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Old 16th July 2012, 04:53 AM   #11
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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. )
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Old 17th July 2012, 12:11 AM   #12
jjman is offline jjman  United States
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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.
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Old 20th July 2012, 11:34 PM   #13
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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
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Old 20th July 2012, 11:55 PM   #14
tsiros is offline tsiros  Greece
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Quote:
Originally Posted by jjman View Post
the JCM900 got most of its distortion from the SS devices in its preamp
are you sure?
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Old 21st July 2012, 12:04 PM   #15
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Quote:
Originally Posted by tsiros View Post
are you sure?
Perhaps he's looking at a different schematic? - looks to me like it's preamp distortion all comes from 4 triode stages.
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Old 21st July 2012, 01:29 PM   #16
tsiros is offline tsiros  Greece
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now that i woke up, i remember some general bitching regarding marshall putting diodes for distortion in some amps and they later changed it.
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Old 21st July 2012, 01:56 PM   #17
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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
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Old 21st July 2012, 11:13 PM   #18
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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.
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Old 22nd July 2012, 03:07 PM   #19
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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.


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!
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Old 28th November 2012, 05:27 PM   #20
JMFahey is offline JMFahey  Argentina
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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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