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5F2A squeal?

Discussion in 'Shock Brother's DIY Amps' started by gonzo1999, Sep 18, 2010.

  1. gonzo1999

    gonzo1999 TDPRI Member

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    I've got a 5F2a I recently built from scratch, and it's up and running and sounds good. The one issue I'm having is that with the controls (vol and tone) up past 3/4 or so, and with no guitar plugged in, it will produce a high-pitched squeal. Won't happen with a guitar plugged in.

    Any ideas on what my problem could be?

    The layout I used is pretty much exactly this one:
    https://taweber.powweb.com/store/5f2a_layout.jpg
     
  2. tubeswell

    tubeswell Friend of Leo's

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    Welcome to the forum. You are getting +ve feedback (instead of negative feedback through your NFB loop). Swap the OT primaries around. (Sometimes they come out of the factory with the wire colours back-to-front). So take whichever OT primary wire that is attached to pin 3 of your output tube, and re-attach it to your B+/*reservoir filter cap node, and take the other OT primary that is attached to your B+/reservoir filter cap node, and re-attach that to pin 3 of your output tube.

    * = the reservoir cap is the 1st (16uF) filter cap in the supply
     
  3. gonzo1999

    gonzo1999 TDPRI Member

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    Huh. Interesting. I'll try that. Thanks for your quick response!

    Is there any way to know ahead of time which wire is which, or is it just trial and error? I just went with what seems to be the convention (brown to B+, blue to pin 3).
     
  4. tubeswell

    tubeswell Friend of Leo's

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    One would like to think that they have their quality control refined enough that it doesn't happen, but the people who put the pieces together in the factory are fallible like the rest of us. You get it from the factory with a brown wire and a blue wire sticking out, and you hope they got it right. In this case there's no way of knowing ahead of time if they got it wrong. It is merely a case of the tried-and-true 'fix it after the problem occurs' method.
     
  5. gonzo1999

    gonzo1999 TDPRI Member

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    Nope. That wasn't the problem. Reversed, it squeals now at low and high volumes.

    Any other ideas?

    I guess I'm ok with it as-is -just have to turn it down a bit before unplugging.
     
  6. tubeswell

    tubeswell Friend of Leo's

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    Then the problem could more than likely be a microphonic pre-amp tube. Have you got another 12AX7 you could pop in to V1?
     
  7. gonzo1999

    gonzo1999 TDPRI Member

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    Yep, turned out to be the tube. I should have thought of that first, I guess. Interesting that it only happened at really high gain settings, with no instrument - well, on second thought, I guess that's not surprising.
     
  8. gonzo1999

    gonzo1999 TDPRI Member

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    Just to update this - changing the preamp tube did not actually solve the problem. I put in a 5751 and that shifted the squeal point a little higher up the dial ( I suppose since it's got lower gain than the 12AX7), but the problem still remains.

    I guess the problem could be in my lead dress somewhere, although I thought I was pretty careful with this. If it is in the wiring path, any ideas where this sort of thing might be happening (i.e. are there any points in the circuit that are particularly sensitive to lead dress)?
     
  9. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    1. Reversed OT would likely squeel at all volume settings.

    2. Microphonic preamp tubes could cause this - or help make it worse if #3.

    3. Crosstalk from poor lead dress.

    It could be other things, but these are the three I'd check first. It looks like you already did #1 and #2, so move on to looking at the lead dress. They should be as short as possible, neat, and tidy. If wires have to cross other wires, or plate cap leads, try to make them cross at 90 degree angles. Keep wires as far from other wires as possible.

    You may need to use shielded wire from the input jack to the board resistors, then from the resistors to the input grid on the tube socket. Shielded wire to/from the Volume/Tone pot might help as well.

    BE CAREFUL, there may be high voltages stored in the filter caps. Use a meter to measure voltages first, if they are very low or zero, then you can safely work on the amp.
     
  10. limbe

    limbe Tele-Afflicted

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    Have you checked that the input jack shorts the signal to ground when nothing is plugged in?
     
  11. gonzo1999

    gonzo1999 TDPRI Member

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    OK, great, thanks for the tips. I didn't used shielded wire on the input or Vol/Tone leads, so that could help. And I'll do some careful inspection of the rest of the wiring.
     
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