5f1 hum

Discussion in 'Shock Brother's DIY Amps' started by jlang, Sep 4, 2018.

  1. jlang

    jlang TDPRI Member

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    Hey all,

    I just finished a boothill 5f1 kit with a classictone 15w OT (http://www.classictone.net/40-18031.html). I’m having a hum that gets louder as I turn the volume up, no cable into the input. When a guitar cable is plugged in to either input the hum is almost gone but still thereat a much lower volume. Also, when I take the 12ax7 out the hum stops completely.

    I’ve also swapped tubes with the same result and checked my soldering so now I’m at a loss.

    I’m using the ground bus and attaching all preamp grounds to that. Could the hum be coming from a loop that is caused by the vol pot and/or switchcraft input jacks not being isolated?

    Here is the layout with the adjustments I made to compensate for the 15w OT. image1.jpeg image2.jpeg

    Thank you in advanced for any insight you might have.
     
  2. clintj

    clintj Friend of Leo's

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    Check and make sure the input jack is actually shorting to ground when the cable is removed. A hum that lessens when a guitar is plugged in would be a symptom of that.
     
    robrob likes this.
  3. Uncle Daddy

    Uncle Daddy Tele-Holic

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    Check the contacts on the shorting jack sockets. Then check the sockets are grounding properly through the chassis.
     
  4. JuneauMike

    JuneauMike Friend of Leo's

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    There are lots of threads just like this one (including one I started). Did you search them out and read them?
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Check your 1M ohm resistors on the input jacks too.
     
  6. jlang

    jlang TDPRI Member

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    Thanks for the replies. Yes I have been reading through everything I can on here. My post was a last resort.

    I don’t think input 1 is shorting to ground properly. When I push the shorting jack the hum stops. How do I fix that?

    I’ll double check that 1M resistor also.
     
  7. Urshurak776

    Urshurak776 Tele-Holic

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    May just be a bad jack. Dave only stocks quality stuff, but things happen. Do you have another jack you can swap?
     
  8. JuneauMike

    JuneauMike Friend of Leo's

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    Just for fun, clip in a jumper from the pot ground to the pot casing, see what happens. That was the culprit in my case.
     
  9. jlang

    jlang TDPRI Member

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    Yeah looks like somehow with my heavy hands I "jacked" it up. I tend to be rough on things. I don't have another around so when I get one I'll rewire and post the results. I'm going to put a new 1M in there as well just in case.

    @JuneauMike I saw the thread on your volume pot grounding and tried that but still had hum. After seeing your thread I ordered a new volume pot and it's on it's way. I'll probably go ahead and wire that in when it gets here. Right now I'm putting my money on the jack because when I push on it most all noise ceases. Just a very faint hum.



    Thanks for help. Hopefully that fixes it.
     
    JuneauMike likes this.
  10. tweedman2001

    tweedman2001 Tele-Afflicted

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    Quite often a little judicious reforming of the contact terminals with needle nose pliers will fix it.
     
  11. JuneauMike

    JuneauMike Friend of Leo's

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    A question I forgot to ask: What type of hum is it? 60hz? 120hz?
     
  12. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    You can test the contacts with a multimeter, although the gap is normally visible..

    Usually that indicates the contacts are sprung - if so I always replace the jack.

    Pictures of the chassis may help as well - especially if it's not the jack.
     
  13. jlang

    jlang TDPRI Member

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    @JuneauMike sounds like 120.

    @Silverface theres a definite gap. If the jack doesn’t fix the problem I’ll start posting some pics.
     
  14. Syrinx

    Syrinx Tele-Meister

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    I would start by testing outlet it is plugged into.
     
  15. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    IMO EVERY wall outlet should be tested - home, gigs, rehearsal studios etc. A $6-7 tester quickly pinpoints missing grounds (3 prong outlets do NOT mean they're grounded!) and reversed hot/neutral - which can cause all sorts of problems. Many players have no clue that AC power outlets have polarity, and reversed outlets can cause hum, ground loops and dangerous conditions.

    Also all fluorescent lights need to be removed from a set of outlets or connections on the same circuit breaker - even those little twisty "power saver" bulbs cause hum.

    Plus computers, monitors, TV's, refrigerators and anything with a motor like a washer or dryer.

    BUT in this case we have identified a sprung jack as a problem.
     
  16. jlang

    jlang TDPRI Member

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    Update: I ended up replacing all three jacks and now it’s quiet as a mouse and sounds great! Thank you all for the help and quick diagnoses. I really appreciate it.

    Now I wanted to pick everyone’s brain once again. I plan to use this amp for harp. I’ve replaced the .22 caps with SoZo .1uf and my filter caps are 22-22-10. Are there any other mods to a 5f1 that might be useful for harp?
     
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