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Thread: Vibro-Champ with static noise

  1. #1
    Member melvin's Avatar
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    Vibro-Champ with static noise

    Iíve got a Blackface Vibro-Champ which had a massive hum- it had a bad filter cap (CE Dist can) that I replaced, but although the hum is now gone, thereís a static that I canít quite get rid of.

    It goes away when I turn the volume down, or ground the grid on the first stage, or lower the wall voltage with a variac below 100 volts- the amp has a replaced power transformer and the voltages are a little high.

    Iíve been able to quiet it somewhat by soldering the ground bus to the chassis.

    Iíve thoroughly cleaned the ground bus, replaced the grounding washers to jacks and pots. Iíve replaced the plate-load resistors and the filter caps, installed a NOS RCA 5Y3GT to get the voltages a bit lower, tried a shielded cable from the input and tried replacing everything in the circuit around V1 (including the input jacks,resistors and the tube socket,tube).

    Havenít tried moving the mix resistors from the inputs to the grid on the tube socket. And the heaters have not been modified, their still grounded at the sockets (thereís an open center-tap on the power tranny).

    Anyone know where this noise is coming from and how to get rid of it?

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  2. #2
    Senior Member SoulFetish's Avatar
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    So it goes away if you short the grid of V1. Does it go away if you put a shorted plug in the input? If tou plug in a guitar and turn the volume all the way down, does it go away? If it does, than the shorting jack probably isn’t shorting and needs to be burnished, cleaned, and maybe retentioned. Or replaced.
    Can you post a schematic please?

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    Bent Member Chuck H's Avatar
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    The volume is after the very first stage, following the tone stack in that amp. So it's really the second stage doing the heavy amplifying of whatever is coming from this circuit. Since reducing the volume OR reducing voltage quiets the static I might think it could be related to shot noise from the CC resistors. I have a theory that this gets worse with high humidity. Maybe blow dry the circuit board for an hour. If you have an electric heater with a blower that's the easy way. If that doesn't work you could try replacing the preamp plate resistors and the tone stack slope resistor with metal film. This is only three resistors and probably what I would try first because the problem, if it IS the CC resistors, could return with prolonged humid conditions.

    I'm just guessing it's humid where you are

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    Member melvin's Avatar
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    The shorting jacks are working and lifting them makes the noise much louder. Shorting after the input resistors removes the noise. And remember, soldering the buss to the chassis made it quieter. So Iím thinking it must have something to do with the grounding.

    The noise is still present with a guitar plugged in.

    Iíve replaced the plate-load resistors, but it could still have something to do with humidity. The screws holding the circuit board down were corroded as well as the casing for the jewel light.
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  5. #5
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by melvin View Post
    The shorting jacks are working and lifting them makes the noise much louder. Shorting after the input resistors removes the noise.
    That's a really small amount of circuitry from the input jacks to the first triode grid. Just the input resistors. Since the shorting jack is working and the noise is still there with the input grounded at the jack, but NOT with the input grounded at the grid. Did you try a different tube in the first position? Even if it's a new tube (especially if it's a new tube?) since the problem started? It seems that any elevation at that tubes grid results in static noises. I don't interpret this as grounding or any circuitry following the tube itself. It pretty much has to be the tube or the cathode circuit (grounding the tube grid shuts down emission from the cathode reaching the plate). But cathode circuits aren't known to cause static in general.

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    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Member melvin's Avatar
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    Solved it! Thanks for the replies. Chuck H, your concern for humidity got me thinking so I removed the grounds to the filter cap can and cleaned the chassis really well at those points, connected the center tap from the power transformer directly to the can and resoldered everything back together. It was a grounding issue, but at the filter caps. The entire chassis must have a layer of corrosion on it and I wasnít thorough enough the first time. The noise is finally gone! Hoorah!

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  7. #7
    Senior Member SoulFetish's Avatar
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    Nice work. Glad it’s all set

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