New project: Marshall MG15CDR

[bluesky6]

I started looking at my new/used Marshall MG15CDR last night.

Nothing wrong with it. So I haven't quite figured out what to do with it. Perhaps socket the op-amps and do some swapping.

Picture of the innards attached. The schematics can be found at amparchives.

The thing at the bottom below the PCB is the reverb unit. It looks rather fragile: hanging from some springs.

The power amp is different from the one on the schematic. This one's an LM1875 which I guess is an equivalent. I like the big heatsink.

I'm taking suggestions for mods/not-mods/experiments

[J M Fahey]

That amp sounds VERY good just as-is.
If anything, I'd try to fit a next-larger size speaker of good quality, such as a good Celestion / Eminence / Jensen MOD / WGS .

[bluesky6]

I've confirmed (2 years of woodworking classes) zero woodworking skills. So a 10" speaker is out of question.

But good point. I'll try the amp with my 1x12 cab to see how the sound changes.

Perhaps a speaker-out socket will be in order.

I have an MG30CFX which I like a lot. That's DSP-based tho' and no schematic so far. So no modding.

[DrGonz78]

I always wanted to add a foot switch to toggle between channels on these practice amps. Not sure if that is practical but just wanted to do that mod. I have a MG15 laying around here... If that mod is possible then I would try it out too on that one.

[Roly]

Socket and op-amp swap away to your hearts' content, but unless the originals were something utterly woeful you are unlikely to hear any real change.  Agree with JM that a speaker change may be more productive.

Reverb tanks are fragile; the other reason they are shock mounted on springs is because they are also quite mechanically sensitive.

[bluesky6]

What's the best speaker upgrade for a Marshall (e.g. MG15CDR and/or MG30CFX)? Celestions? British sound?

I think I've a pretty good handle on Fender amp speaker upgrades but am totally new to Marshalls.

Thanks.

[Enzo]

I don't know. What does better mean?  What is the best way to have eggs?  Scrambled?  Over hard?  Over easy?  Poached?  Better means YOU like the sound better, not whether someone else does.

[bluesky6]

Actually, I don't like eggs. Long story of being force fed eggs as a kid.

But let me rephrase the question since there are no stupid questions. Just misunderstood ones.

For Fender amps, I see generally and that's probably just my observation (via Google) and interpretation of search results, a recommendation for "American" sounding speakers. Something bright with great cleans to match what Fender amps are good at. At least their SS amps.

On the other hand, I am completely unfamiliar with Marshall amps and what it means by "Marshall" sound.

So would "British" sounding speakers be the natural swap ins to keep or enhance the "Marshall" sound in Marshall amps? Or would an Eminence Ragin Cajun be more appropriate for my MG30CFX?

Or you can tell me that there is no such thing as a "Marshall" sound. And that is as good an answer as any.

[DrGonz78]

Do you like steak? T-Bone, Ribeye, New York Strip, Sirloin, or Flank... Etc Etc... Do you like it rare, med-rare, medium, or well done? Do you like A1 or HP sauce or just the natural flavor of the steak? God this site is making me hungry today!

I think the answer to your revised question will still be answered best by Enzo's post. The reason is that I love medium-rare to medium and some like rare or well done. Sure someone might have an opinion about what speaker to put in what but in the end it may not match your taste. The Marshall sound as the phrase goes, to me, means more in regard to the tone stack sounds. It is a scooped sounding amp and the mid range is not very present whereas Fender has very pronounced mid range. It depends on what type of music we are playing. So what type of sound are you going for and what type of music? That may help other's recommend some different speakers for each type of amp. I don't really know myself, but I know everyone has different tastes.

[Enzo]

If you don't like eggs, then that makes my point.  I am asking you for a suggested form of eggs from someone who could't tell me.

There is no single thing you can do to an amp to change its sound that will have more effect than changing the speaker.   The original Marshall 4x12 cans might have mostly used Celestion speakers.  But I have no idea if that is descriptive to you.  Besides, Celestion made/makes many different models of guitar speakers.

The Marshall cabs were closed back 4x12, while in my mind the classic Fender sound is an open backed Twin Reverb.   

Problem is getting into specifics.  We can say something is British sounding  but that usually refers to certain tonal artifacts, but the speakers still come in ranges and certainly various brands and models overlap.   If I tell you women are prettier in North Carolina than in Tennessee, should anyone accept that?  What is pretty anyway?  Anne Hathaway?  Anna Faris?  Kaley Cuoco.  Courtney Cox?   No tattoos?  Covered with them?

So maybe an American sounding speaker is just what you need to go with your Marshally amp, or vice versa.

