Grounding OT

Discussion in 'Shock Brother's DIY Amps' started by since71, Aug 20, 2019.

  1. since71

    since71 Tele-Meister

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    While poking around inside one of my amps (wasn't broke so I'd better fix it) I noticed that the OT secondary negative was grounded in the circuit diagram but I had not done so in the actual amp.

    Can anyone tell me why this needs to be there? It worked anyway.
     
  2. D'tar

    D'tar Friend of Leo's

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    Mind posting the schematic and a photo of the "as built". What rectifier circuit did you use? The electrons really would like a return path to the secondary windings somehow.
     
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  3. King Fan

    King Fan Friend of Leo's Ad Free Member

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    ^^^^ what the smart kid said, including the OT schematic, or at least maker and model?

    Three possibilities... 1) some OTs are grounded thru their case and mounting screws 2) there may be some other ground we don’t recognize as such 3) the amp doesn’t work and you have excellent audio imagination


    Sent from my iPad using Tapatalk
     
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  4. since71

    since71 Tele-Meister

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    Rob'sAO44conversionschematic.jpg The secondary neg now measures to ground. All good.
    Its a conversion of a Hammond reverb amp, about which I have posted quite a lot! It has worked really well, but I think there is more gain than I need, so the first thing I tried was to reinstall the NFB but this results in big time oscillation. Concluding that this is due to a phasing problem I have removed the second stage of the 12AX7. Seems to still work I'll test this tomorrow (its late) and try putting the NFB back.

    And WTH are C10 & R18 doing? I'm going to cut them out....:mad:
     
  5. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The output secondary needs to be referenced to ground for the nfb to work properly. Does the amp still oscillate with nfb attached after you grounded the secondary?
     
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  6. Nickfl

    Nickfl Tele-Afflicted

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    It's a conjunctive filter. Cancels out some of the high frequency signal across the OT primary, probably just for stability in this case, but can also be to cut fizz/ice pick. You don't see them much in guitar amps, but Dr Z does use them in at least some of his.

    Edit: It also apparently changes the impedance curve of the OT and makes it more consistent across frequency, which is supposed to make the amp "smoother". I've tried it on one build and didn't feel like it did all that much, just a light hf roll off. Maybe something I should revisit and play with cap valuesa bit, but you have to use high voltage caps (1kv+) which most people don't tend to just have laying around...
     
    Last edited: Aug 20, 2019
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  7. RLee77

    RLee77 Friend of Leo's

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    If your nfb causes oscillations, it’s positive fb, so try reversing the OT secondary leads.
    As for c10/r18, they’re there to dampen parasitic oscillations.
     
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  8. since71

    since71 Tele-Meister

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    Thanks, but no thanks, that has been done. And put back.
     
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