Amp Squealing. Why?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by willhutch, Jun 24, 2006.

  1. willhutch

    willhutch Member

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    My amp was acting up in a way I've never experienced. Maybe you guys can help diagnose it.

    After powering up at the gig last night, my amp (Savge Rohr 15) started making a high pitched whining/squealing sound. It sounded like high pitched feedback going through a tremolo set for low intensity and high rate. The sound was fairly quiet - you didn't notice when actually playing. The sound got louder/quieter as I adjusted the amp volume or the guitar volume. It went away when I unplugged the cable from the amp input. It got louder when I clicked on my OD pedal. The noise appeared regardless of which channel I was plugged into (this amp has 4 inputs).

    To isolate the problem, I bypassed all effects and plugged sraight in. I swapped cables to exclude that possibility. I bypassed my attenuator. I plugged into various of the 4 inputs into the amp. When a cable was plugged into the amp input, I could make the squeal happen by touching the other end of the cable. But when I wasn't touching it, the sound went away.

    The noise happens in response to input signal. But as I said, I could play over the noise. I just dealt with it and played the gig as usual.

    Tube issue? The amp was working fine at a gig the previous night.

    Anybody experience this before? Any ideas for isolating the problem? The amp is at the venue now and I plan to play again tonight. Maybe there is something I can check/fix before the gig tonight.
     
  2. Blue Strat

    Blue Strat Member

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    If the amp doesn't squeal with nothing plugged in or without the guitar end of the cable plugged in, you should be looking outside of the amp. Cable, guitar, etc.
     
  3. Zelmo

    Zelmo Member

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    Bad power tube(s).
     
  4. CAFeathers

    CAFeathers Member

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    Bad Tubes
     
  5. v-verb

    v-verb Member

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    agreed - check the preamp tubes especially
     
  6. Texasamp

    Texasamp Member

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    The reverb driver preamp tube can cause this. I always keep a good 12AT7 at gigs as well as a matched set of 6L6GC's. I have seen the reverb preamp tubes cause the amp to go from squealing to a loud whistling and is quite embarrassing when it happens on a live set.

    Regards,
    Darrell S.
    Texas Amplification
     
  7. Tallhorses

    Tallhorses Member

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    Without a doubt! Check the pre that drives the trem or reverb
     
  8. Matt H

    Matt H Guest

    with nothing plugged into the amp, the input is shorted to ground... my guess is that the first preamp tube is the culprit here...

    if anything else was randomly oscillating, cahnces are it'd squeal even without a cable plugged in the input.
     
  9. leofenderbender

    leofenderbender Supporting Member

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    Bad reverb/tremolo driver.
     
  10. willhutch

    willhutch Member

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    It appears not to be the tubes. I swapped 'em out one by one - preamp and power - to no avail.

    It makes the ringing sound with the reverb turned down or with the reverb tank disconnected. This amp has no tremolo.

    Blue Strat said to look outside of the amp. Don't think this is it as the problem exists after changing cables and guitars.

    I'll have the amp on my workbench tomorrow in search of the cause. If anybody has any further ideas to help diagnose the problem, please speak up.

    Hmmmmmmmm. Very puzzling.

    The good news is I live in the same town as the manufacturer. He also thought it was a tube problem. I may be bringing it in for a physical.

    Thanks for your suggestions, guys.
     
  11. willhutch

    willhutch Member

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    Guess what, guys!? My amp isn't ringing anymore. I didn't do anything other than bring the amp to my basement.

    Could the problem have been something to do with the power at the venue?? There were some lights on stage close to my amp. They were kind of like Xmas tree lights encased in flexible plastic tubing. Interference? It certainly didn't sound like the kind of buzz or hum you get from flourescent lights or neon signs. There was a power outlet close to my amp with various pices of gear plugged into it.

    Wierd...but good. My amp is OK.
     
  12. Blue Strat

    Blue Strat Member

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    LOL, no one listened to your symptoms. I told you it wasn't the amp ;-)
     
  13. willhutch

    willhutch Member

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    That's the last time I fail to heed advise from Blue Strat.

    Now I know to exclude the possiblity of somekind of interference next time I get a weird sound.

    But c'mon! A high pitched ring?? I never heard that kind of intereference before. Buzz, yeah; hum, sure; talk radio, maybe......but I've never heard that sound.

    Anyway. I'll consider myself wiser for the experience. Thanks all.
     
  14. Blue Strat

    Blue Strat Member

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    It'll be interesting to see what happens when you go back to that room.

    In the meanwhile, are you sure you're using the same cables, guitars, etc. for your test at home? Also, try jiggling the input jack on your amp to be sure it's making good contact and not intermittant.
     
  15. Fuchsaudio

    Fuchsaudio Member

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    Sometimes high gain combined with a cable too close to a speaker or something weird like that can set up a path towards oscillation. It's like RF (radio) problems: Sometimes you get them sometimes not. My old shop (in the house) was less than a mile from a 50-KW AM spanish station. It was so strong, we heard it on the phone before the dial tone started ! My amps never picked it up, yet I get the occassional customer with RF problems... some things you can't replicate.
     
  16. willhutch

    willhutch Member

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    Blue Strat: In answer to your question - yes. I tried to duplicate conditions to get the ringing sound. Same guitar, cable.

    Andy Fuchs: Nice to cross paths with you again.
     
  17. John Phillips

    John Phillips Member

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    I agree with Blue Strat. It sounds like it was nearby electric motor noise, or switch-mode power supply noise, to me - being picked up by the guitar.
     
  18. BJF

    BJF Member

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    Hi,

    How do you power your pedals?

    I'd second that it sounds like leakage from switchmode power supply.
    This can make very funny noises and it's cure is isolation.
    I have a customer that plays the electric violin connected to two preamps in series and Harmonizer, delay onto a Fender amp.
    This type of disturbing sound could be traced in his system to a switchmode powersupply feeding the preamps ( that if connected to guitar would make a bassy fuzz- not so with the violin, but it counteracts what the Fender amp does
    NO POP VIOLIN ALLOWED).

    Nah, I would not suspect the amp unless further investigated.

    Have fun
    BJ
     
  19. willhutch

    willhutch Member

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    I was using a simplified rig with just a single OD pedal. Power supplied by 9V battery.
     
  20. Timbre Wolf

    Timbre Wolf GoldMember Support Group Gold Supporting Member

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    I'm guessing that a dimmer switch was installed on some of that lighting, and it was hooked in to the same power circuit as your amp. I've experienced similar effects from plugging in to power where a cheap dimmer circuit was used on lights nearby. You never know what you're going to get, in some venues. But if something like that happens again, you can try going to another outlet thats on a different breaker, if available.

    - T
     

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