PRS MT 15: FX Loop Hum Solutions

[Bassman1]

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...

 

[guitar_player4_2_0]

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!

 

[50W]

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.

 

[50W]

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.

 

[Wolfhorsky]

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.

 

[guitar_player4_2_0]

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.

 

[50W]

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!

 

[Gus Posada]

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

 

[Emperoff]

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

 

[Motegi]

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?

 

[Motegi]

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.

 

[Motegi]

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.

 

[Motegi]

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.

 

[Motegi]

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

 

[Nicki]

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.

 

[guitar_player4_2_0]

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.

 

[profwoot]

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?

 

[Motegi]

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!

 

[Motegi]

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.

 

[guitar_player4_2_0]

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.