PRS MT15 - Finding and Fixing the Noisy Bits

Lots of hum and buzz from the MT15. This is what I did to fix it.
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[@vincelupone]

Back in 2019 I bought one of these and talked with PRS to have the FX loop repaired under warranty. The amp was fantastic except when anything was in the loop, which had the notorious hum issue. PRS tool the warranty repair, sent the amp back to me quite fast a few weeks later with a list of what they fixed on it, including a complete revamp of the FX loop circuit. I hooked everything up, turned it on, same hum. No difference at all. I didn't keep the amp for much longer and considered it a design flaw. It was great to see you took a look inside, the part about running signal and heater wire held together with a zip tie blows my mind, and you did fantastic work in seeing and rooting out the problem. Thank you for a great video!

[@amp_mechanic]

Thanks Vince.

[@K707OR30]

I had an Archon 50 (mkii) that had this same hum with anything plugged into the effects send. PRS screwed me around on the warranty saying they couldn’t replicate the issue. I watched a video recently on Psionic Audio’s channel with a Sonzera and he was commenting that the transistors they used in that amp for the effects loop combined with the topology/execution of the effects loop circuit, it would always have a lot of noise unless the effects loop circuit was fundamentaly changed. The Archon mki had such a pervasive effects loop hum issue that they warrantied a bunch and eventually just incorporated the fix for that into the actual manufacture of later mki Archons. I love PRS amps but I don’t understand why they hell they can’t manufacture an amp with an effects loop that won’t do this. Everyone else, including “lesser” brands have this figured out.

[@JasonGutierrez]

I just fixed this issue on a friend’s amp. The FX loop hum issue is mainly the design flaw of the ground loop created by the black and red ground wires connected through the circuit boards at both ends of the path to and from the effects loop. Once you remove one of those ground wires (I removed the red one to keep black as ground) the problem is solved. Twisting and cleaning up the heater wire routing should be done as well to further quiet things down and keep those AC wires away from the effects loop signal wires. The shielded leads are unnecessary. Just remove the extra ground wire and cleanup the heater wires and problem solved.

[@amp_mechanic]

This one did need the shielded otherwise I wouldn't have used it. Some are worse than others. Good you got it fixed for him.

[@JasonGutierrez]

@@amp_mechanic did you try it with the cover back on the amp before adding the shielded wiring?

[@BazonBlades]

I have a newer 5881 version of the MT15. I have no noise issues with my example. It is absolutely dead quiet on standby. As it should be. Switched on I only have the typical cascading high gain noise floor, which is actually not that bad compared to many very popular high gain tube amps. Over the last 30 plus years I have owned 10 or 12 high gain tube amplifiers. I know what noise sounds like. My MT15 just isn't that noisy. Lucky me, I know.

[@Soloist1983]

Mine is the 5881 version as well, 2 weeks old. Definitely a bit of noise. It hums in standby and has a lot of FX loop noise

[@Luismanpr]

How could I know which version is the mine one? Thanks, guys!!

[@BazonBlades]

@LuismaRamos , just look at the power tubes. Older versions have 6L6 power tubes, and newer versions have 5881 power tubes.

[@royrobles7423]

I got my prs mt15 Friday March the 29th it is the 5881 model...it has absolutely no noise at all...thankfully !

[@BazonBlades]

@@royrobles7423, awesome to hear....

[@ederiel]

I have the same issues on mine, I swapped 2 tubes. The phase inverter by an 12AT7/ECC81 and le loop by an 12AU7/ECC82 and all the noise are gone.

[@lovecraftmusic8717]

Lost any gain/saturation?

[@ederiel]

@@lovecraftmusic8717 No, it sounds great, it just clean everything You can ear a short sound clip on my last video, in a full mix.

[@lovecraftmusic8717]

@@ederiel yep, still sounds good, thanks.

[@vanespedals1023]

Can someone confirms that using lower gain preamp tubes in those spots fix the FX hum?

[@lovecraftmusic8717]

@@vanespedals1023 i haven't need to... Mine resulted pretty quiet. But to be honest I haven't tried the fx loop yet.

[@royrobles7423]

I got my amp on friday march the 29th...it is 5881....absolutely dead quiet...i am using the fx loop....not using the second channel tho

[@amp_mechanic]

Great!

