What Causes Amp Idle Hum

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ausguitarman, Sep 6, 2006.

  1. ausguitarman

    ausguitarman Member

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    My Fargen has the loudest idle hum, even when switched on standby, that I've heard.

    I've changed all the tubes out, tested it with different cab/leads/guitars and it's still there.

    Any thoughts guys.
     
  2. donnyjaguar

    donnyjaguar Member

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    Is the hum coming from chassis vibration or through the speaker?
     
  3. brad347

    brad347 Member

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    sometimes the power transformer can bleed AC right into the speaker, causing it to hum, if the cabinet is small enough. It's rare but i've seen it happening.
     
  4. NitroLiq

    NitroLiq Member

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    Are you playing in a room with fluorescent lights?
     
  5. ausguitarman

    ausguitarman Member

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    The hum is coming through the speakers.

    I've tried it in a 1/2/4 x 12 with the same hum through all.

    No fluoro's and I've tried it at different locations as well.
     
  6. Sarg_29

    Sarg_29 Member

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    I haven't got the answer for you but my amp does the same thing, I am beginning to think it may be something with the output tranny. Hope you get your answer...
     
  7. trdlasvegas

    trdlasvegas Member

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    I'm really puzzled by this... if the Standby switch actually removes the Hi Voltage from the tubes, then how do you get amp to amplify anything let alone hum???

    Do you have a schematic of your amp? I am really curious what the Standby switch actually does.

    Are you sure your not just hearing the transformer laminations buzzing?

    -Tony
     
  8. fyrwyr

    fyrwyr Member

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    My friends old Super Twin has the same problem, 180 watts of power in a 2x12 combo!!! Other than the constant hum it sounds excellent.
     
  9. donnyjaguar

    donnyjaguar Member

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    I've seen this problem with the Danelctro DM25. The culprit in that case is simply the poor placement of the output transformer with respect to the power transformer. The hum is induced into the output section even in standby (and even with the output tubes removed).

    That said, I don't know much about Fargen, but I'd serioulsy doubt that this is the problem.

    A schematic would help a great deal here.
     
  10. Stormy

    Stormy Member

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    I had the same problem with my old Victoria Lo power Twin. The hum was audible through the speakers on standby. I called Victoria and spoke with Mark Baer and he said it was the position of the transformer.

    I don't know shite about amps except to plug and play so I'm guessing that it has something to do with "magnetic polarity" (told you I don't know sheet, but it sounds likely huh)? :RoCkIn
     
  11. Stormy

    Stormy Member

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    Oh yeah, I ended up selling the Vicky and not just because it hummed, it just sounded boxey to me and couldn't get that amp to sing no matter which guitar I tried; Tele, Strat, or LP.

    You might also check the power tubes and re-check the bias (another nugget of wisdom Mr. Baer shared with me).
     
  12. NitroLiq

    NitroLiq Member

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    Do you get hum without anything plugged into the input(s)?
     
  13. schmit

    schmit Member

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    I have a Fargen 20 112 Blackbird and it also has some hum....not real bad but noticeable.....you don't have it hooked up to an ABY box causing the dreaded ground loop hum do you?

    No schematic, but here's a gut pic that may help
    [​IMG]
     
  14. ausguitarman

    ausguitarman Member

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    Plugged or unplugged - same hum.

    I had a chat with my tech today an he also says that it could be P.T. / O.T. placement.

    I hope not because this is a great amp when cranked. When it's dimed you tend to forget about it but there are a few clean songs I do and the hum is to loud when playing clean.

    I'm at a loss as to what to do as it's so damn good dirty.
     
  15. BENJAMIN FARGEN

    BENJAMIN FARGEN Member

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    It's not the transformer placement......it comes down to low PS filtering and filament noise....or it could just be a bad filter cap can....

    Some of the early MP's used less filtering on the PS......I would have your tech up the 50/50 cap can to a 100uf x 100uf....that will cut the hum in half....and then if you are really picky about SE Class A amp noise he could use 5v rect tap (yellow wires) to create a 6 volt DC filament string for the 2 preamp tubes and you should be good.

    If your tech needs any info feel free to have him contact me via e-mail

    Regards,
    Benjamin Fargen
     
  16. Zelmo

    Zelmo Member

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    Well, there you go!
     
  17. ausguitarman

    ausguitarman Member

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    Thanks Ben.:RoCkIn

    I'll print this off and give it to my tech.
     
  18. rgsss14

    rgsss14 Gold Supporting Member

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    Remind me again how this has an effect on the hum?
     
  19. NitroLiq

    NitroLiq Member

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    You can read this:

    http://www.guitar9.com/studionine7.html

    or...here's the excerpt:

    What made me think of it is at the last ampfest we had, it was held in an industrial space and the flourescent lighting caused the amps to buzz. The same way sometimes an amp would start thumping if someone had a cell phone nearby.
     
  20. Lashing

    Lashing Member

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    I just went thru a simular problem with my Marshall Jubilee. Open up your amp and poke around with a chopstick. Try repositioning wires on the first preamp tube. For me it was a green wire connected to pin 2 of my first preamp tube that was picking up excessive 120 hum from a wire going to the footswitch jack. I fixed the wire against the chassis and 90% of the hum is gone.

    Taking the amp in to see if there not somethign else but I was amazed how much hum I got when I put these wires closer to each other. It was crazy loud hum. Pull them away - less hum.
     

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