What's wrong with this AC30? Weird octave noise

Discussion in 'Amp Tech Center' started by pinkpanda, Jul 12, 2020.

  1. pinkpanda

    pinkpanda TDPRI Member

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    Just bought the amp yesterday. With certain notes there’s some extra low octave noise. Both in normal and top boost channel. Seems to be really prevalent on an Ab. I changed guitars, but still the same.

    Anyone has some ideas on what it might be?

     
    Last edited: Jul 12, 2020
  2. Jon Snell

    Jon Snell Tele-Meister

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    I would replace the EL84s firstly as if the screen grids have loosened over time, they do vibrate and produce an overtone like yours.
    Tapping the valves lightly, with a chop stick or Biro one at a time, should pin point the faulty valve.
     
  3. Dacious

    Dacious Poster Extraordinaire

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    I had a 6L6 in an Electar Tube 10 with a loose element - it would produce almost exactly that tone on low As. Jon's likely right, you have a loose bit in a tube.
     
  4. Javier668

    Javier668 Tele-Meister

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    Your caps are ok?
    My champ silverface when failed played ghost notes.
     
  5. pinkpanda

    pinkpanda TDPRI Member

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    I'm gonna check the tubes this afternoon. Because it's most obvious at a G/Ab, it can be a 100Hz hum. Buddy of mine suggested the caps as well so gonna check that after a tube check
     
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  6. schmee

    schmee Doctor of Teleocity Silver Supporter

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    "Follower" notes are often a result of bad filter caps. If it's buzzing a speaker voice coil rub.
    But yeah, could be a harmonic tube, although that's usually a higher harmonic than a low one.
     
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  7. jrblue

    jrblue Friend of Leo's

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    I'm assuming from the general direction of the posts that this is a used amp. If new, I'd bring it back rather than troubleshooting it, if used, you've been given good advice as to where to start. Those added undertones can be maddening! Let's hope it's just an ailing tube.
     
  8. Doctorx33

    Doctorx33 Tele-Afflicted

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    If your AC30 is a Custom Classic, don’t use the standby. Leave it on all the time.
     
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  9. pinkpanda

    pinkpanda TDPRI Member

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    Thanks everyone for the input! It's an AC30C2 and the reverb tank is dated 2014, so the amp isn't too old. I am in touch with the seller. He wasn't aware of the problem, which I believe. I just checked the tubes, and they look fine. No ringing or anything.

    It's probably an issue with the filter caps, because it really sounds like 'follower notes'. I hope I can get my friend to help me check it out, he often repairs electronic gear, so I have the idea he might be able to help me get it fixed.
     
  10. pinkpanda

    pinkpanda TDPRI Member

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    So checked the tubes. V1 was a bit microphonic but changing didn't resolve the issue. Power tubes were no real issues with either. There's no real 100hz hum or noise and the problem disappears when putting the amp on its front or side. Very peculiar, looks like a mechanical problem and don't expect this will be a real easy fix for myself.

    Anyone else ever experienced an AC30C2 with ringmod feature?
     
  11. allesz

    allesz Tele-Holic

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    From the phone's speaker It does not sounds that bad.

    Did you try tightening all the screws in the cabinet, speakers bolts etc?
     
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  12. milocj

    milocj Friend of Leo's Ad Free Member

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    I know it can be a bit of a pain to access the tubes on some AC30s, but try cleaning all of the tube pins. A spray of good contact cleaner like DeOxit on the pins and then removing and replacing each tube a couple of times should do a decent job of cleaning any layer of corrosion off of the pins or tube sockets.

    I've had this happen a few times and it's usually right around that Ab and A area on the two lowest strings.
     
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  13. tap4154

    tap4154 Doctor of Teleocity Ad Free Member

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    Also double-check that something hanging on your wall or a bookcase or something isn't ringing in sympathy. I have a bookcase near my amps and some of the shelves that didn't have anything on them were lightly vibrating against the back of the bookcase. Made a similar noise.
     
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  14. pinkpanda

    pinkpanda TDPRI Member

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    Haven't, but everything looks really tight. Doesn't sound like rattling bolts to me, but will check them when I have the amp open again.

    Think I'll try this. I don't have DeOxit in home, but do have some Contact Cleaner 601. If that doesn't work, I probably would have to replace tubes I think.
     
  15. pinkpanda

    pinkpanda TDPRI Member

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    OMG, this did the trick. Weird noise is gone! I scraped a bit very carefully of the pins sides. Next, I put some contact cleaner and brushed it with a toothbrush. Put it in and out a couple of times and worked great!

    Only problem now: the top boost channel is not working anymore. Probably one of the tubes I've worked on, but I only worked on the power tubes. Anyone knows which one is the tube for the top boost channel?
     
  16. Jon Snell

    Jon Snell Tele-Meister

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    "Only problem now: the top boost channel is not working anymore. Probably one of the tubes I've worked on, but I only worked on the power tubes. Anyone knows which one is the tube for the top boost channel?"

    The model is AC30/?
     
  17. pinkpanda

    pinkpanda TDPRI Member

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    AC30C2
     
  18. Jon Snell

    Jon Snell Tele-Meister

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    Screenshot 2020-07-20 at 12.52.10.png VR2 is the TB volume, V1 is the first stage and V2 does the business. Check the valves are seated correctly first and then check the valve voltages.
    View attachment 742907
     
  19. pinkpanda

    pinkpanda TDPRI Member

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    Thnx. Swapped V2, did nothing. Swapped V1, did the trick! I think that still the EL84s could use a replacement, but now I know exactly where the problems are of the amp. Thanks all!
     
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  20. Jon Snell

    Jon Snell Tele-Meister

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    Pleased to help.
     
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