HUM in Tube Amp (I Tried My Best)

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  1. DutchGFX
  2. awptickes
    Nah man, sorry. I'm really out of my element with stuff this low frequency.
    I would say that you need some inductive chokes on the B+ supply to the tubes, but I have no idea how that does anything with the audio frequencies.
    Another shot in the dark, how clean is the DC coming into the plate? I didn't see how big your capacitor bank is.
  3. awptickes
    Actually, I can't believe I missed this...
    Is your filament grounded at any point? It looks like you're grounding the negative side of your filament.
  4. DutchGFX

    The filament is not grounded. When I ground the center tap of the 2A3 filament, I get a massive amount of hum. Earlier, I tried grounding the negative side of the supply, which killed the noise (at the time) but then since the cathode was essentially forced to +2.5V, the amplifier no longer amplified.
  5. dsavitsk

    You need to include detailed pictures and descriptions if you want people to be able to make suggestions.
  6. DutchGFX

    I will post detailed pictures ASAP when I get home.
  7. StanD
    Detailed schematics with values.
  8. DutchGFX
    PSU Board
    Both boards connected by a long Belden 1037A cable
    Cable meeting the amp board
    Amp board as a whole, the pairs coming off the front are going to the supply cable and the wires coming off the side are going to the input RCA jacks, which have nothing plugged in to them.
    Amp board top view. all the grounds from each channel are connected to the central bus wire, which is connected to the gnd of the regulator, which is earthed back at the PSU board rectifier. I tried moving my earth point to various locations with no change in the hum.
    My most interesting find was that the cable is dropping 1.6mV on the ground wire (it's 5ft of cable, running about 100mA)
  9. dsavitsk
    Did you add grid stoppers and a grid leak? The grid leak grounds need to be connected directly to the output jack ground which is then connected to the star ground. Star ground needs to be connected to safety earth.

    Do you have PS schematic? You need more than a single cap before the regulator.

    Is there a fuse?

    Some of those solder joints (especially on the wires) look suspect.

    Your plate CCS has an impedance of ~10K which sort of defeats the point of CCS.

    What does your 6sn7 heater supply look like? Is it grounded, or referenced to something?
  10. DutchGFX
    Grid stoppers will be added before I add signal, but for now the noise occurs with no input tube and no signal, so I am fixing the noise before adding anything else.
    Ill add another cap and see what happens. I scoped the regulator and there is no ripple that I can see but ill add another cap before it anyways.
    The 6SN7 heater is non-existent at the moment since the hum occurs without the input tube, I have ignored its supply for now.
    There is a fuse inside the IEC inlet I believe.
  11. dsavitsk
    Can you post an actual schematic? Include all the details -- including all the power supplies and how they are connected. In particular, use 2A3's instead of indirectly heated triodes.
  12. DutchGFX

    What do you mean an actual schematic?
  13. dsavitsk
    What do you mean, what do I mean? The actual schematic. A drawing that includes every component and connection on your project.
  14. DutchGFX

    Oh okay. I'll send that ASAP when I am home
  15. DutchGFX
    This is the current setup. It has NO input tube, no grid leak, nothing that resembles any signal transmission, and it is getting hum.

    Again, I HAVE ONLY INCLUDED THE COMPONENTS NEEDED TO REPRODUCE THE HUM. I have disconnected ALL other components so I can focus on troubleshooting.


    The hum is inaudible with only the Filaments connected, but if I connect filaments and B+, then disconnect filaments, the hum goes away immediately.

    Thank you guys so much for your help!
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