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Debugging a crackling amp

Discussion in 'Amps and Cabs [BG]' started by jmone, Nov 2, 2018.

  1. jmone


    Mar 1, 2010
    It's been happening for a couple days now and nothing has really changed in my setup so I'm wondering what steps I should take to debug the issue.

    It sounds like a needle on the end of a record.

    It's not consistent. Sort of happens and then disappears briefly but I would say it's crackling more than it's not.

    Don't know if it matters but it's a DB751 Head and a DB112 Cab.

    Things I tried:
    • Isolated it was the amp and not cables/bass/pedalboard. It makes the sound with nothing in the input.
    • Not interference from my phone (which normally can be heard through my studio monitors anyway).
    • Raising the master volume does not make it louder. And the sound is present with the master fully off.
    • Raising the gain does not make it louder.
    • Replacing preamp tubes doesn't do anything
    • Unaffected by the Effects loop. I tried bypassing it, putting it in series, in parallel, and raising/lowering the send/return volumes.

    I have another cabinet at my practice space I'm going to test tonight but short of that, anything else to try?
    Last edited: Nov 3, 2018
  2. primusfan1989


    Jan 17, 2005
    new jersey
    That head has 3 12AX7 tubes, id suspect one or several of them are failing and need to be replaced
    kobass and jmone like this.
  3. Wasnex


    Dec 25, 2011
    This sounds like a flaky tube. I don't have schematics for this amp, so I don't know where all the tubes fall in the circuit. To be honest, I suspect all three tubes are before the master volume, but part of V2 and/or V3 could possibly be after the master volume.

    I would swap V3 and V2 with V1 with the gain and master all the way down. If the crackling continues after swapping the tubes around, I would send the amp to a technician. If the problem follows the tube to another part of the amp, replace the bad tube. All three tubes should be 12AX7s.
    jmone likes this.
  4. BogeyBass


    Sep 14, 2010
    Could be tube/tubes

    And the good news is. If it is, the amp could wake up a little. And you will be happy not crackly
    jmone likes this.
  5. jmone


    Mar 1, 2010
    I'll open it up tonight and have a look! Thanks!

    My understanding is V1 is preamp, and one of V2 and V3 is for the Balanced Output...I can't remember the third. I actually have a few spare 12AX7s I can mess around with so I'll have a go.

    I hope it's not the V1 tube because that was the first thing I replaced when I got the amp and bought a Tung-Sol Gold which sounded incredible.

    Anyway it seems to be the consensus and I'd rather it be a faulty tube rather than anything else.

    Thanks to all 3 of you guys!
    Wasnex likes this.
  6. Effects Loop? If yes link Send and Return with a signal cable to exclude that possibility.
    Lobster11, Rockbobmel and jmone like this.
  7. jmone


    Mar 1, 2010
    Even if I don't have anything in the effects loop? Is it normal for that section to remain active with nothing plugged in? I would assume no in series mode but I can run the effects loop in parallel so I'll double check anyway.
  8. The loop jacks have normalising contacts that complete the circuit with nothing connected. These can get corroded causing all kinds of weird and wonderful happenings. The hard wire link bypasses the contacts.
  9. Wasnex


    Dec 25, 2011

    Additional info that may help with the trouble shooting: The effects loop on this amp can be configured as series or parallel, and the effects return level can be turned completely down.
  10. KS78


    Jan 19, 2018
    If you have active bass, check the battery. My TRBX starts crackling when it's going flat.
  11. Raf Seibert

    Raf Seibert

    Dec 16, 2013
    The thing about checking the effects loop is that it’s a quick, easy and cheap thing to check. Might not be it, but I’d check it first.
    Lobster11 and Old Garage-Bander like this.
  12. Thumpin6string

    Thumpin6string Supporting Member

    Apr 25, 2013
    Redding CA
    My first thought too. This should be done first before opening the amp up. The first thing I check for when there is no output, low output or crackling output. Very common.
  13. vvvmmm


    Dec 6, 2016
    Try rotating the knobs (pots) 20-30 times in case they are dirty.
  14. Markamb1


    Oct 24, 2018
    I’m really curious to see what this is. I have some crackling as well. Probably very different reason.
  15. Missing Link

    Missing Link Guest

    Aug 27, 2017
    Probably an output stage problem, BUT here are some things to try: UNPLUG IT FIRST!
    1. ALL jacks. Spray a little contact cleaner(safe for plastics kind) on a plug and insert it in and out a few times. Spin and move it a bit.
    2. Clean the pots with contact cleaner.
    3. Tubes are next. Make sure the amp is UNPLUGGED. Let the tubes cool. Replace and see.
    If you are not very familiar with electronics repair, stop there and take it to a qualified tech. Could be a dry solder joint, or a capacitor starting to fail, or something else. Good luck, and BE CAREFUL! ALWAYS double check that it is unplugged BEFORE you work on it. Even unplugged, the power supply caps can hold a charge and hurt or kill you.
    jmone likes this.
  16. Hand slap

    Hand slap

    Feb 14, 2016
    Service the tube sockets.
  17. Relsom

    Relsom Supporting Member

    Nov 23, 2013
    The Old Dominion
    Don't spray anything into the pots or jacks unless they absolutely need it. You can do more harm than good. With scratchy pots, I'd start by exercising them stop to stop several times as they tend to be self burnishing.
    David Jayne likes this.
  18. Signalsdrone


    Feb 6, 2017
    SW Ontario
    As someone who troubleshoots things for a living, start with the KISS method.

    Open the amp up and look for things that are loose, corroded, out of place or discoloured. Most of the time it is something very simple.
    Bill Whitehurst likes this.
  19. This ^ :thumbsup:
    Rule #1 of electronics trouble shooting.
    Do what’s cheapest (or free) and easiest first.
  20. Radio60


    Nov 11, 2017
    The advice you've been given so far is pretty good, so I'll just amplify (pun intended) some of what's been stated:

    Yes, because the volume/gain controls don’t affect the noise level, the problem is likely near (or in) the output stage. It could be a failing tube or transistor or, less probable, a failing resistor or capacitor. A bad connector, tube socket or loose solder joint is also a possibility. Here’s how I would approach this problem:

    FIRST, disconnect the amp from the wall socket. Open it up and inspect the circuit board and wiring for any obvious problems, such as a loose connection, broken tube, etc. This is a good opportunity to give it some TLC by blowing away dust and reseating connections (e.g. wire harness sockets). If all looks well otherwise, it’s a tube amp, and you have spare tubes, swap them in one at a time, turning on the amp each time to see if the problem is fixed.

    If after doing all of this, the problem remains, affecting a repair will require a higher level of skill and the equipment to go along with it, e.g. a multimeter, soldering iron, various small hand tools, etc. If this within your capabilities, then you already know that having the amp’s circuit schematic will be enormously helpful to do the troubleshooting, staring with the output stage and working your way towards the input.
    jmone likes this.

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