How to improve marshall guitar amp with harsh overdrive?

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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!
 
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!
 
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.
 
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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!
 
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.
 
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?).
 
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 ;-)
 
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Didn't mean to complicate things, I was just following teemuk's lead and trying to offer a simple answer. ;)

Tell you what, though, if you really want to make a silk purse, take a look at the preamp schematics for the Marshall JCM900 Dual Reverb. The normal channel also uses LEDs for distortion, but it sounds really good if you don't go past '6'. I suspect there's less of the clipped signal bled in, and they likely EQ it a bit as well. Might see something there that helps you out.

jmp52-02

(Obviously, the JCM has a few more parts than the MG. :innocent: )
 
I traded in my SS Marshall (Lead Mosfet model) towards a JCM900 since I wanted "the tube sound." I didn't know the JCM900 got most of its distortion from the SS devices in its preamp until some time later. (Very little internets at the time.) Most any distortion/overdrive/fuzz pedal sounds better than my JCM900 did. I sold the JCM900 soon after looking inside it and at the schematic.
 
A friend asked me to fix the "ice pick in the forehead" sound of his KBA10. Quick, dirty and effective - I just kept increasing C8 until it got muddy and then backed off to the next lower value. He's happy and thinks I know more than I do.(Yes, I know the KBA doesn't have an overdrive circuit). Cheers
 
Use of Silicon diodes gives a hard knee to clipping the signal, Leds slightly less and germanium diodes even less. This is due to their respective forward resistances when in conduction. Using the right series source resistor and number of back to back connected diodes, the forward resistance of the germanium diodes can be used to provide a 'rounded clipping' of the signal. Hope this assists in your quest for 'less distortion' Mik
 
Yes. The marshall solid state distortion is way too harsh. Use audiomik's advice to gradually step the square wave with multiple diodes, and this should help you get a slightly smoother sound. Not less, just better sounding.

My experience is that they don't have too much distortion, its just a buzzy, poor quality sound. Not that terrible live, but when you record it, you can really hear it.
 
Just for the record, some 900s are pure tube, some are tube preamps with diodes, and the JCM900 Dual Reverb has a solid state preamp (15 opamps for gain stages, one 12AX7 to properly buffer the tone stack) and I think 9 LEDs (4 arranged in a fullwave bridge) for clipping.

The 900s also went through many changes during the decade they were around; they started with EL34s and Drake transformers but later went to 6L6s (really, the Brit equivalent) as the EL34 supply went downhill.

So please, take care when making generalisations about the 900 range. :judge:


I mentioned them because the DR preamp actually sounds pretty good if you realise it's actually a built-in overdrive pedal for the clean channel and a distortion pedal for the gain channel, and dial them in as such, rather than simply diming them as you would an 800. The gain channel goes to '20' for a reason! :eek:
 
Short answer, get another amp.
You have an entry level, small open back cabinet, puny speaker, 15W chipamp , made by somebody famous for its Metal sounds.
It's in its genes !!! What else did you expect?
Any modding won't change that, unless you fully rebuild it changing preamp, poweramp, speaker and cabinet .... which can be accomplished easier and cheaper following my initial suggestion ;)
That said, Marshall MG15 sound *very good* for a beginner's amp, and you can't complain for providing what's expected from them :)
 
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