Is there really a Marshall sound?  I suppose so.  To me, you say Marshall, my first thought it Jimi Hendrix.   Have there been a zillion players since Jimi that also played one?  Yes.   I just offhand don;t know who they are.

When I get a Marshall amp in front of me, I tend not to like it, I think they are shrill screechy things.   I like a strong Fender amp myself.   But if I were trying speakers, I wouldn;t rule out a "Marshall" speaker, because it might make magic connected to my Fender amp.   Key word there is listening to them.

[bluesky6]

Thanks for the responses.

I can tell you that I don't like eggs because I have tried/been forced to eat them. I can tell you that I like steak (if you count filet mignon steak) because I have enjoyed them. But I can't tell you that I like or dislike a food that I have never tried.

And that's the point here. I've zero experience with Marshall amps. I've 2 now, but have owned them less 10 days! I am also a beginner guitar player.

So... what do I like sound-wise? I don't know. And that's all I'm asking: help me discover tonal options. If we all already know exactly what we like/dislike, won't life be boring like hell?

If I can improve the Marshall-tone on my Marshall amps, I would love to. I just don't want to spend $$ and turn a Marshall into a Fender because I've already enough Fenders.

Anyway, I've been fiddling with the MG15CDR and plans for this weekend are:
1. Swap out the power supply caps to solve the bang/spark noise when I switch off the amp. That mod worked for 2 Fenders. May yet work on a Marshall...
2. Swap in an audio taper pot for the drive channel volume control to have more adjustability. Yes, I continue to struggle with overly loud amps.
3. Swap in a bigger value pot for the clean channel volume control, again to have more adjustability.
4. Maybe do a LED/germanium diode mod like what I did with the Pathfinder

Celestion has an 8" Eight 15 speaker that claims to "upgrade your bedroom blaster with an authentic British tone" (whatever that is, heh?) and that may be a mod down the line. Of course, I won't know whether I'd like that mod until I try it out 

[phatt]

 Hi bluesky6, it's a tough world this GuitTone thang 8|

The SS Small cheap made amplifiers you mention (both brand names) are about as close to the label of Britsh /American Sound as we are to the next inhabitable planet.
Cricky they are not even designed by nor made by those famous people anymore,, mostly gold plated poop is all you get now.

You also mention you struggle with overly loud Amps. hum??

Reality check might be in order;
The overly used term "Marshall sound" is in reference to very early models which where of limited bandwidth and actually altered the frequency response in quite a unique way.
Even today few circuits actually do that sonic trick well.

An early model Marshall (Volume low) is a completely different animal when turned up loud especially through 8 x 12 inch speakers. Note the LOUD bit.
The laws of phizzy things does not allow for those 1 watt micro bedroom thrash machines to impart the same sonic experience no matter how many rack effex you run it through.
---------
Google *Munsons curve* and while you are there research why old 70's hifi amplifiers had a *Loudness switch* which improved the sonic result at low listening levels.
It's not so much the equipment that is imperfect but the way our ears operate at different sound levels.
---------
If you wish to get even close to either F or M sound at low volume, small speaker, wattage challenged equipment then you need to look into cab simulation as well as different gear than those small over rated bedroom amps.
Frankly you maybe better served with a non Guitar amplifier rig.
Try a (preamp for guit) > Graphic EQ > Cab sim then into a power Amp with wattage of choice.

BTW, Watch those small speakers they have close to zero low end response while the high freq rips your ears off,, Which if you are getting the picture is a universe away from the sound you are likely searching for.
Someone once stated that *The Classic Marshall Sound* had a midrange honk like no other amp of that time, which is exactly what you see if you simulate those old circuits.
Sadly until one experiences quite a few bits of gear it's very hard to impart in words or even pictures.

Early M sound had a very narrow band of frequencies while heavily suppressing bands below and above. The combination of circuit, Output transformers used and speakers (add closed back box) form a large part of THAT sound.

As the VERY Experienced chaps here have already mentioned,,just by changing the speakers!!! Yes even in an old Marshall, speakers with wider bandwidth response will seriously alter the signature of the tone produced. 

You need to beg or borrow some real guitar amps,,,real ones and preferably older gear,, pre channel switching and crap.
Hire a sound proof room and crank up some different model gear.
Phil.

[DrGonz78]

So let's look at one of the topics on your list. "3. Swap in a bigger value pot for the clean channel volume control, again to have more adjustability."

