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help me diagnose problem please

Discussion in 'Amps and Cabs [BG]' started by Bayou_Brawler, Apr 26, 2009.

  1. Bayou_Brawler

    Bayou_Brawler The most hurtful thing ever realized

    Oct 23, 2003
    Ann Arbor, MI
    ok i'm sure thic could be 100 different things but....

    over the last 6 months i have been noticing extreme output variation in my rig.....one day it's a lot louder on the same volume than the other (same room). also, a few times at shows (like last night) the out put will drop significantly for 10 mins or so but then come back....

    ampeg svpcl
    qsc power amp
    custom 4x10
    fender jazz

    any ideas guys???????????
  2. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    You just have to check everything in the signal chain first. See if the same problem occurs with
    1) a different cable (easiest fix) try diferent guitar AND speaker cables as well as the cable going from the pre to the power amp.
    2) a different bass
    3) DI into the amp (take the preamp out of the chain)
    4) a different power amp.

    I guess there is a situation where the cab could be causing the problem, but IMO that's much less likely. Try that stuff and at least you will know which part of your rig is causing the problem.
  3. Rick Auricchio

    Rick Auricchio Registered Bass Offender

    The power amp may be overheating and going into a protect mode.

    That "custom 4x10" may have poor efficiency, making you push the power amp much harder than with a better cab. Result? Heat.
  4. Jeff Scott

    Jeff Scott Rickenbacker guru.......... Supporting Member

    Apr 11, 2006
    Your hearing is going!

    Can you hear me..............? :D
  5. MIJ-VI

    MIJ-VI Banned Supporting Member

    Jan 12, 2009
  6. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Bad preamp tube maybe?
  7. bass7858


    Apr 11, 2009
    central florida
    totally dig the band. LOVE train robbery:D
  8. Bayou_Brawler

    Bayou_Brawler The most hurtful thing ever realized

    Oct 23, 2003
    Ann Arbor, MI
  9. Bayou_Brawler

    Bayou_Brawler The most hurtful thing ever realized

    Oct 23, 2003
    Ann Arbor, MI
    this is what my gut is telling me but i have no idea.....never changed the tubes.....never looked at em....
  10. Bayou_Brawler

    Bayou_Brawler The most hurtful thing ever realized

    Oct 23, 2003
    Ann Arbor, MI
    great advice.....

    only problem is i have to wait for it to have the problem before i can look into it....which only happen every now and then,.....in front of a crowd on stage :-(
  11. Mo'Phat

    Mo'Phat Supporting Member

    Oct 1, 2003
    San Diego, CA, USA
    Most definitely sounds like a protect circuit kicking in due to overheating.

    They (power amps) will cut output power to let the circuits cool down. When the thermostat says it's okay, it'll come back on.

    You might have a very dusty fan on your QSC. Break out your screwdriver, open that puppy up and dust it out.

    You might also consider adding a clip-on fan to the back of your rig or adding a space either above or below the power amp for added ventilation.
  12. MIJ-VI

    MIJ-VI Banned Supporting Member

    Jan 12, 2009
    Hi Bayou_Brawler.

    You can't check out the items listed by two fingers offstage or during a sound check? Why not?

    Troubleshooting gear problems is a process of elimination. You don't have to wait until your amp problem reoccurs to verify that the various components of your signal path are functioning properly.


    EDIT: According to its downloadable manual (which includes a BLOCK diagram), the SVP-CL uses (2) 12AX7, (2) 12AU7 pre-amp tubes (~ $10 - $15 ea.) which should be changed by a qualified tech.

    One or more of these tubes may be faulty (as other posters have mentioned) and should be tested & possible replaced.
  13. rbonner


    Sep 25, 2008
    Gain is dropping somewhere. As mentioned it is in the signal chain. Usually if a guitar cord goes bad the output goes completely away.

    With a tube preamp, you could be losing high voltage inside, like it is dropping in half. Cold solder joint causing gain to drop until the connection reconnects. Might be a tube, easy fix.

    The amp would most likely drop offline if it was dropping out not cut power in half.

    Half the speakers could be disconnecting depending on the wiring inside the cab. That would also make things get weaker.


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