Marshall MA100H Mods

[Brad O'Dell]

It all started here...
http://www.marshallforum.com/threads/what-makes-the-jcm800-magic.109040/

I have spent many hours chasing after the information needed to make the MA100H a great amplifier, and a suitable "stunt double" for my '84 2210 JCM800 100w.

I wanted to share that information here because the original thread has become so cluttered with unrelated posts.

Looking back, the original thread was my first post ever, and should have gone here to begin with.

Anyhow JCMDOUG AMPMADSCIENTIST and CADORMAN, as well as others were incredibly helpful and deserve not only my thanks, but anyone benefiting from the information provided here.

1. The first change is the tubes. This is about a $100 investment...

Before...


After...



2. It is also imperative to set the bias.
(This is free if you have a multi meter)

Great Info here...
http://www.marshallforum.com/threads/ma100h-my-first-time-biasing.16597/

3. The next Mod is to remove R86 and replace it with a 500k pot to control the Negative Input.
(This will cost you less than $10)
This knob will "Thin" and "Fatten" the tone of the amp. To mock my JCM800 I fatten it nearly all the way.








4. Next mod is a 500k pot to control the voltage to the phase inverter.
(This will cost you less than $10)
*I will be posting pics of this once I get this mod complete.


5. The last Mod is to replace R84 (470 ohm resistor) with a choke.
You can get one from Mercury Magnetics for about $50 (MAR100-C 3 Henry ) or from classictone for half that. (http://www.classictone.net/40-18058.html)
*I am still waiting for parts on this one and will upload pics once the mod is done.




MORE TO COME SOON!

 

[JCMDOUG]

I use these clips when I attach a resistor in parallel with an attached resistor mounted on board.

 

[JCMDOUG]

So I can hear the change in sound when I do the phase inverter mod.

 

[RickyLee]

I use these clips when I attach a resistor in parallel with an attached resistor mounted on board.View attachment 61589

Are those smaller/have a smaller footprint on the leads than the standard small Radio Shack style test leads?

 

[RickyLee]

@Brad O'Dell

Would you mind posting a close up pic of the transformers with the label? I am wondering if they are similar to what is in my Vox AC 30CH.

 

[Filipe Soares]

I can't wait to see with all mods! when it's done post some sound samples please.

 

[Denis Belacik]

Hi Brad,

have you had a chance to finalize all your mods yet? Recently I bought very cheap MA50H just as a backup for my JVM205H and as it sounds really horrible, I would like to make as many mods as possible on it
I have installed already the choke (no change to the sound at all in my opinion) and now going to replace R86 with 500k pot. Anyway, which pot would you suggest? Linear or logarithmic one?

Thanks,
Denis

 

[nicktgr15]

Any updates on this? I also have an ma50h waiting Has anyone tried any of the above and was any of those a useful additional control to have?

 

[Matthews Guitars]

I had an MA50 for a little while, and I reviewed the schematics for it, and I came to the conclusion that the best way to make it a really good sounding amp is to start by removing and throwing away the circuit board and hand wiring a new turret board into it.

It's sad when a Lead 12 totally kicks a 50 watt tube Marshall head's ass.

 

[Ericur]

I recently modded a ma100h, and now it sounds amazing! The mods I did were cheap and simple, pm me if you whant a schematic with the mods, I can post a soundclip if there is any interest.

 

[FacundoFmg]

I recently modded a ma100h, and now it sounds amazing! The mods I did were cheap and simple, pm me if you what a schematic with the mods, I can post a soundclip if there is any interest.

Hi, Ericur. I'd like to hear clips and to know what mods you did.

 

[Crunchifyable]

Hi, Ericur. I'd like to hear clips and to know what mods you did.

This would be interesting. I could never get mine to sound good. Very mushy, fizzy.

 

[Gunner64]

I have a MA50h with Fresh tad tubes. Clean boost. Hot channel, boost engaged, gain down. Volume up. Un fizzy. Acceptable tone on the cheap imo. Paid $150 for it with the fresh tubes..tubes are almost worth that. Eventually I'm going to monkey with the negative feedback, and the p.i. signal, as I feel this amp doesnt need a shitload of work to sound pretty good.

 

[Crunchifyable]

I have a MA50h with Fresh tad tubes. Clean boost. Hot channel, boost engaged, gain down. Volume up. Un fizzy. Acceptable tone on the cheap imo. Paid $150 for it with the fresh tubes..tubes are almost worth that. Eventually I'm going to monkey with the negative feedback, and the p.i. signal, as I feel this amp doesnt need a shitload of work to sound pretty good.

I could not get a good sound out of the boosted engaged channel but maybe I had a bad preamp tube. I did some tube rolling and found that it's very sensitive to V2 changes.

For the gain very low like AC/DC levels it's not bad. I just didn't like the texture for instance I could get iron maiden tones out of my class 5 with a boost but there was just something wrong with the gain on the MA.

I abandoned my Marshall MA when I moved, left it with family. Sometimes I think about it late at night, what could have been.

 

[shawn w]

Hi,
I just bought a MA50 and I'm interested in the mods. Is there any updates to these posts? Any new ones?
Thanks
Shawn

 

[Jon Snell]

I have just received an MA100H for refurbishing as it failed a few years ago and has been sat under a bench since.
The customer wants it returned to stock and any modifications removed as he wants the original sound.
I have a quad of matched Sovtek Winged C output valves and this is the job for them.
I haven't removed the dust yet but I expect it to be as new, electrically when it has passed through my workshop and waved my magic wand over it.

