# PRS MT 15: FX Loop Hum Solutions



## Bassman1 (May 5, 2020)

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?


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## GoldDragon (May 6, 2020)

Pyle PHE400 on amazon.


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## Bassman1 (May 6, 2020)

GoldDragon said:


> 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.


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## GoldDragon (May 6, 2020)

Bassman1 said:


> 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.


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## Bassman1 (May 6, 2020)

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?


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## viifox (May 6, 2020)

Bassman1 said:


> 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 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.


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## Bassman1 (May 6, 2020)

viifox said:


> 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/
> 
> ...



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.


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## viifox (May 6, 2020)

Bassman1 said:


> 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.
> 
> ...



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!


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## Bassman1 (May 6, 2020)

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!


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## viifox (May 6, 2020)

Bassman1 said:


> 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.


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## Bassman1 (May 6, 2020)

viifox said:


> 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!


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## GoldDragon (May 6, 2020)

Bassman1 said:


> 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.


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## GoldDragon (May 6, 2020)

viifox said:


> 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
> 
> ...



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.


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## viifox (May 6, 2020)

GoldDragon said:


> 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.
> 
> ...


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.


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## viifox (May 6, 2020)

Bassman1 said:


> 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.


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## Bassman1 (May 6, 2020)

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?


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## viifox (May 6, 2020)

Bassman1 said:


> 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.


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## Bassman1 (May 6, 2020)

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!


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## viifox (May 6, 2020)

Bassman1 said:


> 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!


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## Bassman1 (May 13, 2020)

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.


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## Bassman1 (May 13, 2020)

ANOTHER UPDATE:

OK, so I tried the noise gate again in the loop, but this time as the X formation. That didn't solve it either. But, when I ran my attenuator in the loop with short patch cables, that solved it! Sure, if you put your ear right next to the speaker, you can hear maybe a little noise. However, it's faint at best. *So, short cables and the 12AU7/ECC82 tube seemed to do the trick.*

For playing live, I will run 20 foot instrument cables from my board into the FX loop. That'll probably cause some noise, but nothing I would be concerned about on stage. So, that realm is solved too. 

For studio recording, this would really be the last issue to resolve. Namely, how to still use a pedalboard in the loop because one would probably need at least a 10 foot cables each for the send and return. Maybe a buffer pedal could resolve this or perhaps other studio tricks. That is something I am sure can be resolved by the recording engineer or some more head scratching.


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## silbbu (Dec 31, 2020)

viifox said:


> You bet! Here's the video.



Thank you for this! It really helped and I don't know if I'm imagining it but it seemed to smooth out that master volume on lower levels. There seems to be more choice between silent and too loud now.


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## viifox (Jan 1, 2021)

silbbu said:


> Thank you for this! It really helped and I don't know if I'm imagining it but it seemed to smooth out that master volume on lower levels. There seems to be more choice between silent and too loud now.


For sure! I also seemed to notice better volume taper as well.


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## Arthur Saladino (Mar 9, 2021)

I replaced the Fx loop pre amp tube with a 12au7 and dramatically decreased the hum in the loop.. (still a little noise about 2/3rds less)... Im happy I found this forum .. thank you all for your advice


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## Bassman1 (Mar 9, 2021)

Arthur Saladino said:


> I replaced the Fx loop pre amp tube with a 12au7 and dramatically decreased the hum in the loop.. (still a little noise about 2/3rds less)... Im happy I found this forum .. thank you all for your advice




1000% agree with this. I use an ISP G string Decimator II run in an X pattern so that the front of the amp and the effects loop run through the noise gate. That kills any other noise. It's dead silent now with a ton of gain, pedals, and volume.


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## Emperoff (Mar 9, 2021)

Most of these issues:
- "my fx loop is noisy"
- "my clean channel sucks"
- "my dirt channel has too much gain/compression"

Can be easily solved with a tube swap.