[@Soloist1983]

I just bought this amplifier and was definitely surprised that it hums while in standby. Considering exchanging it for an ENGL Fireball 25 instead. I mean, I do really like the way it sounds, but dealing with noise all the time kind of sucks. If it had a built-in noise gate, it would be less an issue, but as it is it's quite annoying. Thanks for the tips, I was scouring the internet for info like this for the first week, wish this had been out sooner, but hey, at least I only had to wait a week :)

[@ivanzivanovic4013]

Lol, yes, that’s why the Engl is almost 3 times the money on the used market. I love the MT15. It’s meant to be a super aggressive amp. would expect noise, and you would also expect some corners cut due to price. But yeah, sure if you want to spend the three times the money buddy. Go for it. Also, the other amp tech on KZbin said this is one of the best built amps. He’s seen. Also, that guy according to his videos seems way more experienced. So I really don’t know who to trust. If you’re going to say, trust the noise, lots of different ants have lots of different noise for different types of issues. You have $3000 Mesa boogie’s that have exorbitant amount of issues. So I guess it’s all in the eyes of the beholder.

[@Bandersnatch786]

The issue is the circuit layout being based off of the Archon but without any consideration for the physical space constraints of a lunchbox. The circuit itself would have been fine but they needed to tweak how parts are layed out on the circuitboard to avoid the electrical interference being picked up by the wires. Most of the time, when you look in any amp, wires get routed on the outer perimeter, or they're generally run away from the circuitboard in the shortest runs possible. The innerds of the MT15 are just a complete mess. Don't even get me started on the linear tapered master volume pot lunacy (I had mine switched to an audio tapered pot to make the volume USABLE). With that said, a noise gate isn't going to solve this problem, internal or on a pedalboard. Believe me, I've tried. The amp sounds great, but the circuit design makes it fundamentally flawed.

[@fabianvasquezjr8852]

Boss ns-2 worked wonders for noise issues in my mt 15 but only when I run it through the fx loop from send/ return on ns-2

[@fergeraci]

Excellent video, thank you very much! Talking about amp design/manufacturing - my primary amp for gigs/record is a Mesa Mark Vii which ironically is at the "doctor" right now (back to Mesa) due to a microphonic component in the reactive load - I got this little fella as a practice amp and $ aside, I am SO sorry PRS decided to use cheap manufacture/assembly for it because this sucker punches above its weight big time. The store had two new in-box... I tested the first one before taking it home at the store and the noise was IMPOSSIBLE... I almost backed out altogether from it but decided to give the second one a shot and now is home with me. No noise compared to the first one. Probably there is a very sound $ reason why PRS decided to cheap out in such a great product - this amp could be easily playing in the Friedman JJ 20w league, which is ~3x its price.

[@amp_mechanic]

Thanks! It really seems to be a consistency issue. As you note, some are fine. It is unfortunate.

[@artytoney2057]

I just bought one of these head , came across a newer at a great deal but of course mine had a terrible hum through effx loop , I took it to a amp tech and hope he can fix but my question is , why in the world does a company like PRS ,who does awesome quality control with their guitars , why they don't correct this issue especially for such a popular high gain amp , it don't make any sense , thanks for great info

[@armymankr]

Wow I glad I came across your video! Was considering buying this amp but guess I'll stay away now.

[@josephkirk939]

I own this amp and it's a freaking powerhouse. I have had mine for 2 years and the only issue I've had are with the blue led lights on the clean channel. This is just one video on repair. Go try out the amp and see for yourself. It's great for a lunchbox.

[@jeffvanbibber6609]

I would not let that stop you from getting this amp. You're missing out if you do. Imo

[@rollirolli2669]

you replaced the wiring of the effectsloop with shielded cable and connected the shield with signalground only on the "mainboard". What about the ground connection of the send- and return sockets, they now have no connection to the signal-ground. Will this work?

[@amp_mechanic]

Yes.

[@danz4904]

​​@@amp_mechanic it looks like the effects loop jacks are isolated from the chassis. Is this correct? If so, how does the send & return jack get ground for their sleeve connection after the wiring modification?

[@amp_mechanic]

@@danz4904 One of the wires is the ground.

[@mjtan6787]

I got the new version with the 5881 tubes and the noise seems to be fixed now

[@amp_mechanic]

Good to know.

[@roberteismann1929]

I received mine last week, February 2024, and it has no noise issues whatsoever. I also do not understand why the effects loop should add to the noise when something is plugged in, as it is always on anyway, the send and return socket are normalled through, so the preamp's sound will always go through the send and return sockets, even if no effect is in line. So maybe the problem is where your effects cables run? Anyway my amp is completely quiet, even with a TC delay in the loop

[@paulauton856]

It sounds like they fixed the issue. I had 2 when they originally came out. The hum was bad enough to make my fillings ache.

