PRS MT 15: FX Loop Hum Solutions

[Bassman1]

So, I recently bought a PRS MT 15 w/ a PRS Stealth 2x12 (UK V30 and Hellatone 25 instead of stock speakers)...it sounds amazing.

However, I noticed that I get a slight hum/white noise when I use the FX loop. I've seen other posts about this before as well. So, it seems to be somewhat of a common phenomenon.

I should add that when I go direct into the amp it is pretty damn quiet. When I plug in all my pedals in front of the amp and engage my noise suppressor, it is SILENT. Even on the lead channel it's surprising how quiet it is (thank you Boss NS-2). So, really it's when I use the FX loop for my EQ and time-based pedals that I hear the noise. It is most noisy when I use my JHS Little Black Box passive attenuator in the loop so that I can play at apartment level volume. Yet, the JHS gets stunning reviews as well and was purchased brand new.

Do you have any solutions to get rid of this noise in the FX Loop? Would a ground lift device like a Ebtech Hum Eliminator / Hum X solve this issue?

 

[GoldDragon]

Pyle PHE400 on amazon.

 

[Bassman1]

Pyle PHE400 on amazon.

Interesting. So, you seem to think it's a ground issue, huh? I tried running my noise gate in the loop both before and after the JHS attenuator. Both did resolve the issue.

I am also now leaning more towards this being a ground issue.

 

[GoldDragon]

Interesting. So, you seem to think it's a ground issue, huh? I tried running my noise gate in the loop both before and after the JHS attenuator. Both did resolve the issue.

I am also now leaning more towards this being a ground issue.

If the noise suppressor in the loop fixed the noise, you already have what you need.

Run the NS-2 in "X" mode using all four inputs. It becomes a double ended noise suppressor. The gating is controlled by the guitar volume, but the actual gate is in the loop and closes signal there.

If that doesnt work, it probably is a ground loop.

 

[Bassman1]

Woops! Sorry, that was a typo in my last post. I meant to say that the NS-2 in the loop both before and after the JHS attenuator did NOT resolve the issue.

It certainly isn't a very loud hum, and it doesn't sound like what other folks have complained about with MT 15. I think you're right in that it is most likely a ground loop issue.

Do you have experience with the Ebtech Hum Eliminator?

 

[viifox]

So, I recently bought a PRS MT 15 w/ a PRS Stealth 2x12 (UK V30 and Hellatone 25 instead of stock speakers)...it sounds amazing.

However, I noticed that I get a slight hum/white noise when I use the FX loop. I've seen other posts about this before as well. So, it seems to be somewhat of a common phenomenon.

I should add that when I go direct into the amp it is pretty damn quiet. When I plug in all my pedals in front of the amp and engage my noise suppressor, it is SILENT. Even on the lead channel it's surprising how quiet it is (thank you Boss NS-2). So, really it's when I use the FX loop for my EQ and time-based pedals that I hear the noise. It is most noisy when I use my JHS Little Black Box passive attenuator in the loop so that I can play at apartment level volume. Yet, the JHS gets stunning reviews as well and was purchased brand new.

Do you have any solutions to get rid of this noise in the FX Loop? Would a ground lift device like a Ebtech Hum Eliminator / Hum X solve this issue?

I actually made a thread a while back on what i did to eliminate the hum...

https://www.sevenstring.org/threads/prs-mt15-how-i-eliminated-the-fx-loop-hum.336431/

The 12au7 preamp tube should eliminate most if not all the hum. If you're still experiencing hum after that, then simply try moving the fx loop cables around.

Also, to make the amp even a little less noisy, put a 5751 in V3. It also makes the gain more useable.

Edit* I just saw that you posted in that thread, lol!

The JHS Little Black Box technically isn't an attenuator, but rather just a volume control. It could be what's causing the issue, but I would still try the 12au7 tube and see if that does anything, because not only did it remove a lot of noise for me, but it also made my pedals sound much clearer, almost as if i was running them in front of the amp. It was totally worth it.

Btw, the JHS pedal is last in your chain, right?

Oh, aside from destroying my tone, that Ebtech Hum elimantor didn't do shit for me, lol! The 12au7 was the secret ingredient.

 

[Bassman1]

I actually made a thread a while back on what i did to eliminate the hum...

https://www.sevenstring.org/threads/prs-mt15-how-i-eliminated-the-fx-loop-hum.336431/

The 12au7 preamp tube should eliminate most if not all the hum. If you're still experiencing hum after that, then simply try moving the fx loop cables around.