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## guitar_player4_2_0 (Mar 9, 2021)

So glad this thread is here, I just picked up a 12au7 from Sweetwater. I was thinking the noise was coming from the power supply to my EQ pedal in the loop, but I guess this is a common problem. I ordered a jhs little black box also, I’m hoping these two things will help tame the volume a bit for bedroom jamming and lower the noise from the loop a bit. Can’t wait to try it out. Thanks!


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## Bassman1 (Mar 9, 2021)

I had the JHS a little while ago for the MT15. It certainly helps use the amp at bedroom levels. Though, it also picked up a lot of noise/interference in certain places in the apartment. It kinda worked like an antenna haha. 

I ended up getting a Two Notes captor load box. The attenuator in it worked far better, preserved tone, and also provided the option to go direct into the laptop for recording or jamming with a cab simulator. Not bad!


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## guitar_player4_2_0 (Mar 9, 2021)

I had a Bugera PS1 reserved but I got the JHS instead. What I don’t like about the two notes stuff is it appears to not have adjustable attenuation levels. Reading the PS1 manual apparently that only cuts the output in half anyways though. I’ll probably get a Captor X eventually, but for now I’m hoping this does the trick. This amp is crazy loud, had a JCA 50 and a 1st gen 5150 III 50 watt and this seems about as loud as those, and harder to get a quiet volume out of strangely.


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## Bassman1 (Mar 10, 2021)

Getting a quieter volume out of it is definitely tough. But, damn is it nice when playing with a drummer ! 

While you cannot adjust the attenuation on the captor, the -20dB on the amp really works well. I found it far easier to find a usable volume once attenuated on the amp itself. 

As for the JHS, or certainly is a good band-aid fix. I noticed some tone suck, but it wasn't terrible. If you're looking for an immediate solution that might not be ideal but gets the job done... The JHS will do that.


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## Gtan7 (Mar 10, 2021)

The 12au7 info is gold (or maybe prozac). I returned a MT 15 with deep regrets because of the noise, after many hours googling noise gates, etc. One of the official PRS forum "fixes" was to swap out power tubes and rebias. I think i will buy another one, both channels sounded great and of course there's the blue and red lights

mandatory safety link for 1st time tube rollers: if they're exposed DON'T TOUCH the CAPS

https://robrobinette.com/Tube_Amp_Safety.htm


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## Emperoff (Mar 10, 2021)

You can also swap V1, V2 or V3 for lower gain tubes if you want to make the gain range on the lead channel more useable.

V1 affects the whole amp, V2 and V3 only the lead channel.


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## 50W (Mar 11, 2021)

I was expecting the loop issue to be a grounding problem. 
Maybe the issue is in that area still, but you can be reduce the effect by putting a low gain tube in for the loop? Am I totally wrong?
Still good that there is a work around!


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## 50W (Mar 11, 2021)

guitar_player4_2_0 said:


> So glad this thread is here, I just picked up a 12au7 from Sweetwater. I was thinking the noise was coming from the power supply to my EQ pedal in the loop, but I guess this is a common problem. I ordered a jhs little black box also, I’m hoping these two things will help tame the volume a bit for bedroom jamming and lower the noise from the loop a bit. Can’t wait to try it out. Thanks!



Have you tried changing the tube yet?
With a good result?
Cheers!


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## 50W (Mar 11, 2021)

Anyway, I tried this today and it really worked!
I had an old 12AX7 with very low output (probably worn down - don't understand - it makes no noises) put it in and it made all the change I needed. 
I have noticed that there can still be a hum but that is if I: 
- use a pedal which I know have produced hum in other amps. 
- put the pedal in the loop too close to the amp and use poor cables.

So I am really happy that I kept this amp. It has a very special voice, actually something I have been looking for all along I think. And you can play every thing from shiny cleans to honky blues tones and all the way to what I guess it was built for - raw metal. Very cool!


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## Bassman1 (Mar 11, 2021)

That real seems to be the big workout around for a lot of folks. The other issue that comes up commonly is the volume pot. If that doesn't bother you, then the amp really is just a gem. I still love mine!


