Questions about the Marshall Lead 12 preamp

[J M Fahey]

It is *very* stupid and *very* dangerous to mount the 120/*240V in the same board as the audio circuits.
You are repairing it *out* of the chassis, over your desktop, or even worse, unmounted but still in the chassis, because you need it still connected to the power transformer, and a small unexpected movement (your attention is naturally focused on some other point you are repairing) and the full chassis becomes live.
Are they *CRAZY*. All I can think is S.O.B.  :trouble :grr
I *always* pull those fuseholder clips, *destroy" the tracks so it can't be re-mounted and add a "flying type" fuseholder, the kind sometimes used in cars, plus adding a glued note that *now* that's the main fuse, no substitutions.

[Rutger]

Hi,

I've finished the amp a couple of weeks ago and it works! It's very nice indeed, even the cleans are very acceptable but it really shines when distorted.

Playing with it, you start to wonder how to make the amp a little more versatile. I would like to make it sound just a little more bright when playing clean, to get a little more 'snap' in the highs. Therefore a bright-switch would be usefull. But I wonder, normally this brightcap is placed on the gainpot, but I don't think that's possible in this circuit.
Is it usefull to place a brightcap on the mastervolume pot? And how can I calculate which capvalue I need? Normally I like something like 100pF or lower.

Another mod I would like to try out is a groundlift on the midpot. Looking at the schematic, there's also a little cap (C10) next to the midpot to the ground. What does this cap do, does it roll of some highs? And how do I calculate which frequencies it rolls of?

[J M Fahey]

Haven't you asked this exact same question somewhere else?
I answered it earlier.

[Rutger]

Well, I've started  a similar topic at musicelectronics. But apparently more people read the some fora as I do

But not at ssguitar, no.

[joecool85]

Juan, it doesn't count if you answered it on a different site haha.

[J M Fahey]

To be fair, I didn't even remember *where* I had answered it.
It might very well have been a double posting inside SSGuitar itself (has happened before)
Sometimes anxious musicians (is there any other kind?  ) post something and not receiving answers after an incredibly long hour  :loco either bump it up or simply post a new topic.
Oh well.

[joecool85]

To be fair, I didn't even remember *where* I had answered it.
It might very well have been a double posting inside SSGuitar itself (has happened before)
Sometimes anxious musicians (is there any other kind?  ) post something and not receiving answers after an incredibly long hour  :loco either bump it up or simply post a new topic.
Oh well.

It's true, I've seen that here before.  Just figured I'd give you a friendly jibe.

[J M Fahey]

Yea, I got it 

[phatt]

Hi,

I've finished the amp a couple of weeks ago and it works! It's very nice indeed, even the cleans are very acceptable but it really shines when distorted.

Playing with it, you start to wonder how to make the amp a little more versatile. I would like to make it sound just a little more bright when playing clean, to get a little more 'snap' in the highs. Therefore a bright-switch would be usefull. But I wonder, normally this brightcap is placed on the gainpot, but I don't think that's possible in this circuit.
Is it usefull to place a brightcap on the mastervolume pot? And how can I calculate which capvalue I need? Normally I like something like 100pF or lower.

Another mod I would like to try out is a groundlift on the midpot. Looking at the schematic, there's also a little cap (C10) next to the midpot to the ground. What does this cap do, does it roll of some highs? And how do I calculate which frequencies it rolls of?

That mid lift trick only works on old fender circuits, I very much doubt it will work well here  but try it and find out.
As to the bright problem ?? Hard to say.
If I get time I'll try a simulation and see what the tone curves look like.

C10 Q,, google *fender tone stacks* or try *tone stack types*
just click on stuff that looks like it might be explaining how they work.

BTW at a glance,, the one you have is not ever going to be a Rolls Royce.
Phil.

[Rutger]

Hey,

Old thread I know, but it keeps the forum clean

Still using this little amp a lot. Never got to experimenting with mods though, allthough I recently I replaced the cheap speaker I used for an Emi Legend 1015... wow, now this little fellow has become a little monster, it rocks hard and loud! Really, really nice Not gigged with it though, the poweramp can put out 20W easilly, so I wouldn't be surprised if it could hold its own.

One thing though, and I wonder what it is and if it can be fixed. When the preamp starts to break up the overdrive starts as a funny fizzy crisp. The same thing when the notes fade out: it ends with a funny fizzy crisp. I think this crisp stays when you drive the amp harder, then it sounds more like overdriven highs which can be nice. But it can be annoying when I play with a little OD, blues for instance, than it's quite noticable.
When playing clean it's not there.

I wonder if it is a characteristic of the chip the preamp uses? It has a LM1458 in it. Can this be fixed by using another (type of) chip?

First I thought it was the cheap speaker, but it's the same thing with the Emi, allthough the Emi is not fully broken in yet (played with it a couple of hours).

[Loudthud]

You might try just about any dual opamp. Install a socket so it is easy to swap parts. The 4558 is popular and of course the TL072 or 5532. Report back on how they sound and if the fizz goes away.