Also, to make the amp even a little less noisy, put a 5751 in V3. It also makes the gain more useable.

Edit* I just saw that you posted in that thread, lol!

The JHS Little Black Box technically isn't an attenuator, but rather just a volume control. It could be what's causing the issue, but I would still try the 12au7 tube and see if that does anything, because not only did it remove a lot of noise for me, but it also made my pedals sound much clearer, almost as if i was running them in front of the amp. It was totally worth it.

Btw, the JHS pedal is last in your chain, right?

Oh, aside from destroying my tone, that Ebtech Hum elimantor didn't do shit for me, lol! The 12au7 was the secret ingredient.

Hey there,

Thanks for your reply, man! This is helpful. I am indeed running the JHS last in the FX chain. I tried the FX loop just with pedals and no attenuator, and it still had a slight hum. Again, it's nothing I would be worried about in band practice or on stage. It isn't that loud. Rather, it is just a slight annoyance.

Interesting that the Ebtech didn't do it for you! If you don't mind schooling me, when I read your thread it sounded like the FX loop is indeed powered by a separate tube. Meaning, the FX loop has a tube it uses outside of the two 6l6s and the 5 other tubes for gain. Is that true?

This is my first tube amp. So, I am not very knowledgeable when it comes to how tubes work and what not. Does changing the FX loop tube to the 12au7 change the tone of the amp at all? Cause I really do love the sound I am getting plugged right in.

 

[viifox]

Hey there,

Thanks for your reply, man! This is helpful. I am indeed running the JHS last in the FX chain. I tried the FX loop just with pedals and no attenuator, and it still had a slight hum. Again, it's nothing I would be worried about in band practice or on stage. It isn't that loud. Rather, it is just a slight annoyance.

Interesting that the Ebtech didn't do it for you! If you don't mind schooling me, when I read your thread it sounded like the FX loop is indeed powered by a separate tube. Meaning, the FX loop has a tube it uses outside of the two 6l6s and the 5 other tubes for gain. Is that true?

This is my first tube amp. So, I am not very knowledgeable when it comes to how tubes work and what not. Does changing the FX loop tube to the 12au7 change the tone of the amp at all? Cause I really do love the sound I am getting plugged right in.

The fx loop preamp tube is one of 6 preamp tubes in the MT15, and it sits closest to the 2 power tubes, behind the front row of the 5 preamp tubes. This is the layout...

V1 – input

V2 – lead gain

V3 – lead gain

V4 – clean gain

V5 – phase inverter

V6 – effects loop (behind front row of preamp tubes)

Swapping out the original fx loop tube with a 12au7 has zero effect on the tone (same with swapping the preamp tube in V3 with a 5751), so it really is an ideal solution!

That's another thing i love about this amp, getting to the tubes is just a matter of unscrewing 4 screws and removing the top lid. Everything is conveniently laid out and easy to get to. Kinda wish all amps had this design!

 

[Bassman1]

Wow! This is really helpful information. I just looked up the 12AU7, and I found an Electro-Harmonix brand one on Sweetwater for $20. That's beyond doable. I assume getting it installed by a tech wouldn't be too bad either. In any case, I think it would be worth it. This amp is just too badass haha.

Thank you so much for your insight and for taking the time to explain that. I am new to Sevenstring, and it's been fantastic to learn more and have conversations like these. Appreciate it!

 

[viifox]

Wow! This is really helpful information. I just looked up the 12AU7, and I found an Electro-Harmonix brand one on Sweetwater for $20. That's beyond doable. I assume getting it installed by a tech wouldn't be too bad either. In any case, I think it would be worth it. This amp is just too badass haha.

Thank you so much for your insight and for taking the time to explain that. I am new to Sevenstring, and it's been fantastic to learn more and have conversations like these. Appreciate it!

For sure! Happy to help.

Even though this is your first tube amp, i really think you're fully capable of doing the swap yourself, instead of having a tech do it. Maybe just watch a couple of youtube videos on how to properly remove and install tubes. Thankfully, it really isn't any harder than replacing a light bulb.

In fact, i can even upload a video for you later today.

 

[Bassman1]

For sure! Happy to help.

Even though this is your first tube amp, i really think you're fully capable of doing the swap yourself, instead of having a tech do it. Maybe just watch a couple of youtube videos on how to properly remove and install tubes. Thankfully, it really isn't any harder than replacing a light bulb.

In fact, i can even upload a video for you later today.

