Marshall Amp increase Bass/low-mid response

[gitaarversterker]

Hi guys I have a vintage Marshall model 2195 (solid state).

I want to mod the preamp/amp to increase the bass/low-mid range response of the amp to get a more rich sound. Now it sounds a bit flat or shrill.

Here is the schematic of the amp.


VR1=channel volume (or gain)
VR2=treble
VR3=mid
VR4=bass
VR5=presence
VR6=volume

I think there are two mistakes in the schematic.

1: C10 (100n) is actually named C8.
2: I think the schematic is wrong about the the way the wiper of VR4 is connected. The standard way would be to connect the wiper after the REAL C10 (47n).


What I planned to do is this.

First check if the preamp is causing the flat sound by bypassing it (directly connect my guitar to the wiper of VR2 taking the signal into TR4 with a 1M ground resistor and a 470n coupling cap).

If the preamp is the problem then I want to increase some caps:

C10 47n > 100n.
C12 47n > 100n.

If the main amp is the problem then I think I should make the following changes.
1: disconnect C16 (presence)
2: Lower the values of C19 and C20 from 220n to 47n

Will my suggestions help solve the problem or am i wrong?
Are there other things to change in the (pre)amp?
And what is the function of RV1 (trimpot).

Thanks in advance guys

[Enzo]

VR4 looks fine to me, for example, this classic, Twin Reverb, look at the tone stack, wired exactly the same.

http://bmamps.com/Schematics/fender/...b763_schem.pdf

The VERY first thing you ought to do is explore different speaker options. That will have the greatest effect on tone. Are you using the 2x12 cab? If so, are the speakers in it wired in phase? Ought of phase speakers sound flat and tinny.

Don't mess with C19, it is your zobel stability network, it prevent amp oscillation, it is not about tone there.

C20? Oh, come on... That and C15 are simply bypass caps from the power rails to ground, they are not affecting tone.

RV1 is the bias adjustment. No, it doesn't affect tone, but it does affect crossover distortion or overheating, depending on which way you turn it too far.

Yes, I think the emitter of TR3 bypass cap ought to be C8, they made a typo.

[Dave H]

If the main amp is the problem then I think I should make the following changes.
1: disconnect C16 (presence)

Don't do that! It will drastically increase -ve feedback, reducing gain and probably make it oscillate. It's C17 you need to disconnect to disable the presence control and reduce treble.

I think the polarity of C16 is incorrect. It should have its -ve terminal connected to TR6 base.

EDIT:C14 is also reversed.

[J M Fahey]

Let's go step by step.

1) "it is what it is" , meaning it's a very good but dated (old) SS amplifier, not much warmth or creaminess built in (and that's an understatement) .
Sounds very good for what it is.

2) now the short answer: tweaking sound is tricky, similar the universal girl's complaint: "if I gain weight it goes to my booty or thighs, if I lose weight I shrink my titties" , here if you pull highs sound becomes muddy, if you increase bass it becomes farty, so you must be very careful and expect little, so as not to be disappointed.
Mind you, you might "improve" sound at home, but then get lost when playing with others.

3) So I suggest the Plan B:

a) leave basic amp EQ as is, but add an effects loop, 2 jacks, between VR6 wiper (Master Volume), and actual Power Amp input, C14 .
I guess preamp and power amp are on separate boards, so there is already a wire joining them, which makes the job easier.

If not, cut the track joining them, scratch a little of solder resist mask so as to have bare copper visible and solder send/return wires there.
You have space on the back panel to add a couple send/return jacks.

b) get a Graphic EQ pedal, and EQ to taste.
Basically increase *a little* the depth and warmth bands (100/200/400Hz) but don't overdo it, to avoid mud/farting.

Resist the temptation to kill highs or sound won't "cut through" when playing along a drummer.

The advantage of adding an external EQ is that it's way more flexible than anything you can do inside, and you can adjust it any time, even in the middle of a rehearsal or show.

To boot, since they are not exactly fashionable these days, I guess you can get a good used one for peanuts.

Absolute best of course would be a 15 or 31 band EQ, which allows tons of fine tuning, and can also be bought used for low cost.

No need for a "guitar" one, PA or DJ or Hi Fi types are perfectly good here.

Hi guys I have a vintage Marshall model 2195 (solid state).

I want to mod the preamp/amp to increase the bass/low-mid range response of the amp to get a more rich sound. Now it sounds a bit flat or shrill.


I think there are two mistakes in the schematic.

1: C10 (100n) is actually named C8. Yes, there's two C10 there, you are right.
2: I think the schematic is wrong about the the way the wiper of VR4 is connected. The standard way would be to connect the wiper after the REAL C10 (47n). Again yes, wiper goes to top leg. Anyway I guess the actual PCB is fine.


What I planned to do is this.

First check if the preamp is causing the flat sound by bypassing it (directly connect my guitar to the wiper of VR2 taking the signal into TR4 with a 1M ground resistor and a 470n coupling cap).
The whole amp Technology is dated, sorry.

If the preamp is the problem then I want to increase some caps:

C10 47n > 100n.
C12 47n > 100n.
No real EQ change, they will let some low bass through which won't help.

