5F11 build negative feedback help

Discussion in 'Shock Brother's DIY Amps' started by Paul Strat, Aug 16, 2019.

  1. Paul Strat

    Paul Strat Tele-Meister

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    489F47EA-FD1C-4CC1-BF38-D5C5E01C1BF3.jpeg A0AB1078-907E-4FDB-A9F8-62F452B2BA07.jpeg C6A7498E-B006-43B4-BA8E-85231704DF31.jpeg C51B1197-C9D9-4A5F-A0DC-4DB5DDBE7995.jpeg 317F8EC3-BE8D-42F7-8D97-04F5F7AC53B8.jpeg F8D8C7DC-E86C-448E-B866-4ACA88FAAEBE.jpeg 6976642C-5FB3-4C3D-8450-13576729EB5F.jpeg AD813533-72E5-4CC6-BC80-79A41A09251C.jpeg Good evening everyone. Building a 5f11 clone and need some fresh eyes please.
    Everything seems to be in spec voltage wise. The issue is, when the negative feedback lead is connected to the speaker jack, it kills the sound completely. Without the NFB lead connected I have sound, but of course it’s not great. All grounds test fine and voltage is within spec for the circuit. Hopefully someone can spot what I can’t.
     
  2. JuneauMike

    JuneauMike Friend of Leo's

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    Why is the speaker jack stereo in some pics and mono in others?

    Also, not sure if that's the correct way to wire the stereo input. I've attached Mojotone's old layout drawing for FYI. Definitely looks different than yours.
     

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    Last edited: Aug 16, 2019
  3. dan40

    dan40 Tele-Afflicted Silver Supporter

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    When you have the nfb wire attached to the jack, are you plugging into that jack or the other jack without the nfb wire? The 3 tab jack is your main jack and must be the one that you plug in to unless you are running an extension cab along with the cabinet's speaker. It also appears that you have not run the jumper wires from the main jack over to the extension jack so it will not have any signal from the OT.
     
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  4. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Sorry...I see that the other jack is for your tremolo footswitch.

    It appears that your speaker jack is missing the ground wire. Is there a ground wire attached to the OT itself?
     
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  5. Paul Strat

    Paul Strat Tele-Meister

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    The other jack is for tremolo pedal.
    There’s only one speaker jack.
     
  6. Paul Strat

    Paul Strat Tele-Meister

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    There are only two jacks.
    The speaker jack and a tremolo jack.
    The speaker jack is a Switchcraft with a tip shunt. It’s not stereo.
    The OT is a Mercury which has the secondary ground already grounded to the body of the transformer.
     
  7. Paul Strat

    Paul Strat Tele-Meister

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    Yes, it’s a Mercury OT. The ground is attached to the OT.
     
  8. Paul Strat

    Paul Strat Tele-Meister

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    I’m using the Mojotone layout with a bias pot added.
     
  9. dan40

    dan40 Tele-Afflicted Silver Supporter

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    The OT secondary must be referenced to ground for the nfb to function properly. Any chance the bottom of the OT is painted or the chassis is painted preventing a good ground? You may be missing the jumper between the two solder lugs on the jack also.
     
  10. JuneauMike

    JuneauMike Friend of Leo's

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    Yeah, you need a shunt from the ring to the sleeve on the speaker jack. And I see the yellow OT wire coming from the grommet to the tip but I dont see the black wire from the OT (which would go to ground on the speaker jack).
     
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  11. Paul Strat

    Paul Strat Tele-Meister

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    I’ll try extending the OT secondary ground directly to the jack across the ground and sleeve, and see what happens.
     
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  12. dan40

    dan40 Tele-Afflicted Silver Supporter

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    If you do get the amp to play properly with the nfb attached, don't be surprised if it lets out a loud squeal when you power it up. With the nfb circuit attached, there is a 50/50 chance that the OT secondary will be out of phase and producing positive feedback instead. Just switch it off and swap the OT leads at pin 3 of both power tube sockets. That will eliminate the squeal and get you up and running properly.
     
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  13. Paul Strat

    Paul Strat Tele-Meister

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    Ok
    I extended the OT ground to the speaker jack and jumpered from the ground to shunt on the jack as well. Primaries seem to be on the correct sockets as there is no squeal or anything to indicate they are reversed. Still no sound with NFB connected. There is sound with NFB disconnected, but low volume.
     
  14. Snfoilhat

    Snfoilhat Tele-Afflicted

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    The new amp/newly rebuilt amp start-up procedures that I like to follow recommend getting every part of the amp squared away before closing a global NFB loop. Or some clever person on TDPRI recommended that. Or both.

    Maybe if you sort what appears to be another unknown issue first, the NFB fix will fall into place.
     
    Last edited: Aug 16, 2019
  15. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Low output volume can be caused by inadvertently sending a bunch of your output signal into the chassis by way of a messed up speaker jack like if the switched ground remains closed even with a plug inserted.

    It could indicate a signal path problem anywhere from the input jacks (have you measured them? Insert an instrument cable into each, taking both the tip to sleeve and tip to V1 resistance?) to the grids of the power tubes.

    Can you do signal tracing or a pop test to get an idea of where the volume loss is taking place?
     
  16. D'tar

    D'tar Friend of Leo's

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    Where are these grounded?
    20190816_221647.jpg Screenshot_20190816-221508_Photo Editor.jpg
     
  17. Paul Strat

    Paul Strat Tele-Meister

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    Under the board
    The B+ and reservoir caps are on a one piece ground to a bolt next to the PT. The 3rd filter cap and the other preamp grounds grounded from the middle input jack with underboard wire per the Mojotone layout.
     
  18. Paul Strat

    Paul Strat Tele-Meister

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    Under the board
    The B+ and reservoir caps are on a one piece ground to a bolt next to the PT. The 3rd filter cap and the other preamp grounds grounded from the middle input jack with underboard wire per the Mojotone layout.
     
  19. dan40

    dan40 Tele-Afflicted Silver Supporter

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    You mentioned in your first post that the amp would play without the nfb hooked up but it didn't sound very good. As the fellas mentioned above, there must be something wrong somewhere else because the amp should sound pretty darn good even without the nfb attached. Many players prefer to run these without nfb because it does allow the amp to overdrive a bit quicker. You may want to start at the input and double check every connection against the layout for any signs of errors or a wrong component value.
     
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  20. D'tar

    D'tar Friend of Leo's

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    Voltages....
     
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