HELP! Bad distortion!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SlyStrat, Jun 9, 2006.


  1. SlyStrat

    SlyStrat Supporting Member

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    I have some nasty distortion that "rides" on top of the notes. Sounds like something is ripping. If I pick the string harder, the distortion sound is more pronounced. It dies out before the note is gone. Seems louder with bass notes. Makes high notes sound shrill.
    Its happening with all my heads and cabs.
    But its more noticeable with my '74 and Heritage H30 speakers. The high end of the H30s brings it out more than the Greenbacks or V30s.
    Could this be an electricity problem? Its the only factor I can think of. Our house was built in the 40s. The wiring sucks. It seems like there isn't enough juice to run everything. My comp screen will flicker when the washing machine runs. Or if my wife vacuums. The lights dimmed a little when the central air came on.
     
  2. bobgoblin

    bobgoblin Supporting Member

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    the house i live in was also built in the 40's, & my ampeg sometimes responds very strangely when the a/c is on, or when the fridge cycles. you may be on to something...of course, my amp doesn't sound like something's ripping, have you checked the speakers? voice-coil rub or small tear? i hope you get it resolved, it sucks to have these types of problems 'cause sometimes it keeps you from playing...best of luck.
     
  3. Blue Strat

    Blue Strat Member

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    What kind of amp?
     
  4. SlyStrat

    SlyStrat Supporting Member

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    Sorry, Marshall heads. '74 and '79 NMV.
     
  5. John Phillips

    John Phillips Member

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    I could be wrong, but that doesn't sound like a supply problem - that normally causes either squashy-sounding distortion (if the voltage is too low or drops under load) or simply noise on the signal.

    Distortion that seems to hover over the top of the note then disappears as the note fades back to clean sounds like an amp problem, even though both amps are doing it. I could be wrong here too, but to be honest what you're describing sounds like how I hear normal non-MV Marshall distortion. I'm reasonably sure it's a characteristic of the way the negative feedback circuit works - try changing the presence setting (in particular, try turning it right off) and see if it changes the harsh overtone.
     
  6. songsmith1950

    songsmith1950 Member

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    So this is on both amps or is it just one amp?

    Songsmith
     
  7. BJF

    BJF Member

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    Hi,

    Though I might be wrong I also think it sounds like an amp problem.
    You might want to clean the speaker outputs, but really this could also indicate an oscillation in the poweramp.
    Many of the NMV Marshalls balance on the fine line of oscillation and it can if balanced rightly give the amp higher gain at the expense of bandwidth.
    If balance is off, oscillation can sometimes be triggered by input signal and die away in the decay. This can be particurlarly noticable if instability is with in the poweramp section. Check outputimpedance selector aswell, as imbalance here can affect poweramp stability.
    Try also tapping each tube indivudually and listen for a ringing sound. Loose elements inside tubes can cause this type of odd distortion too.

    Short input and listen to the noise of the amplifier as controls are dialed an maxed out. If you hear a sudden drop in noise or high whistling sound stabilitymargin is too low.
    There are a number of ways to correct this but if amp was fine before and just gradually arrived at this condition try also a new set of tubes known to be good to be able rule out tubes.

    If you have a scope you might monitor the output, no signal, input shorted condition and then if you hook up a guitar with volume on but strings dampened and listen to the background noise you might find that certain settings make odd changes in the noise- usually combinations of increased presence and treble or volume.

    The poweramp in this kind of amplifier runs a feedbackloop and the presence control defeats this feedback for high frequencies. With volume on max and treble on max there's a highway through amp for high frequencies. This can depending on the phaseangle trip the amp into oscillation.

    Some would put a small cap from input to ground to lower the impedance and increase margin. Another way is to increase value of the cap that goes between the anodes of the phasesplitter. Though under all conditions it is advisable to keep just one dominant roll off or sound can deteriorate.
    The anode to anode cap is inside the feedback loop and decreases gain under openloop conditions such as what happens when presencecontrol is advanced.This rollof then adds on to the rollof of the outputtransformer to form a lowpass filter with resonance due to the fact that the OT is coilbased.
    This resonance needs to be controlled or rather within tolerated limits. all parts inside the feedback loop may affect stability.
    In essence when you investigate a fault in an amp that previously was just fine you'd look at what might have drifted or changed. In this case it might be a good idea to start with the tubes and then backwards.

    If you are not trained in amplifier service you might want to take your amp to a service depau that can place a diagnoze on it

    Just a few thoughts
    Regards
    BJ

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    www.bjfelectronics.com
     

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