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Rushing Water / Crackle Sound on Startup. Stops after 5 seconds.

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  • Rushing Water / Crackle Sound on Startup. Stops after 5 seconds.

    I'm working on a Trainwreck Express build made with brand new components and when I flip the standby switch the amp makes a super loud rushing water/crackling sound for about 5 seconds and then seems to settle down and behave. What in the world could be causing that? Bad filter caps? The caps are after the stby switch so I thought it could be when charging up after the standby switch is thrown. Thoughts?

    Forgot to mention that I replaced all the preamp tubes and it's still doing it.

  • #2
    Could be bad filter cap (s). I wouldn't suspect tubes as they have to warm up more than 5 seconds to start conducting. You could take a 22uf/500v cap and clip it across each filter cap in turn and see if you get a clue. If you clip it across a cap and the noise goes down perceptibly, you get on the trail of the problem. Also chop-sticking may afford some info.


    • #3
      Tube would be my first suspect, but you eliminated that. Next suspect, some plate resistor.

      Does it do it with all controls at zero? Can you zero out ANY controlk and kill the noise? That will tell you how far along the circuit the noise is coming from.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        He never mentioned changing the power tubes, so I bet it is the power tubes.
        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


        • #5
          A quick'n easy noise tracing (without a scope) is to ground grids with, say, a .022 or .047uF x 400V cap , starting from the output and going backwards to the input.
          That way you'll quickly know which stage is responsible.
          Or at least, to split the amp in 2 parts, ground the PI input grid ... and then refine your search.
          Just to add another suspect to the list, it might be a cold solder or some part with oxidized leads which did not get really soldered but just clamped by a cooling solder ring.
          Juan Manuel Fahey


          • #6
            Great! I'll check the power tubes first then the caps. Thanks!


            • #7
              My bad. I see you said standby switch, meaning the tubes were warmed up. Could be output tubes or caps. Good luck!