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## 50W (Mar 11, 2021)

Bassman1 said:


> That real seems to be the big workout around for a lot of folks. The other issue that comes up commonly is the volume pot. If that doesn't bother you, then the amp really is just a gem. I still love mine!



What about the volume pot?
I am not sure I follow. Do you mean it should have been a gain control for clean channel or what? Yes, that would have been great!
But I haven't found any other faults or errors on any volume pots so far. (bought it yesterday)


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## Emperoff (Mar 11, 2021)

50W said:


> What about the volume pot?
> I am not sure I follow. Do you mean it should have been a gain control for clean channel or what? Yes, that would have been great!
> But I haven't found any other faults or errors on any volume pots so far. (bought it yesterday)



He means the amp is stupidly loud since the master volume pot taper is very abrupt (specially for an amp of this wattage).


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## 50W (Mar 11, 2021)

OK, thanks for clarification!
Yes, but I think I can handle it. 
Maybe the volume dropped slightly when I changed that tube?


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## Emperoff (Mar 11, 2021)

50W said:


> OK, thanks for clarification!
> Yes, but I think I can handle it.
> Maybe the volume dropped slightly when I changed that tube?



Maybe, but replace that 12AX7. Completely worn tubes can short and damage components inside the amp. Get a proper low gain replacement that is not worn out. A brand new production preamp tube costs 15$. Don't be cheapo.


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## Bassman1 (Mar 11, 2021)

50W said:


> OK, thanks for clarification!
> Yes, but I think I can handle it.
> Maybe the volume dropped slightly when I changed that tube?




Yes, I meant that the pot isn't linear and goes from quiet to super loud quick. I'm in the same boat as you...it really doesn't bother me. I've had the amp for a year now, and I only have good things to say. 

It handles itself very well with a drummer, and with the right cabinet combination it doesn't need much EQ. It takes pedals like a champ too. 

Now imagine rubbing this in stereo with an Engl Ironball...


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## guitar_player4_2_0 (Mar 11, 2021)

I got a 12au7 from Sweetwater for like $13. Should be here tomorrow, but the JHS black box came today and it works great as a master volume in the loop. The manual said don’t use it with other pedals in the loop, but I’m having no issues running it before an MXR 6 band EQ. Also I tried simply moving the pedals around a bit and that cleared up a LOT of the noise, although there is still some. I’m thinking once I get the tube in it will be almost silent. Thanks for all the advice everyone!


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## 50W (Mar 11, 2021)

Emperoff said:


> Maybe, but replace that 12AX7. Completely worn tubes can short and damage components inside the amp. Get a proper low gain replacement that is not worn out. A brand new production preamp tube costs 15$. Don't be cheapo.



True that. Cheap may become expensive.


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## 50W (Mar 12, 2021)

To keep feeding in experiences for everybodies enjoyment 
I put a Sovtek 12AX7WA - which is a low noise, lower gain tube - in V6 position (the loop) and it works fine as well.
The JnJ ECC83S that was installed from factory is a high gain tube (that I have used in other amps in V1 sometimes also V2 position to improve their gain) and it should never have been installed in the V6 position in the first place. A misstake from the factory.
With that change I have also increased the volume of the amp so now I also have the vol pot problem. But it is OK with me.


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## Wolfhorsky (Mar 13, 2021)

In my case there was no need for tube swap. I don't need it to be dead quiet when I don't play. Maybe I'm old-school. I just use 1Spot power supply for my fx, triple checked ground issues and I use high quality cables (monster, volvox) and it is very subtle. In live setting that delicate hum is unnoticeable.


