marshall MS-2 experiment

[owlexifry]

had this thing sitting on a shelf for years, battery option has never worked in my possession (due to faulty/failed switching DC jack), but recently i finished building a regulated 9V power supply, and generated some interest in seeing what this MS-2 can do, and maybe some mods..


read on some forums that it can sound much better through a real 12" guitar speaker. can be done easily, just need to insert the speaker cable to 'first click' on the 1/4" headphone output (otherwise it shorts out if you insert all the way, due to TRS circuitry on the output).

tried this. works well. yes, it does sound much much better than the tiny onboard speaker. surprisingly good tone in 'clean'(ON) mode. don't really care for the 'OD' mode, it's ok.
i noticed in the clean mode with the volume set to 3o'clock, and with an overdrive in front, i could get a relatively tight and ballsy crunch / lead tone. did not expect this. turns out the MS-2 isn't actually that bad.
but it's very quiet (at least into a 4x12 mesa).

looked at a schematic, turns out there's a 510ohm resistor on the output. so i tried bypassing it.


holy crap, without this resistor, this tiny 9V thing driving a 4x12 mesa cab was so much louder than i ever expected. impressive for a 1/2W opamp chip (KIA6213).
it was actually too loud (for a bedroom/practice scenario), so i tried a few different values of resistor to use to get it to a nice volume.

tried 330R, 220R, and then 100R. 100R was pretty decent. good 'bedroom' volume.

then i decided to install a switch to bypass the 100R resistor for a 'full volume'/'half volume' output option.

i again learned that fully bypassed was just too loud..
astonishing, for a linear dual opamp IC running on 9V (the regulated 9V supply vs battery probably helps)

so i tried a 39R to switch in parallel with the 100R. no real difference.
next lowest value i had was a 3.3R, so tried that. much much louder, and probably only slightly less output than fully bypassed, but i didn't have any other values to try so left it at that (a 10R would probably better).

end result:
turns out boosting one of these with a klone, changing x1 resistor value, and running the output into a guitar speaker cab sounds pretty good. fun afternoon.

here's a little demo

 

[nathan5782]

Wow, good times. My brother had one of those but I'm not sure what happened to it.

 

[gusfinley]

Wow, so maybe these aren't just toys, afterall!

 

[owlexifry]

Wow, so maybe these aren't just toys, afterall!

pretty much my reaction after i plugged it into the 4x12 and turned on the klone..

with the output resistor fully bypassed, i am not exaggerating, it is uncomfortably loud in my 3x3m noise cave/studio.
(the perception of difference between the two volume settings available with that switch is narrowed quite a bit, due to the ‘built-in’ auto attenuation with iphone video recording)

 

[middy]

The first watt does most of the work.

 

[LowCaster]

I’m conducting the same kind of experiment. I started by adding a speaker output bypassing the internal speaker. Loud and clear. One problem: the plastic enclosure is too light and impractical to use. So I’m in the process of transplanting the electronics into a Hammond 1590bb, plenty of room without the speaker. New power input, battery socket, and output. It will be the smallest Marshall head.

 

[LowCaster]

Here it is finished. I really wanted to use that hammered gold paint… Next thing, I may do as OP owlexifry said and add some resistor on the output, (I’m thinking about a potentiometer, but that may be overkill). It may sound silly but once plugged to a cab it gets really loud and has a lot more headroom. I tried it with the power attenuator of my tube amp, and there you go: a really fun Marshall saturation tone at bedroom volume.

Also in this setup it seems to be pushing the battery to its limits quite fast, but to work fine with an AC adapter.

 

[NickK_chugchug]

The only specs I could see on that opamp are:

Power audio amplifier 500mW Supply Voltage Min: 4V Max: 14V
Icc 0: 15mA Iout max.: -Pout max: 500mW

The output stage of the opamp looks like a normal amp in miniature (almost like an old Toshiba 1980s hifi circuit). Note there's no power current limiting from the power pin, but the test circuit is driving an 8R speaker, certainly no resistor in the manufacturer's test circuit but perhaps that's connect/disconnect transient current spike protection.
Also note the manufacturer's test circuit has a 470uF output cap instead of a 220uF. That would help lower end (at a cost of higher current loads).

How long does the battery last at 11?

 

[LowCaster]

I’ll tell you, I played one hour today, no problem. The stock combo sounded so bad I never played it long enough to drain the battery… longer than my patience anyway.

I find a minor difference with the schematics in first post: in my amp the 510R resistor is on the headphones stereo output, but there is no resistor on the speaker output.

When I connect an external speaker to the speaker output (internal speaker replaced by a jack socket), the sound is very loud, quite clear and the typical saturation is lost.

I’m using a 10inch 8ohm speaker. So it’s the same impedance but obviously not the same efficiency/sensitivity as the tiny stock speaker.

Then I followed OP’s idea to add a resistor in series with the speaker, I found 100ohm was too quiet and settled for 50ohm. With my speaker the Overdrive channel sounds crunchy and quite loud, clean can be used too.

 

[sadfield]

Could you use one of these to drive a reverb tank?

 

[LowCaster]

Follow-up: I had a small homemade cab waiting for this to happen: 6inch 8ohm Jensen P6V alnico speaker inside. I played two or three hours, the battery seems to be holding up so far. The amp sounds really good in that setting and is fun to play.

 

[sk8er4life]

That first Video by the OP has got to be the coolest visual of a cranked Marshall I have seen for a while! It's got twilight zone vibez, me likey.