If the main amp is the problem then I think I should make the following changes.
1: disconnect C16 (presence) No need, if you don't like it just turn Presence to 0.
2: Lower the values of C19 and C20 from 220n to 47n
C19 is part of Zobel stability network, don't touch it; C20 is a -40V rail bypass, again for stability, none affects EQ but messing with them might make the amp unstable.

And what is the function of RV1 (trimpot).
RV1 is the bias pot, do not touch it unless really needed.
A too low setting will make the amp (crossover) distort at room levels; too high will overheat and even burn it, so just don't touch.

Thanks in advance guys

[gitaarversterker]

Thanks for preventing me messing with the main amp :-).

About the cab. The amp was connected to a perfectly fine 4x12 marshall cab.

We compared this amp with another marshall and the sound had much less body.

This is why i want to try to get the most out of this amp.

So do you have any advice on what to change

[gitaarversterker]

Thanks for the advice.

But why put the loop after vr6 and not in front of it?

At least i can start by connecting my guitar in that spot and check the tonal "loss" of the preamp.

[Dave H]

b) get a Graphic EQ pedal, and EQ to taste.
Basically increase *a little* the depth and warmth bands (100/200/400Hz) but don't overdo it, to avoid mud/farting.

Good plan (non invasive) I'd try a guitar pedal between the guitar and amp first. The amp's low input impedance could be making it sound 'flat'

[ThermionicScott]

So do you have any advice on what to change

The whole unit between your guitar and the speakers?

[J M Fahey]

Thanks for the advice.

But why put the loop after vr6 and not in front of it?

At least i can start by connecting my guitar in that spot and check the tonal "loss" of the preamp.

The signal before the master volume is always going full blast and easily saturate a 9V powered pedal which by definition has very little headroom, now if you boost the signal it becomes worse.

While after the wiper signal is lower (unless it's on 10) and even if you boost some band(s) you will then lower the Master even more (you are after a tone change, no more volume or gain), so it sort of self compensates.

And I suggested that point because it's after the distortion , so it gets fattened, while boosting lows before it can lead to mud and loss of clarity.

I suggest you download and read, at your own pace, the excellent Teemuk book on SS amps, where he discussed very well how to get *good* SS distortion, including the very good section on pre-EQ and post-EQ around the actual distortion stage.
http://www.thatraymond.com/downloads...ttala_v1.0.pdf

To be more precise, in your amp they cut Bass before distortion, to make it more clear, but did nothing afterwards to improve it, what I suggest will help you tune it to your taste.

There are other tricks, such as using "mixed feedback" , also explained in Teemu's book , but a full EQ goes way beyond that, is adjustable, and worst case can be set to 0 or disconnected.

Plugging your guitar straight into the power amp to check whether Bass is cut or not?
No need to do that, I *guarantee* some Bass was cut (you already notice it) ... and for good reason.

Want to hear the guitar flat?
Easy, plug it into the Bass player's rig and set it with all pots to 10 so it definitely clips.
It will have Bass ... and it will be muddy, compared to a Guitar amp.

Want to boost low/mids after distortion so you check whether it helps?
Easy, wire the Master volume wiper to a .047uF capacitor ( a cheap ceramic is fine) and the free cap leg to a 4k7 resistor, with the free leg soldered to ground, even better through some kind of switch so you can turn this on and off to instantly compare.
Set master volume to 2 or 3 , play, rise volume/gain (not the Master) until it distorts (set it on 10 if you wish) , switch on/off.
Does it help?
Now replace the .047 cap by 0.1 uF , how does it sound?

Only problem is you will lose volume here, because this simple EQ is passive, but is minimally invasive and will let you decide.
In a way it's a "reverse bright" circuit and works similar to rising the first 2 or 3 EQ bands.

[gitaarversterker]

Maybe it's to much to ask but can you add your eq and cap idea into the cirquit? Then i can seethe idea about the loop and cap stuff

[gitaarversterker]

And where exactly is the bass cut before distortion?

[J M Fahey]

C1 + R2 = 1290 Hz

C4 + R8 = 482 Hz

C5 + R9 = 72 Hz

C10 + R13 = 589 Hz

By the way, all of them for good reasons; increasing them will increase Bass, no doubt, but amp will lose clarity; all of them are before distortion (which comes by overdriving Tr3) .

EDIT:

Maybe it's to much to ask but can you add your eq and cap idea into the cirquit? Then i can seethe idea about the loop and cap stuff

Don't make me draw , upload and link
But it's easy: connect the cap from wiper to ground (reverse bright cap, remember?) ... but to avoid losing *all* highs, you go to ground through a resistor.
No need to draw *that* .
And further adding a switch in series with resistor and capacitor allows it to be tested on/off as many times and as fast as you wish.

[loudthud]

I want to mod the preamp/amp to increase the bass/low-mid range response of the amp to get a more rich sound. Now it sounds a bit flat or shrill.

Make C11 smaller, 22n or 33n. Duncan's Tone Stack Calculator shows that will bump up bass and low mids.