Mods for MA50H?

Discussion in 'The Workbench' started by Matthews Guitars, Nov 14, 2019.

  1. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I picked up a cheap but mint condition Marshall MA50H head. I've read up on them and understand that it's probably biased cold from the factory and rebiasing it can warm it up, in at least two ways.

    Since I'm averse to doing any heavy mods on either of my classic JMPs, I'm thinking that modding this entry level amp (Entry level for a "real tube Marshall", anyway...) is something I'd consider doing.

    The basic goal: A browner sound. It has plenty of gain already. Making it warmer and less fizzy at lower volumes would at least be very much appreciated.

    Is there much of a library of mods for this model? Or have people decided it's not worth doing many mods for?
     
  2. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Clearly this isn't exactly a hot topic of great interest. But I've done some research and the MA50 is interesting in that it does exactly what I'd wondered about.

    I was wondering what would happen if you were to take the classic JMP Marshall preamp circuit and reconfigure V2, the cathode follower, as two gain stages and feed that to V3. Sacrifice the cathode follower for a gain stage.

    Well, the MA50 does exactly that. And its sound is FAR from that of a classic Marshall. It's a different thing, and honestly it doesn't sound very good, at least not in the overdrive channel.

    I'm going to do some experiments on it. Change a stage to include a split load resistor, play around with cathode resistor and bypass cap values, and see if I can make it sound better without fundamentally changing the circuit topology.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The trouble with mods is the increased noise. (or the factory noise), microphonic ringing, hiss, hum, etc...
    But I'm working on some lower noise parts, a new master volume design, and a new inter-stage coupling method which greatly increase sustain and smooth out muddy distortion.
    When I have those parts lined up, I will have some pointers for new mods which I can share w/ the forum.
    Parts sources are the major hurtle right now...

    Yeah, I would say the amp has potential for some cool mods. Any tube amp that is cheap has potential probably (like a PV Windsor, etc).
     
  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    When I did my research on this amp I was amused to find out about how it sacrificed the cathode follower for another gain stage.

    I'd wondered what would happen if you did that, since with only three preamp tubes available your options are limited.

    Well, now I know. The character of the overdrive sound is very different indeed. Tone stack values are classic Marshall, most other values are in the ballpark, only the changed V2 really stands out as a game changer.

    I'm now convinced beyond a reasonable doubt that the cathode follower after the clipping stage is of major, even indispensible, importance to the classic Marshall sound.

    The MA50 schematic shows that this amp does things I'd considered doing on my first homebrewed 2 channel amp that I'm still thinking about making.
    Including two tone stacks that are switched in and out depending on gain mode.

    It's an educational circuit, and not too complex for a relative noob. I have lots of electronics experience, but relatively little in tube amps as far as mods and designing are concerned.
     
  5. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    It still has a CF if relay RL1C is toggled
     
  6. Gunner64

    Gunner64 Well-Known Member Premium Member

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    I picked up an ma50h a while back for $150 and it is pretty much mint as well, and it had new tad tubes in it throughout. I found the best "mod" is to turn the gain down, give it some volume and clean boost it with a tube screamer..drive down, level up in the traditional manner.. it turned mine into a Marshall..your results may vary..but give it a try..
     
  7. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I tried that, and yes, it really does help improve its tone. But I found that this particular one has to be cranked up even more to "clear its throat", so to speak, to develop its best tones, than my 1977 2203 does. For it to sound good it's got to be pretty loud. That's fine when I'm home alone, but at other times, not so much.

    What that's telling me is that I'm probably not going to find any quick and simple mod for it that's going to make it sound good at practicing volume.

    So, my best course of action is probably to just sell it. It's not going to fit into my scenario.
     

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