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Old 14th July 2012, 10:26 AM   #1
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Default How to improve marshall guitar amp with harsh overdrive?

Hi - I have a Marshall MG15CDR guitar amp - it's fairly low end I guess

http://www.amparchives.com/Amp%20Arc...%20MG15CDR.pdf

The clean sound is fine and dandy (also has a good sounding tube sim switch)

BUT it has an overdrive input and the sound that then comes out is horrible - it sounds like a ghoul's shriek - and no, I don't like metal. With gain on 0.1 or 0.8 it's harsh - but on 1 to 10 it's hideous.
It has 2 pots on this part of amp: overdrive gain and volume.

So I would like a nice mellow fuzz with a bit of sustain - not this monster.
Any obvious changes I can make to any components?

I've got an external fx box but would be good to make this usable

Thanks!
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Old 14th July 2012, 12:47 PM   #2
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I would guess increase the size of R8 - 560 ohm resistor. C5 also limits your low frequencies so keep that in mind also.
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Old 14th July 2012, 04:53 PM   #3
jjman is offline jjman  United States
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I had a Marshall Lead Mosfet, which is also solid state. The drive channel/mode also sucked. Pedals are the best answer for distortion in a SS amp. Yes, most pedals are SS too but the choices are endless nowadays, including kits if you like to DIY.
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Old 14th July 2012, 09:41 PM   #4
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I found another suggestion online where someone said to change the LEDs in the overdrive circuit to ordinary diodes - you can't see any LEDs on front panel so they must be there to distort...
Printer2: when you say C5 limits lo freq what would you change it to?
would you try 1kohm at R8? (or more?)
thanks!
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Old 14th July 2012, 09:48 PM   #5
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Replace the red and green LEDs with germanium diodes or one silicon and one germanium.
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Old 15th July 2012, 11:24 AM   #6
teemuk is offline teemuk  Finland
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Don't go for lower clipping threshold voltage, it just results to more overdrive. Add more diodes in series instead. Alternatively you can mod a resistive voltage divider to attenuate the signal at the node where the LEDs feedback the signal back to inverting input. Adding series resistance to diodes/LEDs works as well.

Increasing value of R8 is indeed a good suggestion to bring down the overall gain. Likewise you can decrease the value of R6.


You may need to alter the value of R49 to match the resulting signal amplitude to clean channel. Varying the gain and especially clipping threshold will have its effects on signal's magnitude and mods likely send the balance with clean and O/D "channels" off balance (no pun intented).


Radically increasing the value of C8 can help to kill some fizzyness.


All in all, it's a rather simpe design on the overdrive's part so you have very little to work with. Basically you can't really do marvels without extensive modifying, which might prove quite difficult.

You can achieve a lot by modifying rest of the circuit. Basically a lot of the tone is based on shaping the frequency response in the first gain stage, in the tone control circuit, in the 2nd order filters and in the power amp. The O/D is basically just routing the signal through a moderately high bandwidth stage that clips. As is it would be pretty much "fuzz". A lot of the tone shaping towards "metal" territory takes place elsewhere in the circuit... However, modifying rest of the circuit will consequently have it's effects on the clean tone as well.


Ultimately the best "mod" might be just getting another amp that's closer to your actual preferences.

Last edited by teemuk; 15th July 2012 at 11:36 AM.
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Old 15th July 2012, 12:00 PM   #7
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wow teemuk.. thanks very much for your comments.
I have indeed wondered about changing the amp but (mistakenly it seems!) thought a few mods might do what I wanted.
I built 2 fuzz boxes a lifetime ago (30yrs ago) from things published in electronics magazines, so realised that this Marshall cicrcuit was 'minimal'.
As you say looks like it would need trimmers /pots to adjust.
I did 'A' level physics but electronics theory is beyond me !
It's funny on the clean sound - you turn it up and you begin to sound like a "rock guitarist" - which flatters me.. but all in all I prefer a more jazz sound, as "rock" is such a cliche.
Many thanks - I'll try the diodes and capacitor before I swap the amp!
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Old 15th July 2012, 12:06 PM   #8
tsiros is offline tsiros  Greece
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Quote:
Originally Posted by teemuk View Post
Ultimately the best "mod" might be just getting another amp that's closer to your actual preferences.
viable solutions also include:

distortion pedal, effects processor, tube preamplifier,

in order of increasing price

all of which you shall use with the marshall in the clean channel

marshall are horrible amplifiers. their transistor models are worse than the modelling amps from line6 while their valve amps are more expensive than similar (if not better!) from other manufacturers.
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Old 15th July 2012, 03:30 PM   #9
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Quote:
Originally Posted by cretaceous View Post
wow teemuk.. thanks very much for your comments.
I have indeed wondered about changing the amp but (mistakenly it seems!) thought a few mods might do what I wanted.
I built 2 fuzz boxes a lifetime ago (30yrs ago) from things published in electronics magazines, so realised that this Marshall cicrcuit was 'minimal'.
As you say looks like it would need trimmers /pots to adjust.
I did 'A' level physics but electronics theory is beyond me !
It's funny on the clean sound - you turn it up and you begin to sound like a "rock guitarist" - which flatters me.. but all in all I prefer a more jazz sound, as "rock" is such a cliche.
Many thanks - I'll try the diodes and capacitor before I swap the amp!
Marshalls do what they do, which is sound like rock and roll. If you're serious about that sound, then the biggest help you could give that amp is use a Greenback. You'd be amazed at how much it will filter the nasties.

But if you really want a jazz sound, an MG15 is hardly a good platform... it's definitely a "spare any expense" product that barely hints at real Marshall tone, which you don't want anyway. I say, don't muck with it, sell it off and get one of the little Roland Cubes (the bigger the better, but even the little ones will work). They have an excellent modeling setting for the Roland Jazz Chorus, which is rightfully a legend among jazz players, and the "Blackface" setting is likely to make you happier for rock and blues. They'll also give you better "Marshall" tone than the MG will for those angry moments when you want to sound like Pete Townshend (we all have them, don't we?).
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Old 15th July 2012, 06:37 PM   #10
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oh - now you're really complicating my life Keriwena..
I only got this as I needed something in a hurry to play with my mates and it was on the 'bay and local!
and yes my original plan was to get a roland cube

I'm opening it up as it makes a horrendous bang when you switch it off, and I have a hv capacitor ready (from advice gained here) so figured if I have it open, the odd change of diodes/capacitor in distortion circuit would be 5 minutes work

life gets complicated sometimes ;-)

Last edited by cretaceous; 15th July 2012 at 06:40 PM.
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