what can cause a rattling/vibrating sound? Output transformer?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by wsaraceni, Jun 15, 2019.

  1. wsaraceni

    wsaraceni Supporting Member

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    so the amp is a bad cat bc-50. its a 50 watt plexi style amp with a slightly different power supply and a few different values along the way. but essentially a 1987x circuit with the inputs jumpered.

    ive switched tubes and it happens with multiple tubes

    its loud enough to be annoying when playing but i still like the tone.

    when the amp is run into a load i can still hear it, so i know its not in the audio path. i dont know why but i think it's coming from the output transformer. the output transformer has no markings either so i cant tell exactly what it is.

    here is an example. the fan noise you hear is the two notes captor. no matter what load i try it into (ive tried 3) it always sounds the same. .

    the tubes are biased at around 32/33mA with the voltage around 472V. i can get the actual numbers written down at home if that helps.



    where do i start?
     
  2. oneblackened

    oneblackened Member

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    Yes, that's the OT. That's normal.
     
  3. wsaraceni

    wsaraceni Supporting Member

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    So I’d have to replace the OT to get rid of it?
     
  4. wsaraceni

    wsaraceni Supporting Member

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    i should add, ive heard my other amps do this with a load attached when cranked and played silently, but this is louder than the amp when playing only slightly attenuated
     
  5. Tron Pesto

    Tron Pesto Member

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    It would not be out of the question if the noise is due to a bad mechanical connection on any of the transformers and even tube sockets, boards, terminal strips, etc. This not only includes the chassis mounting, but the bell-end fasteners.

    In other words, make sure that with a bolt or rivet is tightened securely.

    I just had something similar happen - and I had checked all of the fasteners to the chassis. I forgot to check the bolts that held the bell ends on the PT. One was actually missing a nut completely. Horrifying - it actually sounded like a loud hum, not a rattle. And I had been running the amp while working on it on the bench for literally weeks - and it was fine. It wasn't until I put it back in the cabinet (different position and vibration from the speaker) that the noise appeared.
     
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  6. eolon

    eolon Silver Supporting Member

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    Hello Saraceni:

    If you are sure it is the output transformer, and not something else, like a loose cab nut or the cabinet itself, then you can damp the xformer a little by coating it with shellac, especially the laminated plates of the core, which is usually the culprit. Use Zinzer Amber, and cut it 50% with denatured alcohol, to seep into the tiny spaces between laminations, Many transformers are coated like this anyway. Depending on your transformer, you may have to remove the cover to get to the laminations.
    Best Regards,
    Don
     
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  7. wsaraceni

    wsaraceni Supporting Member

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    im not sure what it is, but i think its the output transformer. this makes the sound with it out of the cabinet totally so the cabinet isnt the culprit. with any set of tubes i put in it so i assume thats not the source. and with sound coming out of a speaker or without any speaker attached at all. in this video there is no speaker attached so its not vibrating from the sound. it's purely a mechanical noise.

    i actually tried to remove the bell housings from the OT but there is so much of that coating on the transformer i dont know if i can even get them off. the shellac (or whatever it is) on the bolts were hard enough to get off that i need new bolts to put it back together.
     
  8. jthomas666

    jthomas666 Member

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    Just to be sure of the source: If you get a 2 to 3 foot piece of plastic tubing from the hardware store you can use it like a stethoscope. Stick (gently) one end in your ear and probe around with the other to isolate the source of the noise.
     
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  9. oneblackened

    oneblackened Member

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    Potentially, but it's not causing any harm - at all.
     
  10. wsaraceni

    wsaraceni Supporting Member

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    maybe physically. but its also making the amp un-usable audibly
     
  11. pdf64

    pdf64 Member

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    It's pretty common - consider that another one may do the same thing.
     
