"ringing" sound, on certain notes (PR w new OT)

Discussion in 'Shock Brother's DIY Amps' started by LudwigvonBirk, Jul 11, 2019.

  1. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Ludwig, selfish interest, did you ever try swapping the OT leads? I know this isn't squeal, but I've read that postive feedback can be very subtle.
     
  2. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Have not, yet.
     
  3. tubeswell

    tubeswell Friend of Leo's

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    I thought you had already tried swapping the plate leads?. That is the first thing to try. Please Do it and report back.
     
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  4. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Sorry for the unclarity upthread. I have heard OT squeal on a different amp (loud and obvious).

    Anyway, will post findings once I get it on the bench. First step is to try to repro while connected to a scope (which can save .jpeg).

    Thanks for all the support thus far!
     
  5. Wally

    Wally Telefied Ad Free Member

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    Okay, Tubeswell....I am seeing things now. In your first post there you reference Fender’s doing this type of thing in there amps. So, I went to see that and there were those 2000pfd and 1200pfd caps...not 250pfds. That is why I posted my query to you. If you had said in the reference that 250pfd would have been a better choice for for the application than the capacitances that Fender used, I would have, I would have understood that. As for nanofarads, that is a rating that I don’t see used much....for some reason.

    At any rate, thanks.
     
    Last edited: Jul 13, 2019
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  6. tubeswell

    tubeswell Friend of Leo's

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    Sorry about that Wally. Turns out it’s my turn to be confused. Most likely to be straightforward positive feedback after all.
     
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  7. King Fan

    King Fan Friend of Leo's Ad Free Member

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    We'll await an OT lead swap like Mission Control waiting for Apollo 11 to emerge from the dark side of the moon.

    Or was that Floyd? :D
     
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  8. Wally

    Wally Telefied Ad Free Member

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    tubeswell, all I know is that I cut those caps out of those old Fenders and have never encountered any problems due to the absence. I will take it all under advisement, though. I appreciate your insight and knowledge.
     
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  9. tubeswell

    tubeswell Friend of Leo's

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    Well I can confirm that 250pF grid bypass caps are useful for curing ultrasonic ringing where HF phase shift, caused by interwinding capacitance in the OT, interacts with the global NFB loop to result in positive feedback of ultra HF, without compromising guitar amp bandwidth. I have done this myself as a fix to this problem and it works.

    Some tech at CBS Fender probably encountered the problem in some lab test and thought (by way of overkill) that they’d just slap 2nF on all their amps at the time ‘just in case’ and later realised it wasn’t necessary.
     
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  10. dan40

    dan40 Tele-Afflicted Silver Supporter

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    @LudwigvonBirk...definitely try swapping the OT leads first. I built a 6g3 clone last year that was acting very strange but it did not have the dreaded squeal that we normally associate with positive feedback. After several failed attempts at troubleshooting I finally decided to swap the leads. The amp sounded great and all of the weird noises disappeared instantly.

    If your OT leads are cut too short to swap, you can swap the leads at pin 5 instead. This will have the same affect as swapping the primary leads.
     
  11. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    Semi/lame update: I haven't adjusted the amp at all due to time. I played a gig with it tonight (jazz 8 piece + vox), amp sounded very nice and my "problem" was inaudible at the venue, including to me 12" away and listening for it. So I'm going to leave it as-is, until I need tbd additional maintenance (or, need to record, which I don't even wanna). Btw, the Cannabis Rex 12", if you haven't tried one, is pretty tasty.


    Also, if anybody is in, or is coming through Madison, please come to the Zuzu Cafe Jazz jam on Thursday nights. All players and singers welcome. The food and drink is yummy - my pre gig fuel was their icy gazpacho, followed by a primo chocolate cake slice + a double espresso, then 2xPBR during the gig.
     
    Last edited: Jul 18, 2019
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