warwick tubepath 5.1 bass amp

Discussion in 'General Electronics Chat' started by Roque, Feb 16, 2015.

  1. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    Hi guys i got a warwick bass amp with distorted output ,the distortion is not due to overdiving the amp but more like the amp not gettting its max power to throw out any help would be greatly appreciated cheers ...also i have noticed that +96v discharges to +0v while -96v discharges to-36v??? why not -0v
     
    Last edited: Feb 16, 2015
  2. planeguy67

    Member

    Jan 29, 2015
    59
    16
    I'm not familiar with that amp. Is it solid-state or valve (tube) driven?

    The most common way of getting distortion in guitar circuits is to clip the waveform. One way of doing this is overdriving, like you mentioned. You simply crank up the gain until the amp hits its rails (voltage extremes). The other way is "artificially" clip, as in using diodes to chop the peaks off the waveform while it's still very low power (prior to power amplification). Both ways distort the signal by creating higher order harmonics.

    Not sure about the discharging. Are you talking electrolytic capacitors?
     
  3. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    hi mate...the amp is a hybrid amp..preamp is valve and poweramp solidstate...the didtortion is more like a torn speaker type ..the pre amp is ok ...the power amp feels like its not getting enough current so the output is terrible , as if the amp is struggling to drive itself...thought the filter capacitors (electrolytic) 6800uf 100v could be leaky and causing the voltage not to discharge ..swapped them around but the -ve voltage still stays at -36v for days ..even when amp turned off.:-(
     
  4. planeguy67

    Member

    Jan 29, 2015
    59
    16
    Ah, I misunderstood. I thought you were dissatisfied with the quality of distortion you were getting. Ok, so you have unwanted distortion and you're trying to get a cleaner signal, as most basses should sound, IMHO. :)

    Well, if it sounds like a torn speaker, often the surround, and you've verified it's ok, I would suspect the tube, probably a 12ax7 (or Ecc83 for our British friends). However, you said the preamp is ok. Did you verify that through a headphone output or some other manner?

    If the problem is definitely between the tube preamp and the speaker, then the solid-state power section is suspect. It could be a heat problem or downright failed transistor or diode (I imagine we're talking an AB class amp in push-pull configuration). If this particular amp uses an output transformer (yes for all tube amps, maybe for solid-states), then that could be bad as well. Is it not a rare day that one of those fail.

    Lastly, IRT the capacitors, electrolytics can leak in two different ways, physically and electrically. Physically, the electrolyte (usually a waxy substance) literally leaks out like an old battery. Rare with new capacitors, but more common as they reach several decades of age. When this happens, their capacitance decreases significnatly and they lose their ability to filter. Lots of hum and buzz in your output.

    Electrically, caps leak internally, as in there is a minute amount of current flow between the plates, equalizing the charge. Electrolytics are among the leakiest in this regards, hence their use in DC filtering and not precision frequency applications.

    In either case, a very leaky capacitor will not hold a charge very well. I would have to see the circuit and where you're testing to see why you're retaining this negative voltage for so long.
     
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  5. Roque

    Thread Starter Member

    Sep 16, 2014
    68
    2
    hi planeguy ..yeah you right..checked the base of the output trsnsistors and no base voltage present ...then checked the mje340 transistor coz that supplies the .5v base v to the power output transistors in the +ve rail ...found the base emitter open ...very strange as i always thought transistors go short through collector emitter ...but this time base emitter open..replaced with a new one and it WORKS!!!!!.... another one under my belt...thanks a lot for your help appreciate it heaps...cheers
     
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  6. planeguy67

    Member

    Jan 29, 2015
    59
    16
    Excellent troubleshooting and fix. Congrats!
     
    Roque likes this.
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