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SM-900 Vol Flux

Discussion in 'Amps and Cabs [BG]' started by swrbass5, Jul 7, 2013.

  1. Jul 7, 2013 #1
    swrbass5

    swrbass5 Gold Supporting Member

    Jul 18, 2002
    Coastal NC / SC
    Played 6 shows this week, 3 of which, I ended up using the rig for sound (instead of just an onstage monitor). -- [SM 900 / Goliath Sr ]. I own several pre Fender SWR's, but this hasn't come up before- after about 10 mins of play (with the gain and master both around 3/4) the volume just drops out to about half power. A few mins later, the full power comes back (normally after I've ran the master up ) and then blasts my poor drummer! I'm assuming this is the tube needing replacing, but seems like in my 400s and Redhead, the tube made a crackling sound before I replaced those. Wanted to get some opinions before relying on a new tube, cause I won't have a chance to test it out at that vol before we play next weekend.
    Not using any effects. Stingray 5 / L2000 - Both with new batts.
     
  2. Jul 7, 2013 #2
    T-Bird

    T-Bird

    Apr 29, 2007
    Finland (Northern Europe)
    Hi.

    Dirty FX loop switch?

    Run a patch cord there.

    Regards
    Sam
     
  3. Jul 7, 2013 #3
    Sav'nBass

    Sav'nBass

    Jan 18, 2009
    Virginia Beach
    Agedhorse should be along soon to give you a hand with this. Sounds like a tube issue to me although I am no expert on this by a long shot.
     
  4. Jul 7, 2013 #4
    chadds

    chadds

    Mar 18, 2000
    Could be this. Tube is a cheap fix then you have a backup if that's not it.
     
  5. Jul 7, 2013 #5
    georgestrings

    georgestrings Inactive

    Nov 5, 2005
    I don't think the OP is referring to a Genz-Benz product, although it wouldn't be the 1st time Andy has helped out with amps that aren't made by his company...


    - georgestrings
     
  6. Jul 7, 2013 #6
    swrbass5

    swrbass5 Gold Supporting Member

    Jul 18, 2002
    Coastal NC / SC
    I've actually never used the effects loop on this amp. Could it still be something there even though it's not being used?
     
  7. Jul 7, 2013 #7
    georgestrings

    georgestrings Inactive

    Nov 5, 2005
    Sure can - if anything, the fx loop is most likely to cause trouble when rarely being used... It's not uncommon for any amp to have this happen - just patch a cable between in and out, and that will tell you if that's what's happening...


    - georgestrings
     
  8. Jul 7, 2013 #8
    B-string

    B-string

    Nov 21, 2008
    Lake Havasu City, Az USA
    Not using the effects loop is how the problem develops. Other than that a bad master volume control or overheat issue would be my best guesses. 3/4 on the gain and master is running it pretty hard.
     
  9. Jul 7, 2013 #9
    swrbass5

    swrbass5 Gold Supporting Member

    Jul 18, 2002
    Coastal NC / SC
    Thanks ya'll! I'll probly replace the tube and use the patch cable. I had no idea that the loop could cause an issue like that.
     
  10. Jul 7, 2013 #10
    B-string

    B-string

    Nov 21, 2008
    Lake Havasu City, Az USA
    The effects return (most amps use a serial loop) jack has internal contacts. Those internal contacts connect the preamp to the power amp. :)
     
  11. Jul 7, 2013 #11
    JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    If the patch cord in the loop works, then clean out the effects loop jacks with Deoxit because they got corroded inside. Common problem in a lot of amps with folks who never use the loop. And yeah, it could be the tube. They don't always crackle when they go bad. They do a lot of things...work intermittently, add reverb or feedback when they're microphonic, add distortion at lower volumes than normal...
     
  12. Jul 7, 2013 #12
    MuthaFunk

    MuthaFunk

    May 8, 2007
    GTA Ontario Canada
    In addition to the the other suggestions offered here it could be a bad speaker cable, or lose wiring in the cab. If you can rule them out by trying different cabs or cables it would help make sure the problem is in the amp.
     
  13. Jul 7, 2013 #13
    Sav'nBass

    Sav'nBass

    Jan 18, 2009
    Virginia Beach
    My bad.. I saw 900 and thought he was talking about an SL .. not an SM
     
  14. Jul 7, 2013 #14
    swrbass5

    swrbass5 Gold Supporting Member

    Jul 18, 2002
    Coastal NC / SC
    That makes sense JimmyM- most of our gigs are in the Myrtle Beach / Murrell's Inlet (sc) and Wilmington / Carolina Beach / Wrightsville Beach (NC) areas. Probly 80 % of our play time is within 200 yds of the ocean! My drummer has a really nice DW kit and a Sonor kit, but he takes an older kit with him most of the time, for that very reason. I shoulda known better. Lol
     
  15. Jul 7, 2013 #15
    agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Yes, I understand it's an SWR SM-900 but...

    My first guess is that the effects return jack's internal routing switch is intermittent. cycle it a few times with a 1/4" plug and see if that resolves it.