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Name that hum - vibro champ build

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  • #16
    Originally posted by zbones View Post
    hey board, my first post here - although i've been led here for answers for some time!

    i've completed a vibro champ build and i've got some unwanted HUM! I need help identifying it so i can cure it!

    Here's the symptoms:

    - turns up with volume

    - goes away with power tube unplugged

    - becomes horrid and loud with the guitar cord unplugged from the amp!


    anyone experienced this before or have any prescriptions? thanks in advance.
    More pictures - we can't see the input jack wiring. This is very sensitive to HUM if it's done incorrectly.

    It's three basic things:

    A. The input jacks have switches that close when you unplug your guitar.
    Older amps did not have this in the design...it was added later...DOES your amp have the switching contacts on the input jacks???
    One of the input jacks must short the 68K resistor to ground when the guitar is unplugged.
    Otherwise - you will get the HUM you describe.

    B. The filament wiring must be tucked away from the audio path- as far as possible.
    Anytime the filament wiring is near a grid wire, or the audio path, AC will be amplified.
    I SEE from the photos that the filament wires may be too close to the audio wires.
    With the amp ON, MOVE the filament wires with a chopstick, and find the place where the AC filament voltage is bleeding into the audio path.
    By moving the wires around, you can usually find the sensitive spots that are prone to AC noise, and move them accordingly.

    C. The filament wires should be balanced to ground with 2X 100 ohm resistors. Do this at the pilot light location, away from the preamp.

    D. One more source of Buzz, a ceramic disk cap, used on the volume control for the BRIGHT switch.
    Take out the BRIGHT cap temporarily, till you get the rest of it under control.
    Some disk caps will pick up the AC filament wiring, and allow AC into the audio path.

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