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5E3 120 cycle hum

Discussion in 'Amp Tech Center' started by tripley, Feb 4, 2021.

  1. tripley

    tripley TDPRI Member

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    Here is the layout of the board
     

    Attached Files:

  2. archetype

    archetype Fiend of Leo's

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    Thanks. I'd like to know why. Is it because full-wave rectification creates 120 Hertz?
     
  3. Jon Snell

    Jon Snell Tele-Holic

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    Full wave rectification acts as a frequency doubler, hence double mains frequency. Screenshot 2021-02-07 at 08.45.32.png
     
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  4. archetype

    archetype Fiend of Leo's

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    Got it. Thanks.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I really can't tell from the pictures but it sure looks like the input jacks are not wired correctly to the board. It may just be the angles of the photos. Please confirm all is well.

    If all is well with the input jack wiring and if you have checked all the ground connections, I would consider ditching the board connections and using shielded wire for the signal from the input jacks to the tubes. I would also ditch the board connections and use shielded wire to run from the pots to V2 pin2.

    The 68k resistors can be placed on the tube pins (preferred) or on the input jacks (easier). Removing the resistors and signal path from the board would get the input signal away from any stray voltages associated with the board.

    One side of the shielded wire should be grounded to the chassis not both sides.

    Since the chassis is painted, make sure the input jacks are making a good electrical connection with the chassis. Likewise for the power side ground chassis connections.
     
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  6. tripley

    tripley TDPRI Member

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    Thanks I'll give that a shot. The inputs are correct and the grounding has been check a few times and all is good. The chassis is power coated and it's scraped off where ever contact is required so that is all good too.

    I'm also seeing a DC voltage of about 16V on the grid of the phase inverter (the lead of C8) is this ok? I've attached a few pictures of the waveform. The first with the normal volume all the way down, and the second with the normal volume all the way up. This waveform was captured on the output, but I probed all through the signal path and the noise first presents itself after V2A. PXL_20210207_171657601.MP.jpg PXL_20210207_181244588.jpg
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If the reverb is switched out, there should only be the VDC from the grid of the PI at that point. The grid leak resistor is 1M but I would think 16VDC is too high. You may want to check if C8 is leaking.
     
    Last edited: Feb 8, 2021
  8. 2L man

    2L man Tele-Meister

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    Try to look if amp is oscillating very hign frequency. It can be megahertz range and possibly even tens of MHz How high is your oscilloscope capable? I see there is spikes on that low frequency signal. If it oscillates it uses power more than power supply can provide and then you hear it.
     
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