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## guitar_player4_2_0 (Mar 14, 2021)

Got the tube on Friday but finally got around to putting it in today. I noticed a considerable reduction of the background noise, much less hiss and hum overall when not playing. I switched my decimator off and even running quite a bit of gain it’s almost dead quiet. The amount of preamp gain is unchanged. Using the JHS black box and EQ in the loop I also noticed a bit of overall volume drop. The JHS said don’t use it with any other pedal, but it’s working perfectly with the EQ and I’m not noticing any tone loss. I don’t know if the volume drop is due to a less hot signal going through the loop or if it lowers the amp volume overall, I haven’t tried it without the pedals in the loop. Either way, it’s dead quiet and super easy to dial in great sounds at below speaking volume, which I didn’t think this amp would do stock honestly. On a side note I turned the master up to 10 o’clock just to see what it had, that’s the highest I ever had it. I’m using a recto 112 cab and I was getting worried the speaker wouldn’t take anything past that. Whoa.


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## 50W (Mar 19, 2021)

guitar_player4_2_0 said:


> Got the tube on Friday but finally got around to putting it in today. I noticed a considerable reduction of the background noise, much less hiss and hum overall when not playing. I switched my decimator off and even running quite a bit of gain it’s almost dead quiet. The amount of preamp gain is unchanged. Using the JHS black box and EQ in the loop I also noticed a bit of overall volume drop. The JHS said don’t use it with any other pedal, but it’s working perfectly with the EQ and I’m not noticing any tone loss. I don’t know if the volume drop is due to a less hot signal going through the loop or if it lowers the amp volume overall, I haven’t tried it without the pedals in the loop. Either way, it’s dead quiet and super easy to dial in great sounds at below speaking volume, which I didn’t think this amp would do stock honestly. On a side note I turned the master up to 10 o’clock just to see what it had, that’s the highest I ever had it. I’m using a recto 112 cab and I was getting worried the speaker wouldn’t take anything past that. Whoa.



Yes, I can testify that the choice of tube in position V6 will affect the output volume of the amp (all for the better in this case  ), all according to my statements earlier in this thread.
Good to hear that the amp is working fine for you too now!


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## Gus Posada (Dec 15, 2021)

viifox said:


> You bet! Here's the video.




Good evening @viifox ,

First and most importante of all, *Thank you very much* for the video! I've been looking for something like this since I got my amp back in March 2019.

Is change meant to be at V6 or V5? (as stated on the tube chart bellow)

I've been back and forward with PRS Technical Services, since I had to return my first amp because of a noise (even with the Standby).
I got a new amp with lower noise... but with all the troubles that has been described here by other users.

I was told by PRS Technical Services to explicitly not to mess with the "Phase Inverter Tube" V6, since it controlled all we already know about Power section with the fact that they work at a special output wattage;_ this is the tube V6 you replace in the video next to the Power Tubes, but also mention that it is an Effects Loop tube V5 issue on previous messages... you got me confused there._

I replaced the damned tube with a _12ax7a-c TAD Balanced Phase Driver Selection_ but did not work as expected. Later a mix with different TAD tubes (I live in Germany and it is easy to get those really quick and cheap), without success.
Also modified V1 with a lower gain 12at7 to have a better control of the Clean... but the Volume controls started to have crazy behaviors.
At the end I put the stock tubes without event understanding what actually made them "tick" that other same reference tubes did not :s

Now I will install the _JJ ECC82 / 12AU7 Gold JJ Electronic_ in the V6 position and the _JJ 5751 (~ ECC83) JJ Electronic GOLD PIN_ in the V4 as you suggested, and see what happens.

I really hope you can go through my message and send me some feedback / clarification.

Thanks!
Gus


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## Emperoff (Dec 15, 2021)

I find quite surprising that considering everyone says the FX Loop on these is noisy as hell, they don't give a damn about fixing it. They could just put 12AU7 tubes in there from the factory to cover the issue.

But nope. They don't give a damn


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## Motegi (Sep 29, 2022)

I know this is an old thread but I wanted to share my experience for others that may come here looking for advice. I followed the advice here and bought the jj12au7 and did the swap.

The good - the noise from the effects loop was almost non existent

The bad - Immediately I noticed a significant volume drop and the brilliance seemed to be missing a bit from the clean channel. I did a decibel check, put the old tube back in on the same settings and did the same check and the meter confirmed there is a 5-6 decibel difference. Readings were in the 77 range with new tube and 83 or so with the old tube.

