Questions about the Marshall Lead 12 preamp

[Rutger]

Hey,

This thread will last forever, haha. But I'm buisy ordering stuff, including some different types of IC's for this pre-amp and an optional brightswitch.
Furthermore I would like to try some mods on the tonecircuit. At first, the tonecircuit looks like a standard fender/marshall-type tonecircuit, but there is something odd going on: the cap before the basspot is omitted and there is a cap going from the 10k resistor to ground (C10). This cap seems like a fixed lowpass filter to me, but then an awefull lot of bass would be attenuated.

So how does the basspot work, and what does the C10 cap actually do?

[J M Fahey]

1) the cap before the bass pot is there mainly to stop DC from the previous tube plate.
It also has some small effect in the EQ, making the bass pot cut a little bass when on minimum.
Since in this preamp the tone control is fed from pin 7 of IC1, where there is no DC, Marshall can save 20 cents.
In1000000 amps it's a 200000U$ saving, no small amount.
And I'm only half kidding.
2) C10 is not a "lowpass" but a "highpass to ground" meaning it makes easier for highs to get lost.
What for?
Small SS amps are often accused of being buzzy, which in fact is more caused by wimpy ridiculous speakers in shoebox size boxes than by inherent SS "defects".
Typical Fender/Marshall tonestacks , when rising mids, also rise highs ; they are simply a resistor from the Tone Control "ground" to the actual ground.
When you "rise mids" in fact what you do is rise "everything" by reducing the TC attenuation.
But SS amps do not want to rise highs (buzz) so they bypass the mid pot with a cap.
4K7/.022 means around 1600 Hz, so by rising mids you do not raise highs above 1600Hz.
I suggest you do not tweak this amp EQ, the sound is KILLER and has already been optimized and tweaked to death.
The BEST improvement is to use it with a better speaker than the cheap 10" it came originally with.
Since a lot of people replace original speakers everywhere with expensive Celestions, Eminence and Jensen, there are usually available in Craigslist and such lots of still very good original no name Peavey, Fender, Crate, etc speakers for a low price, usually for around $20.
Any OEM 12" is way better than the light G1025 expected by this amp.
2 x 12" even better.
I have tested these little Marshalls into 4x12" 1960 original Marshall cabs: killer sound; more than a Gold record was recorded that way. (Think ZZ Top and others).

[loudthud]

A low pass network attenuates the treble. The mid control brings up the mids because it modifies the lowpass into a shelving network instead of killing all the highs like a simple low pass would. These frequencies are sent to the bottom of the treble pot and proceed to the power amp because the bottom of the treble pot is a high impedance to low frequencies.

[Rutger]

Thanks alot for the explanation! Now I finally understand this circuit. So there is no bass attenuated by a cap at all, and C10 prevents the midpot from influencing the highs.

I've build a bigger (closed) cab with an Emi Legend 10 for this amp, and since I use this speaker it is a little monster indeed, great to rock with!
But does this mean that, now that I use a good speaker, I can omit C10?

I've run this stack in the Duncan tool, and it isnt so different from a typical marshall-tonestack after all, quite nice indeed.
I only wonder if it would be an improvement to place a 220n cap before the basspot, to roll of some lower bass frequencies, just like in most tonestacks. Not that I don't like the bass responce, but maybe it would become a little more tight?

[J M Fahey]

No, leave it as is.
C10 lets your treble pot actually cut highs even if the mid control is up , which may be good if you are playing at bedroom levels.
As soon as you start rising highs, C10's effect disappears.
And adding the 220nF cap before the bass pot will cut some bass in the final sound, (if you wish), but will not make sound tighter, because it's *post* distortion.
Just use it as is, very good sound.

[Rutger]

Thanks Fahey. I will follow your advice and leave it as is.
I played with the tonecircuit some more and found out that it is allready versatile enough, even a fender-middip is possible.

The only thing I hoped for where some better clean sounds, but playing with it I realized this can never be done by this preamp for it is build to do one thing only: marshall rock.
I'll just need to build something else to achieve that.

[Rutger]

Hey, I have another question so I hope you don't mind using my good old topic again...

There's a chance that I will use this amp for band rehearsels. Therefor I would like to add some kind of switching, so I can switch between clean and OD. To me the simplest way is to switch between 2 gain and 2 master pots. So I try to figure out where I need to implement (relay) switches.

But the way the gainpot is placed in the circuit looks a bit odd to me... what is that C3 cap doing in relation to the gainpot?

[J M Fahey]

Mmmmmhhhh.
You can easily switch 2 Master volumes because they are wired like conventional passive volume pots, but the Gain "Volume" pot is active with the center lug grounded, which complicates things.
You would have to swith *both* end lugs at the same time, between the original and "another" pot, and I suspect it will click or thump.
Easiest solution is to build a simple distortion pedal (MXR Dist+ is a classic) and use it for solos.
The combination works very well and is *very* versatile.
Check sites like GGG or DIY Stompboxes for schematic and layout.

[loudthud]

C3 gives a treble boost at lower gain settings. As you increase the gain, the boost moves higher but the amount of boost decreases. At maximum gain C3 is shorted out by the gain pot. It might be annoying at some gains. Feel free to experiment with the value and/or add a resistor in series.

[Rutger]

Thanks again, gentlemen!
I was afraid it wouldn't be that easy so maybe an OD of somekind is indeed a better choice.

[J M Fahey]

Well, the *best* solution would be to build a second channel (á la "SS Dual Rectifier") and switch whole preamps.
Consider it for a future build.