[@royrobles7423]

I got mine march 29 2024...it is totally quiet...so far

[@PK22-b7v]

This is what happens when an architect turned amp designer, designs an amp lol Ridiculous that PRS has not bothered to rectify and fix this well known issue.

[@richardmcgruder5437]

I wonder what mark tremonti opinion is now the noise is a issue after he talked a big game on his amp mt 100 🤨🤔

[@pieve5516]

Great video. Any ideas about excessive hum on just the lead channel?

[@richardmcgruder5437]

Not to mention how much time it took to develope this amp his amps cause he wanted it just right from all the promotional vids. He did i almost bought one however im glad i didnt at least till 5hey fix these issues😢

[@bradclark535]

Great video. Where are you located? Always good to know. A good amp. Tech if I can get your info.

[@amp_mechanic]

H Brad. I'm in Nova Scotia.

[@graemedixon6314]

Hi, i have a 6505mh. After it warms up 15 minutes playing there's a low frequency hum. Any ideas what the problem could be?

[@amp_mechanic]

Check the bias, calculate the actual plate draw. Most of the time I've found the bias will be running hot on at least one or more tubes. There are plenty of other thing it could be but, that's usually my start point. It can also be a bad preamp tube. Easiest way to narrow that down is swap with a know good one and move down the line.

[@Sperzel]

I just don’t understand why they insist on putting the fx loop next to the power transformer. Great job btw!

[@amp_mechanic]

Thanks. Yeah, there's several oddities in the design. Sometimes what works for a prototype or two doesn't work when it comes to full scale production units.

[@Sperzel]

@@amp_mechanic yeah, it’s weird that they didn’t just do some final tweaks and end up with an amazing product. I have the same issues on mine. I’m guessing this operation is not for someone inexperienced? I know how to solder but not worked on amps before.

[@amp_mechanic]

@@Sperzel If you can dress your leads properly and have good heat control, it's not hard to do. I guess you have to decide if you do get stuck or mess it up, do you have an option to get it to someone who can fix it.

[@donjohnson5661]

I keep wanting to buy this amp, but the fact that this isn't even the only video on killing the noise means no thanks

[@azanfour1287]

The clean channel is usually silent, and you can remove the lead channel hiss by connecting a noise gate to the effects loop

[@AJC-jo3ds]

You didn’t really turn the master lead volume up past 1/4…..that’s when these really get noisy with the hum

[@amp_mechanic]

I did for testing it. That may have been off camera. I had shot a clip up about half way but mic was clipping and I didn't feel like grabbing another mic and gaining it down so I just turned the master down and re-recorded.

[@mlonghammer8388]

Those Chinese children were soldering the best they can! How well could you solder when you were 6?

[@amp_mechanic]

oh,well, pardon me....lmao. 😂

[@HooksBill]

Bought this amp before watching all these videos which is unusual for me. It's not here yet, but when it arrives I'm not going to open the box. I'm sending it straight back. A brand new amp with noise like this and the fact that imo after listening to one yesterday, it sounds like a fart can. The cleans are lifeless and uninspiring. It doesn't do rock very well. Even with pedals it sounds like a fart can. It's only good for modern metal, but even then it still sounds like a fart can. I wasted my time buying this. Advice: Don't believe 90% of the demos/reviews you see on KZbin. Just content creators trying to get paid. I'm glad this video showed the lack of attention to detail that was put into these fart cans. At least he got it quieter. A less noise fart can.

[@amp_mechanic]

Yeah, it's not up to PRS standards unfortunately. Hope you can find something to suit your needs.

[@TheMirrorify]

I'm not a content creator, I have this amp paid for with my own money. I have the newer 5881 version, swapped the fx loop tube for a 12au7 and its very quiet. The tones are phenomenal. I get it's annoying to have to swap out a tube in a brand new amp but to me it's worth it.

[@bobbyosborne2375]

I've got one. It's noisy, but it sounds ridiculous. Good clean tone too. 🤷

[@willian5452]

@@TheMirrorifygreat. Did you fix the noise on lead ?

[@TheMirrorify]

@@willian5452 I actually had to return my mt15 unfortunately. I think I was convincing myself the noise level was ok because I loved the tones so much but the longer I had it the more it annoyed me. Mine was a hum type of sound, swapping the tube out did lower it but it was still humming at around 30dB constantly. Tried different rooms, hum eliminators, different cables and guitars, nothing worked. The retailer said they were so sick of them coming back they were considering not stocking them anymore.

[@andreriant8513]

Stop saying 120 hertz please 🤦‍♂️