Need help diagnosing scratchy - static type amp noise

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by redgecko, Mar 8, 2015.

  1. redgecko

    redgecko Member

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    I'm having problems with a continuous static type noise in my Headstrong verbrovibe amp.
    This noise is present whether there is a guitar plugged in or not.
    As the video shows, no guitar or cable plugged in and volumes (both channels) on zero and then increased to full volume. At full volume, the static sound is only slightly louder and a bit more low end but not that different to volume on zero.
    Any thoughts?

     
  2. telenut62

    telenut62 Member

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    None of the tubes putting on a light show?
     
  3. pdf64

    pdf64 Member

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    Most likely either a bad plate load resistor associated with v1, 2 or 4, or less likely a bad tube for one of them.
    Take out v1; noise stop?
    Replace and repeat for v2 and 4.
     
  4. The Ballzz

    The Ballzz Member

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    Have you tried the standard smack the amp on top with a close fist test?:wave Not joking here, if there is any difference or sound produced when smacking, it could likely be simply a dirty tube socket connection. I would remove/replace each tube a couple times and see if there is any change.
    My $.02,
    Gene
     
  5. xtian

    xtian Member

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    Also known as "percussive tuning," an invention sometimes attributed to the Fonz, ca. 1974. Useful in many amp-related repairs.
     
  6. Blue Strat

    Blue Strat Member

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    Not sure what the tube layout is for that amp, but I'm guess that, like a 2 channel Fender, V2 is the one for that channel. Try removing it and see if the noise goes away. If so, either clean the pins and the socket and/or try another tube in that socket. If the normal channel is noise free, swap the V1 and V2 tubes and see what happens.
     
  7. redgecko

    redgecko Member

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    After removing preamp tubes, the noise was much reduced when v4 was removed.
    I've tried cleaning all the sockets with DeOxit and replaced v4 with a new tube but the sound was still present although it doesn't seem so bad now.
    I initially thought I'd fixed the problem as the noise was gone but returned after about 5 minutes.

    Would the next step be to replace the plate load resistors for the v4 socket?
    Also, what would be the best carbon comp resistors to use for this.
     
  8. Blue Strat

    Blue Strat Member

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    The noise seems pretty radical for a plate load resistor issue, but since it's easy...go ahead and replace them. I use carbon film. No sense replacing obsolete parts with obsolete parts which may eventually give you the same problem. Treble caps can cause similar problems but so can almost anything in the signal path.
     
  9. zenas

    zenas Member

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    I'd swap tubes till you know it's not a tube then move on to a voltage check and chopsticking to find the bad part. If it takes a few minutes to start making the noise that could be a resister braking down as it warms up.
    A scope would be faster I'm sure. I really need to buy a scope.

    Seems odd that a resister would be bad on an amp this new though. This could be one of thse things that ends with a lot of changed parts so you may want to take it to a tech depending on how good you are at nailing down the issiue.
     
  10. pdf64

    pdf64 Member

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    It shouldn't be too hard to identify the V4 plate loads, even to narrow it down to the mixer recovery rather than the reverb recovery (assuming that it follows a 2 channel BF Fender type pattern).
    I would use a carbon camp, as this 'last pre-amp stage' is the key point where their voltage co-efficient can add even harmonics (asymmetry) http://www.geofex.com/Article_Folders/carbon_comp/carboncomp.htm
    Noisy resistors can be within spec, in terms of resistance value, so just get it replaced.
     
  11. redgecko

    redgecko Member

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    Thanks for all the help so far. I'm not really confident to change resistors myself but I'm trying to diagnose this as much as possible before taking it to someone, since I don't have someone I can totally trust to work on the amp.

    I did take the amp to someone a couple of years ago to try and fix the problem but it came back more or less the same. It was a bit more intermittent back then so probably harder to diagnose.

    Here's a photo of the board - I see that two of the carbon comp resistors have been replaced with carbon film (I think) resistors. Are these the plate load resistors for v4?
    [​IMG]
     

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