This is a common problem with amps in my opinion if you want to be able to turn the amp up past 2. Now this is only in the respect of bed room playing and it would be nice to have it up to 3-5 volume right? Well I have done a mod a Fender Princeton 112+ that did a really good job. It involves of course changing the pot to an audio taper, but that did not really get it done. So I looked at the input of the amp and tried to increase resistance of the input signal just before the first op amp. This would R1 and R2 on the Fender amp and looking at the schematic for your MG15CDR it looks very similar of course. R1 is 10k and you could raise that to 20-30k and/or R2 (1meg) can be brought down to 750ohms/500ohms. The idea is that at the input of the amp R1 is resisting the guitar signal just a bit more. R2 then lets more of the signal bleed into the ground, thus reducing the signal a bit more. Both of these approaches have made more differences than just changing out the volume pot from reverse log to log.

In your case I get that you just want to increase resistance using higher value pot. However, my example is to show that there are other ways to get similar results.

[Roly]

Someone once stated that *The Classic Marshall Sound* had a midrange honk like no other amp of that time, which is exactly what you see if you simulate those old circuits.

Great post Phil.   :dbtu:

LTSpice, Duncan's TSC, {and the ever-helpful  Xc  =  1/(2 Pi f C)}

[phatt]

Someone once stated that *The Classic Marshall Sound* had a midrange honk like no other amp of that time, which is exactly what you see if you simulate those old circuits.

Great post Phil.   :dbtu:

LTSpice, Duncan's TSC, {and the ever-helpful  Xc  =  1/(2 Pi f C)}

Thanks Mate > Coming from you Sir that means a lot. :dbtu:
Yes on occasion my brain switches on and words come together right, shame my maths does not work as well   :duh
Yes I'm still chipping away at the bottom of the mountain of knowledge but I feel a little more confident of finding my way through the maze of some of those equations you keep posting. lol.
Of course I doubt I'll ever be able to design from scratch with pad and pencil but via those wonderful sim programs I now fully respect why some books rave on about stuff as simple as R/C maths. Sadly I did grade 6 twice and still have trouble retaining more than a couple of numbers in my head so I still cheat a lot by simulating.

For others here who struggle with the complexities of this stuff you are not alone.  :tu:
Even if only armed with the basics there are some Rules of thumb and General guide lines you can find that can open doors to a deeper understanding of how to approach design of your rigs.
What I have learned from Guys like Roly is that most of the magic is actually in the last place most think to look.
I had a bloke some years back fill his bottom draw with many glass bottles all purchased claiming to deliver some kind of magic mojo for his beloved Valve Amp yet he could not get the amp to sound as good as my pile of crap home built job. (made from old Valve radio parts)

In desperation he finally handed me his little Amp and all I did was change the value of a coupling cap, removed treble bleed cap in the signal path and convert the output from fixed bias to cathode bias while dropping the HT supply with a big fat resistor. (Fat resistor was actually Roly's advice way back) :dbtu:
To rub salt into his glossy magazine teckno babble advice which he had vigorously defended for a long time, (insert evil chuckle) I used the original power tubes that came with the amp, the very ones which had been replaced many times over $$$ because of lack of mojo tone.
Needless to say His jaw dropped when he played it,, and He still smiles every time we play together at the same gig.

Sadly a glowing bottle with red dots and some Slick brandnames $$$$$ has far more visual mojo than a mundane looking round blob capacitor or a 100k 1/2 watt resistor Yet that is where all the magic happens just by working with R/C values in the right places will (more often than not) evoke better mojo than replacing perfectly good valves with other perfectly good Valves @ 3 times the price. 

Note this goes for SS gear as well, swapping opamps for better units will do little if the circuit has design flaws that can be fixed by simply altering values on a few R's and C's.
Never underestimate the power in those mundane uninspiring Passive Components they are the foundation of those great tones you call mojo.
Even with limited understanding Once you realize that crucial point you will be way ahead of the pack. The money you save in the process could well pay for your next guitar. 

Give me a bag of R @ C anytime an I'll just leave the self proclaimed vacuum heads to debate the tonal merits of the gold print on a new set of perfectly matched bottles. :lmao:

@ Bluesky6
Sorry for me ramblin a bit off topic but in the process it dawned on me that 
I did mess around with something similar to your Amp a year or so back, Not exactly the same circuit but might give clues as to how to go about simple changes.
This unit also suffered from a sound level mismatch when changing from clean to OD which was improved by my alterations.
The Amp came as is (bare chassis, No speaker) so I can't comment on what it would have sounded like new but as like most small amps they are often close to identical in basic concept and have miles too much hi freq content in the circuit.
With no serious effort to subdue freq above 3~4khZ in the circuit and coupled to a cheap small speaker you have no chance of fat british tone.
Here is that posting with pics and schematic mods.

http://www.ssguitar.com/index.php?topic=2607.0

My amateur techy rant over, Phil.