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Help with strange Princeton 120Hz Hum issue (Fixed Bias)!

Discussion in 'Shock Brother's DIY Amps' started by tweedy_woodpecker, Sep 7, 2019.

  1. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    I have a strange 120Hz hum in a new amp build, that I am unable to track down, maybe you guys can help...the amp is based on a Princeton Reverb schematic (minus the reverb/tremolo in a 5F3 chassis, kind of cramped, but it works) with a few extras (NFB pot, Cathode/Fixed Bias switch, added control for mids, ...) and it works and sounds great.

    The only problem I can't track down is a pronounced 120Hz hum, indepedent of the volume setting and the strange thing is that it only occurs in in Fixed bias mode. As soon as I turn the Bias switch to Cathode biased, the hum goes away.

    First I thought this was a grounding issue, but I change the grounding points of differents sections (especially the Fixed bias circuit) and it did not make any difference. Also moving the heater wires or signal wires does not make any difference. And I already built amps with almost the same configuration (Bias switch, adjustable and balcanced bias) that don't have that problem.

    I attached the layout and a poto of the amp chassis.

    Any ideas? Thanks!
    IMG_6751.jpg tdpri.jpg
     
    Last edited: Sep 7, 2019
  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Does the hun change when you alter the balance pot?
     
  3. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I don't see what location the input and pre section are grounded? Just through the jack?
     
  4. dan40

    dan40 Tele-Afflicted

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    Have you set the negative DC bias voltage at pin 5 to the highest number that the pot allows? It may be that your bias voltage is set too low for this pair of power tubes and they are running extremely hot in the fixed bias mode. Most amps will begin to hum when the power tubes are pulling excessive current and begin redplating. Start with a cold bias and work up from there.
     
  5. tweedy_woodpecker

    tweedy_woodpecker Tele-Meister

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    Thanks for your posts!

    Yes, the pre-ground is grounded with the chassis at the input jack and no, the specific hum does not change with balancing the bias of the output tubes. I set the bias to 70% of max dissipation. (The bias supply is delivering about -39VDC to the output tubes.)

    Since the hum only occurs with fixed bias I will probably change out the diode and cap in the bias circuit to see if there is any hum coming from there. I already could reduce hum with another build going from 47 to 100mf with the filtercap.
    Also, the hum is not terribly loud, it is just that otherwise the amp is really quiet and I would love to get it to be dead quiet at both bias modes.
     
  6. Cfidave

    Cfidave TDPRI Member Silver Supporter

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    Try running a 47/100 cap from the pot wiper to ground, positive side to ground. This would be in addition to your current bias cap.
     
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