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marshall lead 12 more overdrive

Started by stefi83, July 07, 2016, 06:17:23 PM

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Hello, I'm new... a greeting to all you! :)
I recently bought used this amp in pretty good conditions, and I am getting more enthusiast while reading about it in forums and posts.
Now about the overdriven tones, I was wondering what was the easiest way to increase the gain without upsetting the circuit.
Looking to the preamp stage, what do you think about:
- at the first stage, raising the value of R7
- at the first stage, to work on R5 and C1
- at the second stage?
I'd avoid adding diodes, because I don't see diodes in an old marshall super lead, for example.


Hi and welcome,, :)
With R7 larger it will just distort earlier on the dial.
Altering C1 or R5 Will alter the tone a bit. It's just there to bypass a bit more hi freq so altered values will alter the top end response.

To presume that Because your favorite Marshall Valve Amp did not use diodes then your SS amp will sound more realistic if you don't use diodes is a misunderstanding of how the technology works.

Without diodes it's much harder to get SS amps like this one to crunch like a real Valve rig.
My advice,,just buy a pedal you like.


Ok I agree... I'm just trying to understand the circuit, evaluating if I have the chance to tweak its parameters.
From a technical point of view I went to the following points:
- both stages are non inverting, with gain being in the form G=1+Rf/R
- with the supply voltage at +/-16v, the 1st stage is raising the signal with high pass filter and no clip aims
- the 2nd stage has a lot of gain and can clip when driven by VR1, with some kind of band pass
- I draw with computer the transfer functions vs. frequency, separately, considering also the capacitors, and I attach the results.
Any comment is apprecciated.


Hi, I don't know you at all, so take this for what it is worth.

Many people ask for "more gain" when what they really want is different sounding distortion.  Or a more distorted tone if you prefer.  The solution to that is not automatically more amplification, which is what gain is.

SO as phatt mentioned, clipping diodes may well be something to consider.  The fact an old tube Marshall got crunch without them has absolutely nothing to do with this solid state amp.  Diodes come in many flavors, meaning different junction voltages.  germaniums have a very low voltage, while LEDs can have several volts across.  You can pair them up for the degree of clipping you might like.  And no one says you have to use pairs of them, you can clip one way and not the other, or two different types of diode in the opposite directions.

J M Fahey

1) congratulations, your circuit analysis is spot on
2) your frequency response analysis is also fine
3) those curves were carefully tweaked until they got the best sound possible in that configuration.
How did they know that was the best possible ?
Among other factors, because they are Marshall, get regularly visited by the best Guitar players in the World and have the luxury of hving themas bata testers and listening panel, all in one.
Once you got a sound which is liked by, say, Van Halen, Jimi Page , Angus Young , you_name_it ,it´s VERY hard to argue against that.
4) so my suggestion is twofold:
a) leave that wonderful piece of sound processing as is (it does not just "amplify"  by any means) and add clean gain *outside*  with a clean booster
b) do not let any diode get near it.
They are buzzy in general, while here we have Op Amps banging against the rails, a very different mechanism.

Killer little amps, FWIW ZZ Top (and a couple others) are known for gutting one of these and housing it in a Rack cabinet, to drive large external power amps and tons of cabinets.


You mean they cheated and the whole concert was rigged!!!! ???


I'm in the wrong game I should have just made fake gear :lmao:


Many thanks for sharing your comments!
I've been thinking about all your answers, and now I feel more relaxed about leaving it as is and agree eventually to experiment something else outside.
To complete the analysis I add the corrispondig simultation made with a test signal of vi=200mv SINE at 1KHz in input, maximum gain and vo2 before the tone controls. If I've done correctly I find it very simmetrical and squared. I see those rails pointed by Fahey, too :)


Hi in my opinion yo should leave it as it is, more gain will spoil
this amp. All you would get when adding more is the sound like one of the smaller 90s valvestates, these can be bought all day for very small change.


In this case, instead of modding the amp, I think you can get a good bit more gain by using either a boost, an overdrive, or an EQ pedal in front of it. That's certainly gonna give you the "kick" you need.

I mention the EQ pedal because those can also boost your signal, and you can work the tone by adjusting the equalization.

If you don't have any of those, you can easily build a boost or an overdrive and there's tons of projects online.