Modifications for Marshall MS2

[Big Monk]

So a few thoughts:

1.) Playing this through a real speaker was an eye opener.

2.) It seems you could play with a number of garden variety transistor boost topologies to replace the "preamp" here but in general any boost or even overdrive circuit could do the trick.

3.) I have to review the tone control topology a bit as I can't quite pin it down.

4.) The power amp eludes me a bit. I'm admittedly a bit behind the times with opamps. Obviously, I have found out that the limiting resistor off Pin 6 to the speaker can function as a master volume of sorts but other than that, there are a lot of capacitors there which I'm unsure of as to what they are doing.

5.) I'd prefer to keep the little guy in its faux Marshall shell, but I'm not opposed to boxing it up in something more pedal like. 

[PRR]

> It seems you could play with a number of garden variety transistor boost topologies to replace the "preamp"
> I have to review the tone control topology a bit as I can't quite pin it down.
> The power amp eludes me a bit. .....the limiting resistor off Pin 6 to the speaker can function as a master volume ... there are a lot of capacitors there which I'm unsure of as to what they are doing.

The preamp is not "magic"; rather it suggests severe penny-pinching. (Note the huge range of hFE on that part; this is lucky-dip designing, or rather "Sort by cost, low first".)

The tone control echos "Big Muff" but has been beaten-down to a low impedance because the power amp input is so low.

The power amp is nothing-special. How much do you know about power amps? Series resistance is just like using a miles-long speaker line; sure it won't be as loud. The only key data is the internal gain-set resistor: 15k, like the input bias resistor.
https://pdf1.alldatasheet.com/datasheet-pdf/view/69377/KEC/KIA6213S/+4J_J8UhwC/1KIDvtBv/1+/datasheet.pdf

[Big Monk]

> It seems you could play with a number of garden variety transistor boost topologies to replace the "preamp"
> I have to review the tone control topology a bit as I can't quite pin it down.
> The power amp eludes me a bit. .....the limiting resistor off Pin 6 to the speaker can function as a master volume ... there are a lot of capacitors there which I'm unsure of as to what they are doing.

The preamp is not "magic"; rather it suggests severe penny-pinching. (Note the huge range of hFE on that part; this is lucky-dip designing, or rather "Sort by cost, low first".)

The tone control echos "Big Muff" but has been beaten-down to a low impedance because the power amp input is so low.

The power amp is nothing-special. How much do you know about power amps? Series resistance is just like using a miles-long speaker line; sure it won't be as loud. The only key data is the internal gain-set resistor: 15k, like the input bias resistor.
https://pdf1.alldatasheet.com/datasheet-pdf/view/69377/KEC/KIA6213S/+4J_J8UhwC/1KIDvtBv/1+/datasheet.pdf

I think mods to the preamp booster and tone stack are straightforward.

Any insights on the various capacitors in the opamp stage?

[niektb]

Of course I can spam all possible modifications of the circuit, but is there anything in particular you would like to improve? Is there something you don't like about the current state?

[iainpunk]

if it were my tiny amp, and i was determined to mod it, i'd add a seperate tone stack to the overdrive channel, and some big muff style clipping diodes on to the transistor. maybe add a gain recovery stage for the loss of volume due to clipping and tone stack insertion loss.
i'd also consider replacing the whole pre amp with some nice opamp gain, gives a bit more room for sound tailoring.
another option is cmos, for the smooth clipping character, and you can re-paint the box like a SUNN Beta Lead, since it also has cmos distortion!

this thread reminds me that i have a 3 watt Epiphone ''amp'' i got with my shitty epiphone explorer, i threw in a speaker output jack, and in my upright 4x12, its loud enough that the people 3 houses over complain about the volume!
its the little white thing. its 3 watts, and quite loud with the original plastic speaker, that's also the guitar i have. its really light weight, great neck and frets, i put in more money on mods than it originally cost me... thursday, im adding a neck pickup (i got off of WISH.com)



y'all motivated me to go and play the mighty 3 watts this afternoon. the internal distortion is great, and the tone control is extremely powerfull when plugged into a real cabinet. (with its original speaker, it has only one sweet spot, the rest is crispy black metal or only muddy flub)

cheers

[Big Monk]

Of course I can spam all possible modifications of the circuit, but is there anything in particular you would like to improve? Is there something you don't like about the current state?

It sounds best above 5 on the "Volume" control (really Gain) and with the Tone control up pretty high. It's accentuated in the highs, however, in this arrangement. A touch harsh may be the right description.

I would imagine the tone circuit would be a good place for alteration but i'm also curious about the frequency shaping in the Opamp power stage as well.

I'm not expecting anything major here. This really is a one trick pony so i am simply looking to shape the frequency response of that one trick.