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  12. wsaraceni

    wsaraceni Supporting Member

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    But is it caused by the transformer or by the circuit? Is it something like x/100 transformers are loud and you most likely won’t get it or is it something like this design will rattle more than others. Also, since the transformer is unmarked, what do I order as a replacement. A regular 50 watt Plexi replacement?
     
  13. Dan40

    Dan40 Supporting Member

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  14. wsaraceni

    wsaraceni Supporting Member

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    Yes. This amp was from before they purchased the company and they have to info on it.
     
  15. Blue Strat

    Blue Strat Member

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    Common for this particular transformer in this amp or...?
     
  16. wsaraceni

    wsaraceni Supporting Member

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    this is what im trying to understand as well.



    and just to be clear to everybody. i've heard a little transformer buzz when playing a cranked amp into a load. the thing is, at anything over a whisper volume, the speakers are way louder than that.

    this is louder than the speakers at a "loud bedroom volume"
     
  17. HotBluePlates

    HotBluePlates Member

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    It's "Magnetostriction" and is normal. It's more common on transformers and chokes that are not potted or impregnated with shellac, which is why @eolon was describing how to impregnate the transformer with shellac.

    But @jthomas666 was describing a way to be sure it's the output transformer, rather than the choke, etc, as your description makes it sound like the OT is well-enough coated with shellac/varnish.

    Does this really make the amp unusable? In your video, the transformer buzz is about the same volume as the acoustic noise coming from your guitar strings. And this is a 50w amp that you're attenuating down to being silent, or only as loud as your guitar strings??

    I can see how this is disconcerting if you didn't know it's something normal that happens with transformers. I can also understand it being annoying if you're trying to be totally silent. But this amp is also made to be blasting 100-115dB out 2-4 speakers while the transformer buzz is happening...
     
  18. wsaraceni

    wsaraceni Supporting Member

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    ill get a tube this weekend to check to make sure it's not the choke. like I said, I have had other amps that I've heard the same thing when playing into a load and using the line out for silent recording.

    the video does not actually pick up the sound that well but yes it makes the amp unusable. I do realize it sounds about as loud as my guitar strings but it is definitely much louder. not sure why both the fan and strings sound louder. maybe just the direction of the microphone on my phone. i'll see if I hook it all back up if I can get a better video. I normally play a komet 60 into a power station 2 into a 2x12 with blues or a 4x12 with v30s. the komet cranked pretty good and the power station 2 volume around 11 o clock. at zero volume I can hear the same sound, but slightly. so not quiet, but not hurting your ears loud. this is still very audible with the settings set the same way. im not complaining really. I bought the amp used. I guess im trying to narrow it down the the choke or OT at this point, but mostly im wondering if it is the OT, can I just order a replacement and if so, what would be a good OT to go with.
     
  19. HotBluePlates

    HotBluePlates Member

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    Why is the noise objectionable? That is what I think I don't understand, and I'd like to understand rather than dismiss it.

    I'm imagining a situation where you're doing silent recording but have headphones on to listen to the track. In such a scenario I don't see what the problem is, so it causes me to think this is a case of unrealistic expectations. But I may have this all wrong, and that's why I keep asking.
     
  20. wsaraceni

    wsaraceni Supporting Member

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    oh. im not using this to record silently. I actually don't record anything. on occasion, when I need to play silently, i'd run the amp into a load. the load would go into a two notes C.A.B. and I would have speaker and mic IRs there and listen to the headphone out. I don't even use this amp for that setup but I could without issue even with the buzz.

    ive tried that same setup, into a load silently with all 3 of my amps. one I hear no buzz. on I hear some quiet buzz only when everything is cranked to 10, this one is very loud buzz.

    the issue is when i'm playing through the speakers. like if im trying to practice or play along with a backing track or song. that's when the buzzing is loud enough that it is distracting. like I said. a little sound overpowered by any decent volume would be fine. but this is much louder than that. I have a small drum set, a Ludwig questlove kit, in that room. if someone was playing those drums, and I was playing this amp. everyone in the room would still hear the buzzing.
     
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