So the MT-15 still had problems

Discussion in 'Amplifiers' started by TRJC24PRS, Apr 28, 2019.

  1. TRJC24PRS

    TRJC24PRS Possibly A New Member

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    As the title states, my MT15 was still making crackling noises. Called PRS and the only information i got was i needed to replace one of the 12ax7 tubes. Now to my understanding theres 6 pre-amp tubes in the amp and i have no experience in tube amp work. So my question is, am i supposed to put the replacement tube in a specific slot or does it matter where it goes.
     
  2. Black Plaid

    Black Plaid just another Alan

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    It's *probably* the tube in position 1, as that's typically the one that works the hardest.

    I don't have an MT 15, so I can't tell you which one that is.

    What you can try is swapping them out with the new tube one by one, and stopping when the problem is fixed!
     
  3. Van Noord

    Van Noord New Member

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    Black plaid is right, replace each tube one by one with a known working tube.
    V1 is the tube closest to the input jack.
     
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  4. TRJC24PRS

    TRJC24PRS Possibly A New Member

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    Yup that was the one. Thanks!
     
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  5. JJJ

    JJJ asleep

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    if you google prs mt15 tube layout, you find a link to a thread on this forum and there is a layout PDF uploaded.

    Bit of a weird layout, because furthest from the power tubes appears to be a phase inverter (usually closest to the power tubes as it's part of the power section). Those usually get worked harder than any of your preamp tubes and can play some games when they go bad, so maybe worth swapping that out with a new one to see if it helps. Depending on how loud your crackling is, and whether it is present all the time (ie, when you switch between channels) you can narrow it down to v1. Because that's the first two gain stages, and noise there will be amplified by subsequent stages. That also means if it only crackles on a drive channel, it might be a later tube and not your v1. But the easiest way to figure it out, if they're all the same tube type, is to swap each one out one at a time with a known good/new spare.

    Rustling noises IME are usually a preamp tube, although I have had popping and farting and carrying on from bad power tubes too, fwiw
     
  6. Hellion

    Hellion New Member

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    Were you getting those crackling noises even when standby switch was off? Heard if for the first time yesterday. Bought the amp last week. I don't understand how a new amp can have all theses issues. Was the problem solved when you replaced the preamp tube closest to the input jack?
     
  7. TRJC24PRS

    TRJC24PRS Possibly A New Member

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    Yes sorry for the super late reply, but yes it would happen when standby was off, but replacing that one tube definetly fixes the problem
     
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  8. Hellion

    Hellion New Member

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    Ok gotcha. Thanks man
     
  9. TRJC24PRS

    TRJC24PRS Possibly A New Member

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    no problem
     
  10. elvis

    elvis Hamfisted String Banger

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    This is a great example of how an amp gets labeled as problematic when the actual issue is that tubes are terrible quality.

    One reason that certain amps can have more issues than others is that they have more gain, so the tube noises become louder. High gain amps are sensitive to noise.
     
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  11. JJJ

    JJJ asleep

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    standby switches cut the high voltage but leave the tube heaters on only, so I'm not sure how it would make the noise ? Glad to hear it's behaving now though
     

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