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Mosfet tubelike behavior and m-pulse preamp

Discussion in 'Amps and Cabs [BG]' started by Scutterflux, Jan 28, 2004.


  1. Part 1:
    I seen a post from I can't remember who, but they stated that a mosfet power section (solid state) will behave like tubes.

    Does anyone know what he was talking about?

    Part 2:
    I found that the m-pulse sounds really wicked if you just crank the first stage of eq and then bring the gain up to before it distorts, or so. You can get a real nice clean tubey bottom, and when you start to climb the neck a smooth distortion starts to blend in.

    Is the first stage of eq on the m-pulse a tube eq or a solid state eq, that is does the eq run before or after the preamp tube? The mesa website says the m-pulse has a 4-stage vacume tube preamp, maybe this helps!

    Thanks to anyone who knows the answers to these trivial questions. :bassist: :confused: :D
     
  2. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    The M-Pulse, like my Walkabout, uses two 12AX7 (or similar sized) tubes. One drives the preamp section, and the other loads the MOSFETs in the power section. The EQ section is solid state, and sits between these two tubes. It is the preamp tube that offers the overdrive when you crank the Gain knob. I agree that you can get some absolutely killer sounds that way. In fact, I am considering using my Walkabout with my rack rig or iAMP 800 purely as an overdrive unit. Or possibly to power a "dirty" cab, while I use the other for a clean cab or two.

    Hopefully, this helps.

    Tom.
     
  3. So is it possible that I'm clipping the eq? Can you do this? or am I driving the tube before the power section?

    I think it sounds good, I just want to know what I'm doing, and if it's dangerous. Clipping solid state makes bad square waves and driving a tube makes fat waves, right?
     
  4. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    If you are hearing a tube overdrive kind of sound (which it sounds like you are), this is from the preamp tube, and it is an intentional option. Is the Gain knob cranked, or nearly all the way up? If so, then that's it for sure. I cannot imaging clipping the EQ section on that unit.

    Personally, I really like the tube overdrive that you can get out of those amps.

    Tom.
     
  5. The gain is fairly low, say 10 o'clock. I imagine that some of the signal is being rerouted back to the tube after the first stage eq (I'm an idiot and sort of just figuring that this is what's going on even though I have no engineering training). It's a pretty smooth distortion up high.
     
  6. As for your part 1: MOSFETs are transistors, and to my knowledge they don't sound like tubes at all. To my relief.

    Any transistor circuit can have tube-like behaviour, but mosfets are no different than "normal", bipolar transistors.

    MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
     
  7. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    Wow, you've got me there. On my Walkabout, I don't get any overdrive until the Gain knob is nearly all the way wide open. Are all of your EQ knobs cranked? What are the rest of your knob settings? How about the Solo "boost", is it on (you have an M-Pulse, right?)?

    Tom.
     
  8. lneal

    lneal

    Apr 12, 2002
    Lee County, Alabama
     
  9. Phat Ham

    Phat Ham

    Feb 13, 2000
    DC
    MOSFETs have similarities to pentode (power) tubes in their load line characteristics. When FETs were first developed, some people thought they would be able to replace power tubes because of the similarities in load line characteristics, but that never happened.