MARSHALL MG-50dfx ultra-gain modification!

The Marshall MG-series amplifiers
 The Marshall Mg-series amplifiers are some of the best selling guitar amps on the market. Their success is probably a good example of how marketing works. The 4th MG series generation has now been released.
  As a tech, I would say that these amps have potential (especially with a few mods) but the Mg50/100/250 models are quite overpriced.







 



Techincal specifications: Looking at an mg-50 circuit board, an amp tech can easily notice that the electronic components are of a medium quality.  For instance, the resistors are rated at 1/8W and the heatshink of the power amp is a 2"x2"x1" (inches)non-solid metal block. Such a block is not at all sufficient for cooling a 50W solid state(chip) power amp!... That's why there is a tiny fan on the heatshink. The only problem is, the fan is not working properly in many cases...

 The MG circuit: the circuit of an Mg amp is pretty basic and it's all designed using operational amplifiers. Moreover, the preamp structure is exactly the same for the MG-15 and the higher power models. As a result, the preamp tone of the amp is totally based on the resistors that define the gain and some capacitors of the filters. The power supplies use small filtering capacitors which results in poor bass response, hum, noise and even radio signal interference.

  The speaker and the cabinet construction also affect the amp's overall tone. Mg-50/100/250 models use Celestion custom designed speakers or the Eminence Legend 12" which is made in the U.S.A.! In my case, the MG50 has a 12" Eminence speaker in a totally closed cabinet to increase its bass response. The lower cost models use 8" and 10" OEM speakers.







 The circuit schematics can be found here:http://rapidshare.com/files/442528220/Marshall_MG15DFX.pdf
http://rapidshare.com/files/442528221/MArshall_MG100DFX.pdf
http://rapidshare.com/files/442528260/Marshall_MG50dfx.pdf
http://rapidshare.com/files/442529394/Marshall_MG250dfx.pdf

Marshall MG series fan problem:
 Throughout the world, there are many MG amp users that have encountered problems with their amp's cooling fan. The reason why this small 5cm fan malfunctions is the continuous use of it when the amp is on. Here is the original fan part.
 The fan reaches its full output a few minutes after the amp's start-up and maintains its speed. We cannot really do anything about this. If the fan was set at moderate rotating speed, the power amp would melt down after an hour or so. Once the fan goes out of order, we should replace it immediately with a 5cm or 6cm Marshall or equivalent PC case fan, depending on the MG model. It costs about 5-15 euros and is very easy to install. All you need is a screwdriver!

 The best solution would be to replace the heatshink and the fan altogether with larger ones. A much larger aluminum heatshink could cool the power amp without the use of a noisy fan.
 If the fan on your MG amp is still working but still makes weird noises, it's probably the dust that is piling up inside the fan or its power supply unit. The cleaning of the fan is a must.
  As for the power supply, here is a good trick: The fan and the footswitch are sharing the same power supply rail. So if the fan is malfunctioning, disconnect the footswitch and you may solve the problem.
 The reason why this happens is that this power supply rail can only produce 2Watts of power. The fan needs about 1.2W of power and the footswitch about 0.5W. These are nominal wattage ratings. At the amp's start-up, the capacitors, the power amp and especially the fan(as any D.C. motor) require more current and power(maximum ratings). That's why the footswitch should be connected very carefully a few minutes after the amp is turned on. The best solution would be to modify the power supply by installing larger filter capacitors plus higher wattage resistors and diodes.

 Looking for extra gain!   I recently thought it would be a good idea to modify the stock circuit of the Mg-50. I had to add more gain! I searched for the schematics and compared the Mg-50 and Mg-15 amps. The preamp of the Mg-15 consists of two 4558 op-amps (same as the Mg-50) and has much more gain and "marshall roar" than the mg50/100/250 models and that's a fact. The gain of the 2nd overdrive stage in the mg15 amp is set to x1000 (max). The gain of the Mg-50 2nd stage is set to x500, it's half the mg-15's gain!...  The mg-50 and bigger models of the range are designed in a conservative way to reduce the feedback and noise at high volumes. That's because these amplifiers have small p.s.u. filtering capacitors and no screened cables inside. So, it is only natural for an mg amp to pick-up radio signals and noise.

The ultra-gain mod! (see the MG-50 schematic link for reference).
 Here is how I conducted the modification of the MG-50 circuit. First, I disconnected the R7(270k) resistor and replaced it with a 1N4001 diode(it does not matter which way the anode is). I then bypassed  the R1(10k) resistor with a jumper wire. This mod increased the overall gain of the overdrive channel but I wanted more!
 That's why I put a 1k resistor in parallel to the R6(470r) and another 1k in parallel to the R8(1.5k) resistor. Then, the R3 (10k) resistor was connected in parallel with a 10k one. Now, the MG-50 is ready to shred at maximum gain!

 Notice: to keep the hum and noise low, you should apply aluminum tape (or foil) at the inner sides of the slot where the amp chassis is placed in the wooden cab(read on, "farad cage"). I sprayed some a Uhu quick spray glue on the inner wooden surface of the cab and then put on sheets of aluminum foil. That shielding technique is common among amp builders.

 "But, what's a farad cage? Does my amp have one?"
A farad cage is simply a metal box/chassis with 6 solid metal sides that is electrically conductive and connected to ground(earth).
The "farad cage" protects the circuit board by external rfi(radio frequency interference) and electromagnetic fields that cause noise, hum and harmonic distortion problems. No magnetic field can interfere with a circuit that's inside such a box.

Science content:
To understand how the operational amplifiers work and how to set their amount of gain, you should check the link below. It's also very helpful to see how the electronic passive or active filters work.
http://www.electronics-tutorials.ws/opamp/opamp_1.html
wiki links:
http://en.wikipedia.org/wiki/Operational_amplifier
http://en.wikipedia.org/wiki/Operational_amplifier_applications


Electronic Filters:
http://en.wikipedia.org/wiki/Electronic_filter
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/highpass/hpf.html
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/experiment/lowpass/lpf.html

Enjoy!
More modifications coming soon...

-Thanos