Hum From 5E3P with no tubes installed

Discussion in 'Shock Brother's DIY Amps' started by Cfidave, Sep 13, 2019 at 6:03 PM.

  1. Cfidave

    Cfidave TDPRI Member

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    I have a very low volume 60 cycle hum. This is with No tubes at all in the amp. It has to be AC getting into the output some how. I have tracked down all sources of AC that I can think of:
    Transformer mains are putting out 320vac each, bias is 50vac, heaters are 5.4 volts & 6.5 volts. I have done the following:

    Rechecked grounds, HV center tap, Heater center tap, Bias ground. HV center tap is grounded at first filter cap.
    Tried different house circuits, no change. ( other amps don't hum at any house location)
    My PT & OT are rather close so I shielded with copper foil & metal plate both transformers, no change.
    Paralleled another 50/100 bias cap with the current two caps, no change.
    Power supply caps all read good, and grounds seem good.
    Ran heaters off of a 6 volt battery, no change at all.
    Re-soldered the entire rectifier leads.


    I can probably live with this hum as you need to be within 2 or 3 feet of the amp to hear it and it does not change with volume, when tubes and guitar are connected.. I would love to know how this is occuring, very strange.
     

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  2. RottenTheCat

    RottenTheCat Tele-Meister

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    Not nearly enough room between the power and output transformers. Even with perpendicular iron you going to get some cross coupling
     
  3. Wally

    Wally Telefied Ad Free Member

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    Will a shield in between the two transformer do any good?
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    If you really want to know...try a shielded piece of metal between the transformers.

    If you REALLY want to know, either disconnect the OT and wire it from the front just for testing, or desolder the existing OT and grab a spare and connect it from the front. If that resolves the issue, there's your problem.
     
  5. Cfidave

    Cfidave TDPRI Member

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    Tried the shield, no change. Actually built a shielded box to go around each transformer.
     
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  6. Cfidave

    Cfidave TDPRI Member

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    I do think there is some coupling going on, everything else has been eliminated that could inject AC into the OT. I will try another OT wired from the front.
     
  7. Wally

    Wally Telefied Ad Free Member

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    A bad filter cap????
     
  8. Wally

    Wally Telefied Ad Free Member

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    What is that brown wire just above the first filter cap....and is making contact with that ground...if so, should it be?
     
  9. dan40

    dan40 Tele-Afflicted Silver Supporter

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    I built an amp last year that had a similar hum issue. I used the headphone listening trick on the OT and no matter where I moved it on the chassis, the hum continued. I finally found that tilting the OT a few degrees from upright completely eliminated the noise. I ended up having to fabricate some washers to fit under the feet of the OT so that I could securely mount it in the "tilted" position.
     
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  10. ArcticWhite

    ArcticWhite Tele-Meister

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    I know a guy who elevated his OT to solve a similar problem. Try shimming it up an inch or so?
     
  11. Uncle Daddy

    Uncle Daddy Tele-Holic

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    Sometimes it's just bad luck with a transformer. I built one "right" last year that had the dreaded hum; I fitted the 1/10th power switch mod that eliminated it at room level.

    Recently I rebuilt another using some left over parts, exactly the same schematic, and no hum whatsoever.
     
  12. Cfidave

    Cfidave TDPRI Member

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    I believe you are looking at the center tap for the PT. It is actually grounded at the first filter cap.

    Very interesting on elevating the OT. I will give that a try.
     
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