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5F2-A Hum Problem

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  • 5F2-A Hum Problem

    This is my first guitar amp build, based on Steve Milberger's schematic and layout. Star grounding (deviating slightly from the layout with the input ground, vol/tone grounds, and output xfmr and jack grounds all going direct to the star), shielded input wires, things I've used with hi-fi amps without issues. This unit hums though, with or without an input. To see if I could isolate the hum I tried grounding at the coupling cap input to the 6V6 - quiet, the coupling cap input to the second stage -quiet, which makes me think it originates in the first stage. Any thoughts on where or what to look at next. The amp sounds great otherwise - volume and tone work well.
    Thanks
    Johno
    Junior Member
    Last edited by Johno; 10-28-2009, 04:18 AM.

  • #2
    I never ground power-supply and signal path together. I'm far more in favor of keeping those grounds as far apart as possible. Try separating all the signal path grounds.

    I put my signal path grounds on the input jacks and it's very quiet.

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    • #3
      Further info: I have used isolating 1/4" jacks for input and speaker.
      Thanks Tele-cat for your response.

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      • #4
        Does the volume control kill the hum when it's all the way down? Where is the cathode of the input stage grounded? Where is the 8uF filter cap for the preamp stage grounded? Is there a center tap on the heater winding or a virtual (two 100 ohm resistors) center tap?
        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
        REMEMBER: Everybody knows that smokin' ain't allowed in school !

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        • #5
          Volume does not affect hum. Cathode of input stage is grounded on the eyelet board ground bus (which also attaches PS caps and has a single connection to the star ground) per Milberger. Although the PS xfmr has a center tapped heater winding, it is unconnected and a virtual CT with two 100 ohm connect to the 6V6 cathode.
          Any help Loudthud would be appreciated.
          Johno
          Junior Member
          Last edited by Johno; 10-28-2009, 04:19 AM.

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          • #6
            I searched for Milberger's info and didn't find anything. Are the schematic and layout posted anywhere online? Apparently he's active on alt.guitar, a site I'm not familiar with. The Fender info is easy to find.

            I'm shooting in the dark here. Where are those isolated jacks grounded? How did you deviate from Milberger's grounding scheme.
            WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
            REMEMBER: Everybody knows that smokin' ain't allowed in school !

            Comment


            • #7
              The hum is gone!
              I cut the ground bus on the eyelet board, which separated the audio cathode resistors from the PS caps, and connected the audio ground side to the input jack. Thanks Tele-cat.
              I was wrong about the volume control not affecting the hum. It did vary with volume.
              The Milberger schematic and layout are not on the web any longer it appears. It used to be at diycustomamps.com.
              Thanks for your responses guys.

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              • #8
                Originally posted by Johno View Post
                The hum is gone!
                I cut the ground bus on the eyelet board, which separated the audio cathode resistors from the PS caps, and connected the audio ground side to the input jack. Thanks Tele-cat.
                I was wrong about the volume control not affecting the hum. It did vary with volume.
                The Milberger schematic and layout are not on the web any longer it appears. It used to be at diycustomamps.com.
                Thanks for your responses guys.
                I'm glad that worked out for you. I got my info from Curt Emery who has all his shit in a bucket for sure. When I built my Super, I took it to him for a once over. He found a few mistakes and explained grounding to me. On the Deluxe I just finished, I used his grounding scheme pretty much to the letter, adapting it to a Deluxe which is a much simpler amp.

                Enjoy your new amp!

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                • #9
                  Cool. You have to separate your preamp section grounds from the power supply grounds. Otherwise it's hum city.

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