Really? Wow well that's pretty cool. In that case, a $20 fix is truly nothing. I appreciate your willingness to upload a video. That is really kind, man!

 

[GoldDragon]

Woops! Sorry, that was a typo in my last post. I meant to say that the NS-2 in the loop both before and after the JHS attenuator did NOT resolve the issue.

It certainly isn't a very loud hum, and it doesn't sound like what other folks have complained about with MT 15. I think you're right in that it is most likely a ground loop issue.

Do you have experience with the Ebtech Hum Eliminator?

I have had the Ebtech and the Pyle. They are identical functionally, except the Pyle can also accept XLR and it costs 1/3rd. Does not color tone.

 

[GoldDragon]

The fx loop preamp tube is one of 6 preamp tubes in the MT15, and it sits closest to the 2 power tubes, behind the front row of the 5 preamp tubes. This is the layout...

V1 – input

V2 – lead gain

V3 – lead gain

V4 – clean gain

V5 – phase inverter

V6 – effects loop (behind front row of preamp tubes)

Swapping out the original fx loop tube with a 12au7 has zero effect on the tone (same with swapping the preamp tube in V3 with a 5751), so it really is an ideal solution!

That's another thing i love about this amp, getting to the tubes is just a matter of unscrewing 4 screws and removing the top lid. Everything is conveniently laid out and easy to get to. Kinda wish all amps had this design!

Why would swapping the loop tube solve the problem? Is it a bad tube? A lower gain 12au7 will just make the noise seem lower and may reduce the volume of the amp.

If he swaps tubes between the loop and one of the others, he can diagnose if the noise goes away.

If the noise gate does not fix the noise, that is probably a ground loop. A noise gate would clean up tube noise.

OP,
First thing to do is swap loop tube with one of the preamp tubes if identical. If that doesnt fix it, dont go buying a new tube.

 

[viifox]

Why would swapping the loop tube solve the problem? Is it a bad tube? A lower gain 12au7 will just make the noise seem lower and may reduce the volume of the amp.

If he swaps tubes between the loop and one of the others, he can diagnose if the noise goes away.

If the noise gate does not fix the noise, that is probably a ground loop. A noise gate would clean up tube noise.

OP,
First thing to do is swap loop tube with one of the preamp tubes if identical. If that doesnt fix it, dont go buying a new tube.

No, it's not a bad tube. I've owned 3 of these MT15 amps, and I've replaced the fx loop tube every time with the exact same results. The 12au7 lowers the noise floor (that's the real issue) and DOES NOT lower the volume of the amp. If you were to put the 12au7 in say v1? Then yes, I'd expect the volume to drop, but that's not what happens when you replace the fx loop tube with a lower gain preamp tube.

The Archon is the exact same way.

 

[viifox]

Really? Wow well that's pretty cool. In that case, a $20 fix is truly nothing. I appreciate your willingness to upload a video. That is really kind, man!

You bet! Here's the video.

 

[Bassman1]

You weren't kidding, huh. Looks very simple.

It looks like you are using the JJ brand 12au7 tube (ECC82). Do you recommend just the standard tube, or one that has speciacl gold plating and what not?

 

[viifox]

You weren't kidding, huh. Looks very simple.

It looks like you are using the JJ brand 12au7 tube (ECC82). Do you recommend just the standard tube, or one that has speciacl gold plating and what not?

Nah. The standard JJ will do the trick.

 

[Bassman1]

Sweet. I just ordered one on Musician's Friend. Hopefully it does the trick! Thanks again for your help. This was informative beyond words. I will update in a week once I get the tube!

 

[viifox]

Sweet. I just ordered one on Musician's Friend. Hopefully it does the trick! Thanks again for your help. This was informative beyond words. I will update in a week once I get the tube!

You're welcome! Good luck!

 

[Bassman1]

UPDATE:

Got the JJ 12AU7 tube in the mail this morning. While the noise isn't completely gone in the FX loop when I use a passive attenuator or my normal FX, there is a very noticeable difference. I would say that alone got rid of most of it. There is definitely still SOME noise though. After trying out a noise gate, there wasn't a difference either.

I have yet to try high end cables like Mogami or Monster in the loop to get rid of the remaining noise. However, I would've imagined a noise gate would have solved that if the remaining noise was a cable issue. The noise sounds like radio interference more than anything. I would say the hum is gone now actually, and now there is just that distant radio noise in the background. Again, not a deal breaker.

Still LOVE the amp. It's just a beast. I think live it won't be an issue at all now. Recording might catch some of the noise, but I am sure there are ways around that too.