Weird Frequency Specific buzz/crackle

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

  1. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

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    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

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    I'd be suspecting a noisy tube.
     
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    You've tried different speakers as well, I'm assuming?
     
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  4. clintj

    clintj Friend of Leo's

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    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

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    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.
     
  6. bblumentritt

    bblumentritt Tele-Afflicted Platinum Supporter

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    Interesting turn of events.

    The amp used a split grounding scheme, with the cap can, the output jack, and the cathode resistor all grounded at the cap can, and the rest of the circuit grounded at the input. Eliminating the ground point at the cap can and chaining those to C3 made a huge difference.

    The other thing that helped was shortening some of the coupling cap leads and re-soldering them.

    The most interesting part is that the resonant frequency is no f# 185 or 370 Hz, but not as bad. Play an F# and static comes out of the speaker, regardless of which speaker I use or whether the amp is in a cab or on a test bench.
     
  7. philosofriend

    philosofriend Tele-Holic

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    Wouldn't you try turning the signal down and/or tapping more lightly until the tap test gets more specific?

    Interesting to consider that ground currents could oscillate at one note. Makes sense if ground currents are coming through a capacitive and an inductive source.
     
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