 

[Ericur]

Ok, sorry for the absence guys.

SO,,, here are my mods, and why to do them;

First the simple stuff;
There is a (i think) purple wire in the amp running across/along other wires in the amp, its a AC wire and it introduces a TON! of noise, isolatet it, and zip tie it away from all other wires.
I have read posts that the MA is just a noisy amp, but that is just not the case, and please use a wooden chopstick or equivalent to prod around in the amp, not your fingers when isolating the humming cable.

Clean channel;

Remove c5; this is a permanent bright cap, this will make your guitar sound clean and sparkly, but the amp wont take pedals at all, remove it and it will be a nice pedal amp.
Remove C117, this is just there to compensate for the bright cap, it bleeds of a lot of high end from the signal.
Change c1 from 100nf to 22nf, this is to big and just allows to much bass into the clean channel.
Optional; change r6 to 100k, this will voice it closer to a fender, and fenders do cleans very well and take pedals very well.

Distortion channel;

Change c3,c10,c12,c13 to 22nf, this will beef up your tone, and make it more Marshally.
Remove c9, this bleeds of a lot of high end, and is necessary in these types if preamp circuits to tame the harsh fizzy tones, and the mods we are doing here are negating the need for it.
Change r164 from 150k to 470k, this will balance the volume between the channels, as the clean channel is WAY louder than the distortion channel.
Optional; if you want a little less gain, change r23 to 2.2k, 820ohm is a little aggressive bias for a 12ax7, and 2.2 will cool it down a bunch and remove background hiss.
Optional; add a 10pf 50v cap between pin 2-3 on v2a, this is a known Mark Cameron mods, and will sweeten a high gain distortion.

And thats it! and everything is reversible if you dont like it.

My motto when modding amps is not to make a whole new amp, but to bring out potential, and functionality.
If you bring me a Fender twin and you want me to make it sound like a Marshall, ill tell you to buy a Marshall.
But if you bring me a Marshall MA100/50 and ask me to make it more usefull, ill tell you; give me an hour!

 

[shawn w]

Ok, sorry for the absence guys.

SO,,, here are my mods, and why to do them;

First the simple stuff;
There is a (i think) purple wire in the amp running across/along other wires in the amp, its a AC wire and it introduces a TON! of noise, isolatet it, and zip tie it away from all other wires.
I have read posts that the MA is just a noisy amp, but that is just not the case, and please use a wooden chopstick or equivalent to prod around in the amp, not your fingers when isolating the humming cable.

Clean channel;

Remove c5; this is a permanent bright cap, this will make your guitar sound clean and sparkly, but the amp wont take pedals at all, remove it and it will be a nice pedal amp.
Remove C117, this is just there to compensate for the bright cap, it bleeds of a lot of high end from the signal.
Change c1 from 100nf to 22nf, this is to big and just allows to much bass into the clean channel.
Optional; change r6 to 100k, this will voice it closer to a fender, and fenders do cleans very well and take pedals very well.

Distortion channel;

Change c3,c10,c12,c13 to 22nf, this will beef up your tone, and make it more Marshally.
Remove c9, this bleeds of a lot of high end, and is necessary in these types if preamp circuits to tame the harsh fizzy tones, and the mods we are doing here are negating the need for it.
Change r164 from 150k to 470k, this will balance the volume between the channels, as the clean channel is WAY louder than the distortion channel.
Optional; if you want a little less gain, change r23 to 2.2k, 820ohm is a little aggressive bias for a 12ax7, and 2.2 will cool it down a bunch and remove background hiss.
Optional; add a 10pf 50v cap between pin 2-3 on v2a, this is a known Mark Cameron mods, and will sweeten a high gain distortion.

And thats it! and everything is reversible if you dont like it.

My motto when modding amps is not to make a whole new amp, but to bring out potential, and functionality.
If you bring me a Fender twin and you want me to make it sound like a Marshall, ill tell you to buy a Marshall.
But if you bring me a Marshall MA100/50 and ask me to make it more usefull, ill tell you; give me an hour!

Awesome! Thanks for your response. Do you know if the MA50 has the same topology? Any sound bites?
Thanks again,
Shawn

 

[Ericur]

I believe the schematic says MA100/50 MA50, so they should have the same preamp layout, and sorry no sound bites, it was a customer job, and he was very happy BTW.
Double check with your amp and the schematic for the MA100, so the values match.

and if you want it even closer to a JCM800, let me know, good luck.

https://bespin.federalproductions.com/mycloud/Schematics/Marshall/MA100%20%20%20MA50%20Circuit%20Diagram%20VERSION%201.pdf

 

[shawn w]

I believe the schematic says MA100/50 MA50, so they should have the same preamp layout, and sorry no sound bites, it was a customer job, and he was very happy BTW.
Double check with your amp and the schematic for the MA100, so the values match.

and if you want it even closer to a JCM800, let me know, good luck.

https://bespin.federalproductions.com/mycloud/Schematics/Marshall/MA100%20%20%20MA50%20Circuit%20Diagram%20VERSION%201.pdf

Awesome! I'll try the changes you've listed so far and see how it goes. I'm getting the amp tomorrow, so hopefully before too long I'll have a chance to dig into it.
What would you do differently for more of a JCM800 sound?
Thanks
Shawn