This was all playing at bedroom levels, I turned it up a bit to try and compensate but the cleans just didn’t sound the same. I am not sure if I could have gotten the same sound had I really cranked it, but at this point I prefer the minor hiss and hum over the volume drop and different clean sound I heard with the 12au7 tube.

This amp seems to be sensitive to the cable placement, to get the least amount of noise I followed the instructions in the manual and tied the send and return cables together about 1 ft from the amp and then made sure to route them to the right away from the power cable and transformer. There is still noise and it’s annoying if you focus on it, but if you just play and don’t worry about it you hardly notice after a few minutes. My 68 custom deluxe reverb has about the same level of annoying hiss and it doesn’t even have an effects loop. 

I am also a bit curious about the original poster of this solution saying they had gone through 3 of these amps. Was there some issues with the amps after the tube swaps or did they just decide they wanted one and then didn’t want one 3 different times, gave some away as gifts perhaps lol?


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## Motegi (Sep 29, 2022)

I am also confused by the official prs diagram. Is the effects loop tube the stand alone tube close to the power tubes or is it the last tube to the left when looking straight at the amp? Have we all been changing the phase inverter tube and not the effects loop tube? I emailed prs to ask.


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## Motegi (Sep 29, 2022)

Prs initial response, I am trying to get clarification on the effects loop tube placement still:



> Thank you for writing PRS Guitars. The MT15 packs a punch with multiple gain stages, components and transformers in close proximity to each other can cause some noise. Cable placement can make a surprisingly large difference with this amp. Please make sure you do not have pedals sitting on the amp or right next to it which can induce some interference. We recommend using straight cables as 90 degree cables rout the wires extremely close to the back of the amp and tend to induce hum much more easily. Hooking up multiple things through the same circuit (outlet), the amp, power conditioner, etc can cause a ground loop or interference and would need to be isolated from ground. You could try replacing the effects loop loop preamp tube with something a 12AY7 or 12AU7, which is a lower-gain tube but preserves other properties that matter to the circuit.


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## Motegi (Sep 29, 2022)

Well no word back from PRS on the tube placement yet so I tested the 12au7 in the front left 12ax7 spot, seemed to have the exact same effect as swapping the tube near the power tubes. I am talking to myself now on a year old thread so I will just leave it at this for anyone that ends up here In the future. I’m keeping the original tubes in.


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## Motegi (Sep 29, 2022)

Official response from PRS:

*The effects loop tube is the last tube in the front preamp tube row, on the opposite end of the input.*

So it appears previous people were mistaken here about which tube was the effects loop tube, everyone has been changing the phase inverter apparently lol


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## Nicki (Oct 1, 2022)

Bassman1 said:


> Yes, I meant that the pot isn't linear and goes from quiet to super loud quick. I'm in the same boat as you...it really doesn't bother me. I've had the amp for a year now, and I only have good things to say.
> 
> It handles itself very well with a drummer, and with the right cabinet combination it doesn't need much EQ. It takes pedals like a champ too.
> 
> Now imagine rubbing this in stereo with an Engl Ironball...


The master volume pot on the amp actually IS a linear taper. I though initially it had a wierd taper on it and brought it to an amp tech. I asked to have it swapped to a linear taper only to be told it already was a linear taper and if I wanted controllable volume, it would need to be swapped to an audio tapered pot. So that's what I ended up doing. I made a thread on it in this forum a while back. It's in my post history somewhere but I'm sitting on the porcelain throne with my phone right now so I'm not gonna dig for it lol.


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## guitar_player4_2_0 (Oct 1, 2022)

I’ve since sold my MT-15  but I can confirm it was crazy sensitive to cables producing noise. I had an EQ and JHS black box in the loop and had the cables hanging over the top of the amp. TONS of noise, was driving me crazy. Moved the cables one day and it totally stopped lol. 

