Marshall MG10 modification

This is the Marshall MG10cd guitar amplifier. It's a really small amp with a 6.5" speaker, two channels with gain, volume and "contour"/EQ controls. It's a cheap unit that sounds better than most solid state 10W amps of this price category. The new MG10CF model does NOT share the same electronic circuit as the older gold-face MG10cd model. The new model is built with surface-mount components and it has a different pcb. The tone of the mg10cf is really close to the old model but the modifications I did only apply to the old model pictured bellow:

Here's the inside of the old MG10cd model:




And here's the inside of the new MG10CF model:

Source: http://www.twotrackmusic.com/ttmforums/index.php?topic=1032.0

There is a very thorough analysis of the MG10 electronic circuit here:
http://www.electrosmash.com/marshall-mg10

I modified this amp based on the analysis and the schematic diagrams provided by ElectroSmash.

Here's the schematic diagram of the mg10cd:

External Cab output!

This is an essential modification that can be done in most combo amplifiers. I wanted to hear the small mg10 amp through a 2X12" cab loaded with celestion V30 speakers. The tone would definitely be different. I didn't want to drill the tiny chassis of the mg10 but I also had no use for the headphone output jack. The amp sounds terrible through headphones by the way... This mod was easy. I removed the back of the amp (4 bolts), then I disconnected the speaker and removed the chassis of the amp from its wooden cab. I then removed the knobs and the washers that hold the main PCB of the amp in place and traced the headphones output jack. This is a stereo jack.



Here are the steps: 
I replaced the R41 resistor with a jumper wire/short circuit. Then, I scraped the trace of the pcb that connects the middle pin of the stereo headphones jack to the hot/tip pin. The middle pin is not connected to anything now. Check the pictures bellow:


That's it! It's done! The amp can now be connected to an external cab via the phones jack. The internal speaker will be muted when an external cab is connected. I hooked it up to my 2X12" cab and it sounds huge! The tone is way better and of course the sound pressure level went up!
Important notice: Do NOT drive 4 ohm cabs with this little amp. The power amp runs colder with 8-16ohm cabs (even 4X12" and 1X15" ones). The low impedance loads (4 ohms or less) could damage the chip of the power amp.

Tone mods:
Here are a few more modifications that change the tonal characteristics of the amp. These mods do not alter the tonal character too much but they do provide extra gain and reduce the fuzzy treble.

Extra gain:
I replaced the R1 (10kOhms) resistor with a 100ohms (100R) one. This is how you get a couple dBs of a bright boost. It's a subtle modification.
Then, I replaced the R4 (6.8kOhms) resistor with a 10kOhms one. This will provide the amp with a clean boost. The clean channel does not sound overdriven at low volume settings but it sounds crunchier when you crank the volume up.
I also removed the R9 resistor for less OD/Clean channel crosstalk.
I removed the R6 resistor in order to increase the gain of the OD channel a bit more, without affecting the clean channel.
Less nasty/fuzzy treble:
The extra gain increased the fuzzy highs a bit so I had to replace the C22 (4.7nF) capacitor of the power amp with a 47nF one (code: 473) to reduce the treble.

Here's the modified schematic for the mg10cd amp (open in new tab for full size view):

How does it sound?
The amp sounds like a proper amplifier head unit via the 2X12" cab now. The tone is really good. The clean channel has plenty of punch and it sounds like a 5w tube amp when the clean level is cranked up. The OD channel sounds full with plenty of gain on tap even for dropped tunings and metal solos. It can also sound bluesy when the gain is set at lower levels.






Cheers,
Thanos