5F1 Kit Build - no sound

Discussion in 'Shock Brother's DIY Amps' started by brackp, Feb 14, 2019.

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  1. brackp

    brackp TDPRI Member

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    Yeah I have another cap I can put there, I'll steal one from my buddies :)

    Is the 181V going to pin 1 of the 12AX7 not a bit odd? Shouldn't that be identical to pin 6? Will see about putting together a signal tracing jig.

    Thanks for all the help so far everyone!
     
  2. jimgchord

    jimgchord Tele-Holic

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    This won't help your current issue, but do yourself a favor and create more space between that cathode resistor and the bypass cap.
     
  3. brackp

    brackp TDPRI Member

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    Thanks, will do, do you mind if I ask why that is?
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    The resistor gets hot and will cook the cap over time. To be clear, he's talking about the Cathode resistor on the 6V6.

    Measure the voltages on both sides (power supply side and tube side) of both 100K resistors feeding the 12AX7 and see what you get.
     
  5. brackp

    brackp TDPRI Member

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    The 470 ohm resistor right?

    I measured the two 100k resistors
    Power supply side is 225V
    Tube side of R7 is 150V
    Tube side of R5 is 70V
     
  6. jimgchord

    jimgchord Tele-Holic

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    I should've specified, thank you. Yes the 470 ohm large resistor.
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Yup - the big 470ohm guy.

    Those seem low. It also seems odd that you had B+3 at 291, which is the wire that fees the two 100K resistors on the 12ax7, but you said the power supply side (top) of that resistor pair was 225V. So.....re-check the following:

    -B+3
    -resistance to ground from pins 8 and 3 on the 12ax7
     
  8. brackp

    brackp TDPRI Member

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    Hmm, possibly (probably) some bad soldering or measuring on my part.

    I gave it a go again.

    B+3 - 263V (251 going to R7, 177 going to R5)

    Pin 8 Resistance 2.2 ohms
    Pin 3 resistance 1.5k

    Oh and there was some slight noise from the wires when measuring R7, like some arcing going on there. I resoldered it but it's still there
     
  9. jimgchord

    jimgchord Tele-Holic

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    Y
    Have you tried swapping pre amp tubes?
     
  10. brackp

    brackp TDPRI Member

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    yes i have :) and the power and rectifier tubes
     
  11. D'tar

    D'tar Tele-Afflicted

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    We are missing the almighty 6v6 pin 8 voltage here.

    And I do not see a ground wire for the 6v6 cathode resistor.
     
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  12. brackp

    brackp TDPRI Member

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    Whoops, pin 8 is 18v

    The cathode resistor is ground to input jack 1
     
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  13. brackp

    brackp TDPRI Member

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    BTW I'm trying to find a proper scope to do the signal trace, don't feel too comfortable poking around with a home made jig right now
     
  14. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    what do you mean by "proper scope" ?

    You make that cable from the image posted early and send that signal out to another amp to listen. No tools required. just make sure you touch the capacitor end of the probe to the amp chassis after each spot you touch to discharge it.
     
  15. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Pin 8 resistance on v1 is off. Should be reading 1.5k like pin 3 is. May be a bad solder joint on pin 8.
     
  16. D'tar

    D'tar Tele-Afflicted

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    NFB and speaker will give this reading. May want to disconnect for troubleshooting.
     
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  17. dan40

    dan40 Tele-Afflicted Silver Supporter

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    Corliss had asked for a pin 3 and 8 reading on the 12ax7 in post #27 I think. If brackp was reading 2.2ohms on pin 8, that should be the issue I hope, but it could also be a measurement error. Brackp, can you confirm that low reading on the cathodes of V1? Both should read 1.5k since they are tied together.
     
  18. harleytech

    harleytech TDPRI Member

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    Should be good... Make sure the speaker jack is grounded like the input jack too... Run a wire from the speaker jack ground to the chassis...
     
  19. D'tar

    D'tar Tele-Afflicted

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    Dan, 5f1 v1 cathodes...:)

    V1b has its own 1,5kr which is where the nfb is injected. That low r is reasonable. I would recommend disconnecting the nfb lead from the speaker jack or wherever it is and recheck the cathode resistance on v1a, v1b, Also check 6v6 p8 to ground resistance.

    Also check the jack terminals to ensure the tip connection is actually the tip, ground is ground and shunt is shunt. That doesn't look like a switchcraft and may have different terminal configuration. Not same as robs layout etc.

    When probing for voltages you should hear something... Scratching noise on 6v6 plate and grid. If you cant hear anything I would tripple check the output jack.
     
    Last edited: Feb 15, 2019
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  20. boredguy6060

    boredguy6060 Friend of Leo's

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    The fact that it is silent when probing the all the pins on the 12AX7 and the 6V6 leans toward no output on the 6V6.
    Any contact with pins 2 and 7 on the preamp tubes should get amplified if the output section is working.
    It should be scratching and popping very loudly when probing around 3,4,5 on the 6V6, if output section is working.
    I know you have, but recheck every connection to make sure that everything is going where it’s supposed to go and that all solder joints are solid.
    Good luck,
     
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