Is there a fix yet for the MT15 noise/hum?

Bump!

Long time Mesa player, I received my first PRS amp today, the MT15, and I'm having the hum issue. For mine, it's not a tube, it's the effects loop.

When you plug a guitar into the input with nothing in the effects loop, the amp performs perfectly. The clean channel has zero noise. However even putting a patch cable in the effects loop elicits 60Hz hum immediately. Running anything in the loop, pedals, rack effects, other preamps, or just a cable = 60 Hz hum.

It's not a tube or the transformer, otherwise it would be making the noise with nothing in the loop. So is there a fix for this? The amp sounds amazing and this is a fix that would make the amp nearly perfect.
 
There is a very good chance that ther FX loop tube is bypassed when there is nothing in the loop. Try swapping V6, that is the FX loop tube.
 
Just tried it, swapped out V6 for another 12ax7 I had here in the house, and it didn't fix the issue. So it's not tubes. I'm wondering if it's a bad ground in the effects loop or if there's an internal part causing interference.
 
The patch cable from FX send to return probably rules out external sources. It's possible that there is a loose ground connection in the FX loop circuit, I had that happen on two amps. But it seems consistent with the other amp owners' experience.

Best to contact PRS directly.
 
Miscellaneous response, about other, but similar things...A couple people mentioned placing pads between the heads and cabinets, and Les' pucks, which look pretty cool.
I have been folding up a Yoga mat (about $5.00 at the general store "5 Below") to put between my bass heads and cabs. But mostly due to limited space in my dungeon.

If you have room, most of what I've read states its better to keep heads off of their cabinets, as to minimize the vibrations, as Tubes can be very sensitive, especially when warmed up.

PRS has always been good about fixing issues, so I assume they will be doing something to correct the ongoing MT-15 issues.
 
The patch cable from FX send to return probably rules out external sources. It's possible that there is a loose ground connection in the FX loop circuit, I had that happen on two amps. But it seems consistent with the other amp owners' experience.

Best to contact PRS directly.

SO here's an oddity. When I use the FX loop on the MT15 with a speaker cabinet, I get the 60 Hz hum immediately, however when I run the output to a Torpedo Captor and into my PC, no hum from the FX loop.

I'm stumped. I guess it's awesome for recording and would be fine at a gig, but lower moderate volume playing gets the hum.
 
I use an ISP Decimator II GString to help reduce the hum noise, which is effective but chokes the sustain a little. My lipl send and return runs through it as well as direct to the front input. Hope this info helps some.
 
I use an ISP Decimator II GString to help reduce the hum noise, which is effective but chokes the sustain a little. My lipl send and return runs through it as well as direct to the front input. Hope this info helps some.
so I tried a preamp tube swap out as suggested in the forums and put a mullard at7 in the pi position and a Jan GE 5751 in the V3 position. the only change was a reduction in gain not a reduction in the hum noise, which I find is more prevalent when I am connected to the effects loop. thinking of trying the Jan GE in v6 position. thoughts?
 
I’m posting this to help others with the hum issue. If you’ve tried the usual process of elimination, and you still have hum, you will have to send it in to PRS service to get it fixed.

The PRS service dept. is excellent, and they were very helpful and pleasant to deal with. My amp is on the way back from them, and they said they “revised the ground scheme” and “power transformer lead dress has been updated” , and that my amp is repaired, sounds good, and is in spec.

I haven’t got the amp back yet to confirm, but I have faith that they fixed the hum.

Obviously not everyone is having this issue, but if you do, and you’ve eliminated the usual suspects, I advise you contact PRS.
 
Being that the first post is from 2018, does anyone know if this is a common problem among the last batches of MT15s?
 
Hello,
Signed up to share my own experience :)

I tried the MT15 in January and was blown away, So bought it the next chance i got and it was my first tube amp :rolleyes:

Once home and in the office/bedroom environment and plugged into my cab (112 celestion vintage 30 @8ohm) I noticed a buzzing while only on standby regardless if the guitar was plugged in or not.
I borrowed a 16ohm 112 cab and there was no buzzing.

So I took it back the store and plugged into a 4x12 8ohm but had no buzzing (at least it couldn't be heard) so i requested a 112 @8ohm cab and sure enough it buzzed!
Again this was all only in standby...
so in the end I returned the amp :(

Im now debating weather to get another MT15 or the 5150 lunchbox. :confused:
 
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so I tried a preamp tube swap out as suggested in the forums and put a mullard at7 in the pi position and a Jan GE 5751 in the V3 position. the only change was a reduction in gain not a reduction in the hum noise, which I find is more prevalent when I am connected to the effects loop. thinking of trying the Jan GE in v6 position. thoughts?
waste of time, IME hum comes from the circuit not the tubes. Tubes might have a little noise, hiss, pops and farts or be microphonic but you aren't going to fix hum in a modern circuit board amp by rolling nos tubes
 
I purchased an MT15 early on when they came out and heard about the noise issues. I plugged it in for the first time and it was super noisy for all of about 30 seconds, and then the buzz went away and it’s been totally quiet, like lead channel cranked with barely a hiss quiet ever since.
 
waste of time, IME hum comes from the circuit not the tubes. Tubes might have a little noise, hiss, pops and farts or be microphonic but you aren't going to fix hum in a modern circuit board amp by rolling nos tubes

That information is incorrect. It depends on whether the hum is 50/60 cycle hum (buzz) or 100/120 cycle hum (a higher-pitched hum). Of the two, 120 cycle hum is the most difficult to eradicate. I just created a thread on 50/60 cycle hum. If amp has buzz, then a preamp tube swap can often eradicate them. However, one cannot use just any tube. The tube has to have a spiral-wound filament.
 
UPDATE - I had a local tech take a look at my MT-15 and it turned out to be a ground wire issue. Simple an cheap fix. The awful hum is gone and the only noise is the normal high gain hiss on the lead channel. I put my ISP Decimator in the loop and its dead quiet.

Loving this amp now!
Glad to hear that! Enjoy!!!
 
UPDATE - I had a local tech take a look at my MT-15 and it turned out to be a ground wire issue. Simple an cheap fix. The awful hum is gone and the only noise is the normal high gain hiss on the lead channel. I put my ISP Decimator in the loop and its dead quiet.

Loving this amp now!
Nice!
 
Hey all. There is a local guy selling one for a decent price. Aside from asking him and hearing for myself in person to see if him exists, is there anything else I should ask or hear for myself? Is there any evidence that the newer produced MT15's have been rectified out of the box, or if the older productions have a sure-fire fix? Before I drop many dimes on this thing, I don't want to get stuck with a problem and no fix.

My rig runs a Boss NS-2 and I use the gate in a way that uses the FX loop in a creative way, which sounds to my exact liking. So having an amp with a FX is essential requirement, preferably without a hum.

Finally, just curious for those with a hum, have any of you tried using a load box of some sort just to see if there is a change?
 
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