Weird Frequency Specific buzz/crackle

Discussion in 'Amp Tech Center' started by bblumentritt, Feb 12, 2019.

  1. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

    Dec 3, 2012
    Austin, Texas
    I've got this Marshall 18W TMB two-channel amp. Channel one has no issues in and of itself. The TMB channel, on the other hand has this very specific rattle sound when I play anything near D3 (146.8 Hz).

    At first, I though it was a cabinet vibration. I eliminated that.

    I put the chassis up on the test bench, made sure all the screws/nuts were tight, made sure the tubes were seated, the pins were making contact, and the tube clamps weren't rattling.

    So, here's the deal.

    When I play a D at moderately loud volumes, I get a sound that sound like a physical vibration, but it's coming through the speaker.

    I pulled out my chopstick, and put my signal generator at 440Hz, then 220Hz and then 146.8Hz. At 146.8Hz everything I touch creates a rattle through the speaker. Every jack, every resistor, every capacitor, when I tap it with the chopstick, I get a buzz/rattle coming out the speaker; it sound something like static. It's not totally specific to 146.8Hz, but it's in that neighborhood. You definitely notice it playing guitar through there every time you play the 4th string open D or 5th sting at the 5th fret D.

    I'm still looking, but at this point, I haven't figured it out, yet.

    Has anyone had a similar experience.
     
  2. LeDocteur

    LeDocteur Tele-Meister Silver Supporter

    358
    Oct 20, 2009
    PA
    I'd be suspecting a noisy tube.
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Sep 13, 2008
    Lansing, MI
    You've tried different speakers as well, I'm assuming?
     
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  4. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I'm still trying to wrap my head around this. So on the lead channel, passing roughly 150Hz signal will cause a resonance or similar in the whole chassis, even with it physically isolated from the speaker cab?

    That is REALLY odd.

    The only thing I can think of, and this is probably conflating a few things that don't apply, is maybe a transformer vibration? I've heard a few OTs "sing" when power testing on a dummy load, maybe this is a related phenomenon.

    Sent from my Pixel 2 using Tapatalk
     
  5. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

    Dec 3, 2012
    Austin, Texas
    corliss1 I've used two different speakers.

    clintj Yes, at ~150Hz sine wave input from a signal generator, with the chassis on a workbench and connected to a speaker, anything I touch in the chassis with my chopstick, results in a static noise from the speaker, that sounds as if you had a combo amp, and hit a note and had a physical vibration in the cab.

    I'll double-check the transformer, and the grounding scheme.

    Thanks.
     
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