"Fuchs ODS reverb squeal"

Discussion in 'Amp Tech Center' started by telemonade, Jul 28, 2018.

  1. telemonade

    telemonade TDPRI Member

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  2. Digital Larry

    Digital Larry Tele-Afflicted

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    Wait till you're my age, it won't bug you as much. Seriously, I could not hear it. My son on the other hand said THAT IS LOUD.

    If you unplug the spring return completely, does it change anything? It's pretty easy to get replacement spring assemblies for about $20. But of course you don't want to waste $20 if that's not really the problem.
     
  3. telemonade

    telemonade TDPRI Member

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    Ha! I didn't think about the frequency of the ringing. Even when I unplug the RCA jacks of the spring reverb, it makes the ringing noise. One time I accidentally plugged the RCA jacks in backwards, the ringing stopped, and it made a buzzing noise.
     
  4. thunderbyrd

    thunderbyrd Poster Extraordinaire Silver Supporter

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    write to fuchs and see what they say.
     
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  5. JD0x0

    JD0x0 Poster Extraordinaire

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    Contact Andy Fuchs, that's a pretty bad oscillation.
     
  6. telemonade

    telemonade TDPRI Member

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    I did write to Fuchs. Very nice, but couldn't diagnose without looking at it. They offered to fix it, but shipping and bench fees are pretty expensive for me right now. I'm guessing at some point I'll need to have a professional look at it.
     
  7. Digital Larry

    Digital Larry Tele-Afflicted

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    Here's my theory. Whether the spring is connected or not to the return doesn't matter. I think the output coil of the reverb spring is open. When you turned it around, the input coil (being good) made a connection which stopped the oscillation, though it buzzed. Different reverb springs have isolated or grounded inputs and outputs and different impedances. One thing you could do if you have an ohm meter is to check the resistance on the output. Normally I'd think it would be from the tens to a few thousand ohms depending on its design.
     
  8. telemonade

    telemonade TDPRI Member

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    Interesting, I like that theory. It still makes the ringing even when both RCA jacks are unplugged though. My first thought was to replace the reverb tank, but even with it unplugged from the amp, the ringing continues. The only time the ringing stops is when the reverb is plugged in backwards. I don't have an ohm meter, is there another way I could look into fixing it?
     
  9. Digital Larry

    Digital Larry Tele-Afflicted

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    The ringing (in my theory anyway) is because the input to the reverb return circuit is open. If it sounds the same whether the reverb spring output is plugged in or not. At that point it wouldn't matter whether the reverb spring input is plugged in or not.

    You can see if there's a number on the reverb pan that matches anything you can find here https://www.amplifiedparts.com/products/reverb_tanks

    and take a $20 (or so) gamble, which is what you might pay for a cheap ohm meter.
     
    Last edited: Jul 29, 2018
  10. telemonade

    telemonade TDPRI Member

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    I might go that route, thank you! Any other thoughts or suggestions are welcome.
     
  11. schmee

    schmee Poster Extraordinaire

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    with no reverb connected it rings? you have a microphonic tube. The reverb circuit just enhances it.
     
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  12. JD0x0

    JD0x0 Poster Extraordinaire

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    Sounds like the recovery/Mixer stages. Could be a bad tube, but I always assume people check the basics, when starting threads like this, and that's REALLY bad feedback for even a severely microphonic tube, IMO.. It could be a failed bypass cap in the recovery stage. I know in Fenders if the cathode isn't fully bypassed the reverb will start ringing like crazy, usually it starts with the dial around '6' though.
     
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  13. Wally

    Wally Telefied Ad Free Member

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    It sounds like tech time, imho....unless you are willing to do some studying. That is a high dollar amp that deserves experienced tech work, imho.
     
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