[Big Monk]

if it were my tiny amp, and i was determined to mod it, i'd add a seperate tone stack to the overdrive channel, and some big muff style clipping diodes on to the transistor. maybe add a gain recovery stage for the loss of volume due to clipping and tone stack insertion loss.
i'd also consider replacing the whole pre amp with some nice opamp gain, gives a bit more room for sound tailoring.
another option is cmos, for the smooth clipping character, and you can re-paint the box like a SUNN Beta Lead, since it also has cmos distortion!

My preference would be simple mods that don't add much to the existing footprint or board (i'd like to keep it in the little faux Marshall shell). Tweaking the existing boost "preamp" would be a good start I think and reshaping the response of the Tone control and maybe playing with the opamp frequency shaping as well.

I'm really a vintage dirt guy and transistor circuits are really my jam. I'm not very familiar with opamps and the mods required to alter the response and tone of them.

[iainpunk]

if it were my tiny amp, and i was determined to mod it, i'd add a seperate tone stack to the overdrive channel, and some big muff style clipping diodes on to the transistor. maybe add a gain recovery stage for the loss of volume due to clipping and tone stack insertion loss.
i'd also consider replacing the whole pre amp with some nice opamp gain, gives a bit more room for sound tailoring.
another option is cmos, for the smooth clipping character, and you can re-paint the box like a SUNN Beta Lead, since it also has cmos distortion!

My preference would be simple mods that don't add much to the existing footprint or board (i'd like to keep it in the little faux Marshall shell). Tweaking the existing boost "preamp" would be a good start I think and reshaping the response of the Tone control and maybe playing with the opamp frequency shaping as well.

I'm really a vintage dirt guy and transistor circuits are really my jam. I'm not very familiar with opamps and the mods required to alter the response and tone of them.

that power amp is nor an strict op amp. it a power amp, with feedback partially done internally. if you want to mellow it out even more, you can change the 470pf cap across pin 1 and 4 to 1n, this cuts down on the gain of higher frequencies.
if you want less gain from the power amp, the 47r comming off of pin 3, make it a higher value for less gain.

cheers

[Big Monk]

if it were my tiny amp, and i was determined to mod it, i'd add a seperate tone stack to the overdrive channel, and some big muff style clipping diodes on to the transistor. maybe add a gain recovery stage for the loss of volume due to clipping and tone stack insertion loss.
i'd also consider replacing the whole pre amp with some nice opamp gain, gives a bit more room for sound tailoring.
another option is cmos, for the smooth clipping character, and you can re-paint the box like a SUNN Beta Lead, since it also has cmos distortion!

My preference would be simple mods that don't add much to the existing footprint or board (i'd like to keep it in the little faux Marshall shell). Tweaking the existing boost "preamp" would be a good start I think and reshaping the response of the Tone control and maybe playing with the opamp frequency shaping as well.

I'm really a vintage dirt guy and transistor circuits are really my jam. I'm not very familiar with opamps and the mods required to alter the response and tone of them.

that power amp is nor an strict op amp. it a power amp, with feedback partially done internally. if you want to mellow it out even more, you can change the 470pf cap across pin 1 and 4 to 1n, this cuts down on the gain of higher frequencies.
if you want less gain from the power amp, the 47r comming off of pin 3, make it a higher value for less gain.

cheers

Sounds good. I'll target tweaks to the Pin 1 and 4 cap, tweak the boost stage a little, and mess around with the tone control. Those 3 things should be easy and fruitful enough!

[niektb]

I think I would insert a buffer in the clean path (or maybe in both, doesn't really hurt I guess). Maybe a Fetzer Valve in the clean channel? (if that isn't too much hassle)

[Gattoconglistivali]

I'm not sure how is the MS2, it's just a simple clipping stage into power amp; and straight into power amp in clean mode. There's no effort of voicing anything.

The 5150 micro stack is more fun, it has a preamp and some says it doesn't sound bad.

So I got very interested of what I can get from this little toy. I made a very simple mod only costs less than $3: add 3 jack, send, return,and speaker out

Putting a tube preamp(which doesn't color nor overdrive the signal), and cab sim in the fx loop, add a centaur(clone by nux) brfore the amp, and output to a hi-fi speaker, this is how it sounds like:
https://app.box.com/s/4r9kq6qqt0762shpkp1m4ckbza3pz30h

[Big Monk]

For the past few days I have been playing this through the clean channel with the Volume and Tone full up through the 10" Celestion G10N-40 speaker. I have to say, I don't think this needs the preamp section at all. It has a pretty good non-master volume Fender/Marshall thing going on and it takes pedals, especially Fuzz very well.

So, what I may do is work a little on the tone control and some of the filtering and see about double it up ala the RoG Little Gem Mk II to get closer to 1.6 W out of it. I may just order up a few KIA units or simply build a double version of the MS-2 into a stompbox with a modified tone control on the input.