I wish I had it to look at where I put the 12au7, but I’m pretty sure it was V6 lol. Oh well, the dude who bought it tried plugging it into a combo amp instead of a cab, I’m sure tubes are the last thing on his mind rofl.


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## profwoot (Oct 1, 2022)

I'm about to sell my MT15 due to the fx loop noise (and I just bought a 5153...), so this thread had me excited to try to salvage it. Then the thread continued and I got confused.

If I go ahead, any reason to use a 12au7 in V5/6 and a 5751 in V3/4? Could I just use two of the same tube?


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## Motegi (Oct 1, 2022)

profwoot said:


> I'm about to sell my MT15 due to the fx loop noise (and I just bought a 5153...), so this thread had me excited to try to salvage it. Then the thread continued and I got confused.
> 
> If I go ahead, any reason to use a 12au7 in V5/6 and a 5751 in V3/4? Could I just use two of the same tube?


I guess it depends on what you are trying to do. If you want to tame the gain a little it sounds like the 5751 in v3 is worth a shot. I wouldn’t put a 12au7 in v3 though As that might take too much of the gain away or cause some issues. If you want to get rid of some noise in the effects loop then use a 12au7 or a 12ay7. I am assuming 12au7 will reduce noise more as it has a lower gain factor, but haven’t tried a 12ay7 yet.

*The 12AX7 has a gain factor of 100, while the 12AT7 has 60. 12AY7 has 40. The 12AU7 has the smallest, at just 20.*

I went ahead and put the 12au7 back in the effects loop at v5 and it really does cut down on the noise, I don’t notice so much of a tone drain either at the moment. But I did have to turn the volume up more after the swap. I was able to get the clean channel to sound really close to my 68 custom deluxe reverb, so I will probably sell the deluxe. I have a Princeton reverb that I use as well. I just ordered a few more tubes to play around with it, a 5751 for v3 to see how it affects the gain and a 12at7 for the phase inverter at v6 as I have seen In other videos. Also bought a 12ay7 as prs suggested and will see how that is in the effects loop vs the 12au7.

This amp is a pretty good deal as the head plus cab cost less than most other lunchbox heads out there, so I am going to mess around with it a bit. I bought a Mesa mini rectifier and matching 1/12 cab before the MT15 and I didn’t like the sound at all and returned it, it was almost 1k more too!


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## Motegi (Oct 2, 2022)

guitar_player4_2_0 said:


> I’ve since sold my MT-15  but I can confirm it was crazy sensitive to cables producing noise. I had an EQ and JHS black box in the loop and had the cables hanging over the top of the amp. TONS of noise, was driving me crazy. Moved the cables one day and it totally stopped lol.
> 
> I wish I had it to look at where I put the 12au7, but I’m pretty sure it was V6 lol. Oh well, the dude who bought it tried plugging it into a combo amp instead of a cab, I’m sure tubes are the last thing on his mind rofl.


Any particular reason you sold it? Just curious.


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## guitar_player4_2_0 (Oct 2, 2022)

Motegi said:


> Any particular reason you sold it? Just curious.


It’s been a bad year financially. Been through a couple jobs and sold most of my gear. I’ve had a bunch of amps over the years and the MT-15 was def one of my favorites, not even considering the price. I’m really hoping the MT-100 is import and under 2k, because I really want one. I just hope they work out the loop noise a bit better.


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## Motegi (Oct 2, 2022)

guitar_player4_2_0 said:


> It’s been a bad year financially. Been through a couple jobs and sold most of my gear. I’ve had a bunch of amps over the years and the MT-15 was def one of my favorites, not even considering the price. I’m really hoping the MT-100 is import and under 2k, because I really want one. I just hope they work out the loop noise a bit better.


Sorry to hear that man. Hope things start looking up for you!


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## Nicki (Oct 3, 2022)

It's so interesting that people are having the fx loop be the cause of the noise. When I took mine back to the dealer over how loud the noise on mine is, after they talked with the PRS rep, I was told it's basically because they have the 50w archon preamp circuit design and didn't space the components out enough to prevent them from spilling a bunch of electromagnetic noise into each other.

