Amp sudden microphonics/nasty feedback problem...please help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Young Angus, Sep 21, 2006.

  1. Young Angus

    Young Angus Member

    Messages:
    1,054
    Joined:
    Jul 13, 2004
    Location:
    Down Under
    I rehearsed last week and all was well...come this week after not even having touched the settings on my amp because I didn't play it since, I turn it on and straight away it starts howling at the moon (if it could see it anyway). There was just uncontrollable feedback everywhere, even when on the clean channel with a light gain from a pedal. I also swapped guitars to make sure it wasn't them and nope, same thing...all with humbuckers mind you (one JB and one BBpro). I couldn't even silence it by playing or muting the strings, it just went bananas and I didn't even have the gain cranked as I never do anymore.

    This was on my ENGL Savage 120 by the way. What could cause this? My guess is a tube that's gone bad/microphonic...the question is though would it be a preamp or poweramp tube, and can I pick it just by looking closely at them? I didn't notice any strong glowing or anything.
     
  2. B Vance

    B Vance Member

    Messages:
    1,978
    Joined:
    Dec 20, 2004
    Location:
    OHIO
    It could be a preamp tube. Some things to look for. Are you using a guitar cable as opposed to a speaker cable (I've done that before and the feedback is out of control). Try changing your cord first. Is it happening in both channels? If so it would most likely be a shared tube like your phase inverter, rectifier tube or possibly your power tubes. Also, try playing with the tube caps off. Sometime the caps can come loose and rattle against the tubes causing unwanted noise that can come through.
     
  3. Trout

    Trout Member

    Messages:
    7,575
    Joined:
    Jun 23, 2006
    Location:
    Illinois/Far West Burbia
    Sounds like it might be a microphonic tube( Pre-amp). I just had a similar issue with some sovtek sprial filly 12AX7's.

    At the closest possible volume to feeding back, Take a #2 pencil and lightly tap each preamp tube,or ping it with a fingernail, If you hear it at the speaker, Its likely you found the source "IF" thats the problem. I recently had a silver mica cap go south that caused some freaky stuff like that also.

    Generally speaking of course, I go around all my preamp tubes in a new build and ping them with a fingernail and listen. Its amazing how bad some can be and still work, Yet others will not work at all. Especially old long plates.
    Trout
     
  4. onemind

    onemind Member

    Messages:
    3,588
    Joined:
    Oct 28, 2004
    Location:
    Oceanside, NY
    An easy quick check for preamp tubes would be to swap positions, V1 is usually the main culprit, swap it to another spot and see if the problem abates or is lessened
     
  5. brad347

    brad347 Member

    Messages:
    4,817
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    How about just pulling tubes out one by one, starting from the first preamp tube.

    An overly microphonic tube will usually seem to howl on both channels,
    because the volume control is usually before the tube, so you can turn the volume all the way down and it will still howl away.

    Also, I hate the "ping" test. It causes people to throw away perfectly good tubes. If a tube is microphonic, it is NOT bad necessarily. Only if it's "overly microphonic." I prefer the sound of somewhat microphonic tubes over totally 'dead' tubes. I think most guitarists would agree in a side-by-side test. If a tube does not "howl," then it's fine. If it "howls" then obviously it has to go. I love a slightly microphonic old long-plate.

    Also, no harm intended, there is a virtually identical thread on here about every week or two. Maybe we should make a sticky about it, saying "before you post, swap all yr tubes"
     
  6. Chris Scott

    Chris Scott Silver Supporting Member

    Messages:
    7,768
    Joined:
    Sep 1, 2006
    Location:
    Playa Jaco, Costa Rica
    +1 on microphonic tubes-- just had a PR come thru with all tubes replaced by a well-meaning tech. Client complained that it wouldn't feedback on certain magic notes anymore! --swapped out V1-3 for originals (thank god the tech didn't toss 'em) and voila- magic feedback re-appears!. My bassman is the same-my favorite 12AY7 howls at me and I won't swap it until it dies a proper death...
     
  7. Young Angus

    Young Angus Member

    Messages:
    1,054
    Joined:
    Jul 13, 2004
    Location:
    Down Under
    Okay well I pulled all the preamp tubes one by one replacing them in turn with a good tube and still no answer. One thing I did notice though was that when I had the amp on and at feedback volume, i stepped on my cord and it fell out...and no matter how much I touched the tip of the cord it would not feed back at all even though it made some horrible noises...but that probably doesn't mean anything!
     
  8. Young Angus

    Young Angus Member

    Messages:
    1,054
    Joined:
    Jul 13, 2004
    Location:
    Down Under
    Also I tried the tap test on the tubes and the only one that I can hear at the speaker is V1...but I can hear that tap from V1 no matter what tube I put into the amp so I'm guessing V1 is just a sensitive spot in the amp where you will hear a tap anyway and it doesn't mean something is wrong. Is that correct?
     
  9. Blue Strat

    Blue Strat Member

    Messages:
    30,049
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Agreed, but if the amp is howling, it's a valid test to help you zero in on what might be the problem tube.
     
  10. brad347

    brad347 Member

    Messages:
    4,817
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
  11. JJman

    JJman Member

    Messages:
    992
    Joined:
    Mar 12, 2006
    Location:
    New Jersey
    Do the testing with nothing plugged in. Or will it feedback only with a cord plugged in? Could be a bad cord.
     
  12. Young Angus

    Young Angus Member

    Messages:
    1,054
    Joined:
    Jul 13, 2004
    Location:
    Down Under
    It will feedback only with a "guitar" plugged in...but it does it with every guitar...???
     
  13. Young Angus

    Young Angus Member

    Messages:
    1,054
    Joined:
    Jul 13, 2004
    Location:
    Down Under
    No more suggestions...my amp guy is really far away :(
     
  14. brad347

    brad347 Member

    Messages:
    4,817
    Joined:
    Feb 8, 2006
    Location:
    Brooklyn, NY
    so tubes didn't fix it?

    maybe all yr guitars have un-potted pickups.
     

Share This Page