "New" Gain pot mod: The "volume" pot will now become the new gain pot. It's like having the gain all the way up while using the master volume.
 The volume pot resistance value is about 50kOhms. You can replace this volume pot with a B250K linear one. This will provide for a better control of the volume level. It will also increase the overall gain and output level when set to max.

  Master volume-attenuator:  Well, it's not like we're dealing with a 50W Marshall head but 1W of output power can still be pretty loud for practice. Turning down the volume pot of the amp will also reduce the gain level and that's not what we're looking for in this article!
 We can use an external volume pot as an Attenuator! A 4,7kOhms Linear pot, rated at 1W, could be connected in series with the speaker(s). This will do the trick.

You can even create a simple attenuator using a 5W/20ohms resistor and a switch wired in parallel. This attenuator network will then have to be connected in series with the onboard speaker of the ms-2. See picture below:
 
To avoid under/overloading the IC power amp, I connected a 47Ω/2W resistor in parallel with the speaker (+/-) terminals. The 47Ω resistor is the load the power amp sees even if we disconnect the speaker/cab.

 Cabinet out: the classic stereo "headphones" jack will be used to connect an external cab if needed. As you know, this stereo jack disconnects the internal speaker(s) once the headphones are connected. Here's how you can turn the headphones output into a cabinet-output. Locate the R12 resistor next to the headphones jack. It's the only resistor on the "out" pcb board.
All you have to do is to place a jumper across the "R12" marked resistor or replace the resistor with a jumper.
 In order to connect the cab's mono cable to the stereo output jack, you should slide the cable's tip only halfway in the jack. This will mute the internal speaker(s), giving a "mono" output signal to the external cab. You can replace the stereo jack with a mono one or fiddle with its metal pins. I used the original stereo jack like that and it worked.
 Be careful! Don't connect any headphones or pedals/amps on the "cab output". The new output level is powerful enough to destroy them!

 BASS cut!: The amp might sound ok and high-gain with a single coil pickup or a vintage humbucker but an EMG or other high output pickups can produce many bass frequencies and overload the amp. For this reason, you can replace the "R1", 1kOhm input resistor with a 1nF up to 100nF capacitor. This will for a high-pass or low-cut input filter that will reduce bass frequencies.

 The jumper used to bypass the R1 resistor in the schematic forms an all-pass filter while an input capacitor forms a high-pass filter that will reduce bass. Smaller cap value = less bass and a more trebly sound (VOX style, etc).

 Your tiny Marshall is ready to rock! It's got the classic British overdrive sound and it can really roar when connected to an 8-16Ω, 4X12" cabinet!

P.S. There's a video of a guy playing with an Ms-2 connected to a 1X15" PA Peavey cab on youtube. Check it out:
http://youtu.be/zki8djHoVLk

Cheers,
Thanos
guitardreamer.gr@gmail.com