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5F2A help needed

Discussion in 'Shock Brother's DIY Amps' started by backalleyblues, Aug 5, 2020.

  1. backalleyblues

    backalleyblues Friend of Leo's

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    Hey Gang!

    Well, I finally finished up the amp I built for my wife's birthday, a 5F2A for her harp playing (mainly). Went to fire it up, and I got this weird and loud phasing sound-was affected a bit by the volume and tone controls, but not a lot. Also got some VERY distorted guitar through it, that was nowhere near the volume level of the phase sound.... arrggh!

    Obviously, there's a ground issue in here somewhere, what else should I be looking for? Since it's a birthday gift, I can't obviously work on it while she's around, so any troubleshooting will have to wait til morning-in the meantime, please let me know what to look for!

    Thanks,
    Franc Robert
     
  2. D'tar

    D'tar Friend of Leo's

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    Sounds like you may have positive feedback through the NFB circuit which would require you to swap the OT primary wires.

    To check the health of the whole circuit I recommend a complete voltage chart and a few photos.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Is that a "Mississippi saxophone" or a stringed harp?

    Tell your wife *Happy Birthday* from the shock brothers.:)
     
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  4. backalleyblues

    backalleyblues Friend of Leo's

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    Here's some pics of the guts, not the cleanest job on earth, but better than my first try-the one hanging green wire is a ground wire, which I did attach after noticing it. Did a test and I'm still getting the whacko feedback, though it's not phasing now... Will look at the OT wiring, it's a ClassicTone multi-tap tranny...

    Franc Robert
     

    Attached Files:

  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Pease follow the advice @D'tar gave. "Sounds like you may have positive feedback through the NFB circuit which would require you to swap the OT primary wires."
     
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  6. backalleyblues

    backalleyblues Friend of Leo's

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    OK, just swapped the OT primaries, still got the feedback-also tried disconnecting the negative feedback loop, that didn't do anything either...

    Franc Robert
     
  7. D'tar

    D'tar Friend of Leo's

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    Is this just the amp with nothing plugged in? Or feedback with an instrument?
     
  8. D'tar

    D'tar Friend of Leo's

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    where is this grounded? I would ground this with your PT center tap.

    upload_2020-8-6_12-33-5.png

    upload_2020-8-6_12-33-43.png
     
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  9. D'tar

    D'tar Friend of Leo's

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    Grounding to the pot.... you may need a little more heat for a solid connection. Perhaps you could move these to the eyelet?

    upload_2020-8-6_12-36-48.png
     
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  10. D'tar

    D'tar Friend of Leo's

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    leave the NFB disconnected until you get things straightened out. Then it will be obvious if you have a phase issue when reconnecting.
     
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  11. D'tar

    D'tar Friend of Leo's

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    It looks as if you have the 6v6 cathode connected to the nfb reistor and the nfb connected to the big cathode resistor?

    upload_2020-8-6_12-43-51.png
     
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  12. backalleyblues

    backalleyblues Friend of Leo's

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    Yup, that was the culprit alright! Switched the wires, and now it's working fine!!! Debating on whether to leave the NFB in or not-I played it with no NFB, and it distorted a lot quicker, and felt quite a bit looser-I'll let my wife decide on that, especially since it's just one wire to remove...

    Thanks again for the help!

    Franc Robert
     
  13. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Dig the purple cab and pink knobs! Got more pictures? :cool:

    I went back and forth on the NFB in my 5F2A, too. Was thinking of putting in a switch, and ultimately decided to tweak the circuit for more gain at the second preamp stage and reduce the NFB by about half. I like it.
     
  14. backalleyblues

    backalleyblues Friend of Leo's

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    Ask and you shall receive!!!

    Pink and purple are her favorite colors, and the heart on the grille is part of her signature-I felt a pink cabinet would be too much in this case.

    As for the NFB, I'm thinking that a switch would be the way to go too-it sounds really good with the NFB and a guitar, so having a switch (call it a harp-guitar switch) might just work...

    Franc Robert IMG_5724.jpg
     

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  15. Commodore 64

    Commodore 64 Friend of Leo's

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    That's a cool gift man. Good job. The harp players i know prefer 0 NFB, if they even notice at all.
     
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