PRS MT15: how i eliminated the fx loop hum

Discussion in 'Gear & Equipment' started by viifox, Jun 2, 2019.

  1. viifox

    viifox SS.org Regular

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    Alright, so I love these amps, but the one thing that's always bothered me about them is the noise/hum from the fx loop. I've read similar complaints online from several other people, so here's what i did to eliminate the hum...

    1. I put a 12au7 tube in V6, which is for the fx loop. This alone got rid of most of the hum. It's a much lower gain tube, which makes sense as to why it quiets things down.

    2. The cable that goes into the return of the fx loop needs to be elevated or out of the way somehow, because it causes interference, which also creates that hum. You'll have to get creative on how to elevate it.

    Well, that's really it. I'm not an amp tech by any stretch if the imagination, so I have no idea if putting a lower gain tube in v6 will cause any harm to the amp (i doubt it though, and i actually got the idea from an amp tech), so don't come to me if your amp blows up, lol!

    All i can say is that by doing this, the annoying hum is gone, and the fx loop sounds so much better. My pedals sound cleaner, and clearer. My Bigsky would make a loud hissing sound when cranking the mix knob, but it's so much quieter now, and the detail for my fx pedals is much more apparent (it's more like running pedals in front if the amp now).

    Anyways, i hope this helps!
     
    op1e, Edika and The906 like this.
  2. MaxOfMetal

    MaxOfMetal Likes trem wankery. Super Moderator

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    They probably should have gone for a solid state loop vs. tube driven. Using the lower gain tube doesn't remove the hum as much as lowers the gain, so lower noise floor. It's a popular technique for trying to quiet noisy tube loops. Since it only drives the loop it won't change the gain structure of the amp itself.

    As for the cable, have you tried a different cable? Sometimes that's all it takes. Cables are basically antennas for signal interference, so a better shielded one is ideal for some applications. It also could just be a packaging issue with the amp, the arrangement of components inside. Make sure to use instrument cable too, not speaker, as far as the loop goes.

    Either way, congrats on solving your problem.
     
  3. viifox

    viifox SS.org Regular

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    Good to know I'm not gonna blow up my amp, lol!

    Yeah, the cables I'm using are high end Mogami shielded instrament cables.
     

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