Now mind you, I'm running mine through a Fender Bassbreaker 112 which is a cheap and noisy can for such a high gain head. When I tried mine through the matching PRS cabinet, it was way less noisy so cab components definitely have an effect. I should really try mine in my Orange 212 to see how noisy that one is.


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## rbdoggface (Oct 6, 2022)

I did the 12AU7 swap yesterday and it helped a little. Went from a VERY annoying hum to a mildly annoying hum. Then I tried some other tips PRS support told me to try- use shorter cables with no right angles and secure them together (ziptie, velcro) to limit interference. The cable change made a big difference. I am going to get cables as short as possible and better quality than the Amazon specials I have now. With these changes, it is no worse than the hum you get out of many high gain amps turned up.


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## Emperoff (Oct 6, 2022)

rbdoggface said:


> I did the 12AU7 swap yesterday and it helped a little. Went from a VERY annoying hum to a mildly annoying hum. Then I tried some other tips PRS support told me to try- use shorter cables with no right angles and secure them together (ziptie, velcro) to limit interference. The cable change made a big difference. I am going to get cables as short as possible and better quality than the Amazon specials I have now. With these changes, it is no worse than the hum you get out of many high gain amps turned up.



Ok, so the solution is not being able to use long cables? Excuse me but that is lame as fuck. Why not fix the issue everyone has been complaining about since the amp was released instead? 

There are a fuckton of lunchbox-style amps and this is the only one with this issue.


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## rbdoggface (Oct 6, 2022)

Emperoff said:


> Ok, so the solution is not being able to use long cables? Excuse me but that is lame as fuck. Why not fix the issue everyone has been complaining about since the amp was released instead?
> 
> There are a fuckton of lunchbox-style amps and this is the only one with this issue.


I don't disagree. Unfortunately there are too many products these days with the same types of "workarounds" straight out of the box. I am sure there is a factory level solution to the problem.


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## MatrixClaw (Oct 6, 2022)

Granted, I don't use the FX loop much, but I plugged a delay into mine this afternoon and it's dead silent with the stock tubes


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## rbdoggface (Oct 6, 2022)

MatrixClaw said:


> Granted, I don't use the FX loop much, but I plugged a delay into mine this afternoon and it's dead silent with the stock tubes


I know. I hate being one of the people that has the bad luck. I’m tired of dealing with it with so many electronics. Guitar amps, computers, TVs. Why are so many manufacturers oblivious to problems so many users of their products have? PRS do say that I could send it in because it is under warranty. But I have read of other people who got replacements that had same issue. Been there done that with other products.
The thing with this amp is- I really like it and the loop is the only issue. Honestly, I have never had an ear good enough to notice the difference in pedals in the loop versus all through the front, anyway.


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## Motegi (Oct 7, 2022)

rbdoggface said:


> I know. I hate being one of the people that has the bad luck. I’m tired of dealing with it with so many electronics. Guitar amps, computers, TVs. Why are so many manufacturers oblivious to problems so many users of their products have? PRS do say that I could send it in because it is under warranty. But I have read of other people who got replacements that had same issue. Been there done that with other products.
> The thing with this amp is- I really like it and the loop is the only issue. Honestly, I have never had an ear good enough to notice the difference in pedals in the loop versus all through the front, anyway.


I can tell a big difference with just a delay before the loop vs in the loop on the dirty channel. It’s almost unusable in front as it seems to create this washy echo effect. Before the clean channel everything is good.


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## rbdoggface (Oct 7, 2022)

So I tried a different tube swap that was recommended in another group. Put the 12AU7 in the phase inverter slot. Problem is pretty much gone now.


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## profwoot (Oct 8, 2022)

Can confirm that the fx loop hum is reduced quite a bit more by changing out the phase inverter tube (V6) than the fx loop tube (V5). I don't know enough to speculate why that would be.


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