MT15 strange noise kicking in (no FX loop issue)

Goran Lorencin

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Joined
Feb 27, 2021
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Ljubljana, SLOVENIA
Hi there everybody, well after almost a year of owning the small beast and trying to solve the FX loop (which I still did not fix) this happened ... any ideas as to what is happening here?

 
Well, now is looks like FX loop issue of some sort. Getting constant buzz and big drop in guitar signal, switching half power switch does nothing ... BUT when I bridge the FX loop with regular 3ft guitar cable everything seems to be fine, no buzz and guitar signal normal. But I can not use the FX loop the way you are supposed to use it :)
 
Have you checked the tubes?
I have, swapped, turned and so on ... but the "proof" of normal operation with the bridged FX loop tells me that it must have something to do with the FX loop not being grounded properly or maybe that it has a bad jack plug or that ground somewhere else needs to be redone, all basically minor things, just have to get them fixed. Also measured the buzz freq and it is 50Hz, EU mains freq. It is strange that the buzz started to appear sporadicly, also with the use of the 1/2 power switch, so it just did not dawn on me to check the FX loop loop.
 
I have, swapped, turned and so on ... but the "proof" of normal operation with the bridged FX loop tells me that it must have something to do with the FX loop not being grounded properly or maybe that it has a bad jack plug or that ground somewhere else needs to be redone, all basically minor things, just have to get them fixed. Also measured the buzz freq and it is 50Hz, EU mains freq. It is strange that the buzz started to appear sporadicly, also with the use of the 1/2 power switch, so it just did not dawn on me to check the FX loop loop.

Often that means a ground loop somewhere in your signal chain. Or...maybe the amp needs to be looked at by PRS. What do I know? Not much.
 
Often that means a ground loop somewhere in your signal chain. Or...maybe the amp needs to be looked at by PRS. What do I know? Not much.
I hear you Les, and right you are. But you probably did not read my post about me finding out that when bridging the FX loop all is back to normal and the one where I talked about the PRS Europe being so kind and offering to send the MT15 schematic to a service shop of my choice (in my country, as the one in Germany is still quite far from my place). So I'm getting closer and closer to getting it fixed :) and maybe even a bit better hopefully, as it would also be great to get rid of the switch pops when turning it on and when using the 1/2 power switch. Looks like I'm learning about tube amps and electronics along the way, which is also a good thing,
 
UPDATE

Well ... it looks like it was an FX loop issue in a way, but the kind where I had to replace the FX loop jack sockets ... will see tomorrow if I solved the issue. It sure sounded like there was no noise when I turned it on two hours ago, right after replacing the sockets. It seems that the original sockets did not make a strong enough contact, these new Neutrik sockets make a much better connection.

--------------------------- the next morning -----------------------------

Yep, just played my little beast, silent as a sleeping kitten. With and without FX loop. Hope this is the end of my torment ... and did not need to take it to a service shop, well done Goran. Although I must say it would help to have some experience with desoldering pcb mounted parts beforehand, I would have done it in half the time, but now I have experience ;)
 
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Hi Goren.

I signed up just to ask you some questions :) I recently got a MT15 in a trade and it's got a good amount of hum.

Did yours have any noticeable hum when turned on with nothing connected (except cab)?

I've read a few of your posts regarding the MT15. This post where you mention you swapped 3 tubes and most of the noise went away, is this referring specifically to the FX loop noise or the general hum without anything connected?

I also get very noticeable pop sounds when switching between full/half power and switching to standby and turning the amp on/off. Do you think these are related to a grounding issue?
 
Hi there Weirdoku,

congrats on your purchase :) hope it'll give you much pleasure.

First about swapping tubes, if you have a bad tube, there is a chance of getting some hum, but those predictions were in the early days of me owning the little beast. To rule out a bad tube issue you can visually inspect all tubes, watch the filament when turning on the amp and roll them to see if there are changes in sound when you try out different tubes in a specific position. I think I learned a few things since then (and that is just a few) and found out that there was a bit more to it than just swapping tubes to resolve my issues. True that you can have microphonic (kinda more sensitive) tubes that can generate some noise, but those are usually large plate tubes of which there aren't any in the original configuration of the MT15. There really should not be much difference in the amount of hum on the clean channel if you use for example a 12AT7 instead of 12AX7 or a different brand of 12AX7, there probably will be a bit less noise on the lead channel as the 12AT7 (or 12AY7, 12AU7) is a lower gain tube than the 12AX7 ... but that usually is not the cause of the more prominent hum/noise that should not be there. Keep in mind that there will be a lot of noise on the lead channel if you have the gain knob turned to the max and the master above 20-30 percent.

Just to be clear, if you are in a "special tube/sound searching mindset" it can appear as if you can hear some differences in sound and you'll say "wow, I'm on a path to the ultimate amp sound" ... and in the end you'll find out that you were swapping the tubes in V5 instead of V1 (as I did) and will feel like a fool because you spent 2-3 hours of rolling different tubes in the FX loop position (V5) and not the input position (V1) because you thought you really heard THA difference ;) ... like I did. I really felt stupid. V1 is the closest to the input jack. And so it sometimes happens that someone writes false positives on forums.

In my case it could also be about being in a different room/building when trying out stuff and using a pedal board and so on ... you should always do all the testing in the same place, with the same gear ... I had some discrepancies in my journey that could have made some difference, but I was still too green to know that.

To make a long story short ... there are a few things to check before making conclusions:
- first start with good mains ground, look for possible ground loops, look for other electric devices near the amp, those are very basic things
- make sure that the cables are all good, all the tubes are well seated in the tube sockets
- check if everything is firmly connected ground wise on the inside (disclaimer: you are doing it at your own risk, there are deadly voltages present in specific circuits of the amp, but you can leave if off for a while with standby engaged to drain the caps, there are other methods of doing safety checks as well ... read some posts, watch some YT videos)
- try the FX loop trick, bridge the FX loop with a patch or guitar cable and see if the hum goes away ... in my case it was exactly that ... it looks like the jacks were "bad" and apparently did not make sufficient contact when nothing was plugged in and so I replaced the FX loop jacks with different ones and it did the trick ... although I don't know why I could "fish out" the noise with using the half power switch a couple of times (before I replaced the FX loop jacks, now it is silent)
- I also checked the inside for bad solder jobs and joints and refreshed some solder spots
- you can also visually inspect the caps and resistors to see if any show signs of damage
- also check this out https://robrobinette.com/Amp_Stuff.htm he's got plenty of good advice on how to go about troubleshooting tube amps

I still do not know how or why it all started, but I THINK I fixed it ... if not I will do some more research or take it to a qualified technician and let him do his or her magic, but for now it is very quiet.

Hope anything I wrote helps and that you resolve your issue.

Take care


p.s.: regarding the pop sound, if it is just a pop sound when switching and no other noise is there, then just leave it ... I think you could put a small resistor in like the ones you use to drain the big caps without making sparks and it should help with that ... but I think I also noticed a small decrease in the popping sound after replacing the FX loop jack sockets, so go figure ... hope someone more knowledgable can say more about this
 
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Hi there Weirdoku,

congrats on your purchase :) hope it'll give you much pleasure.

........

Take care

Man thank you so much for the reply!

I'm definitely not in the mindset of swapping out tubes in search for the ultimate tone lol. If the solution (f partially) to solve the hum is to swap out tubes then so be it. I like how the amp sounds by default, my goal is to solve the hum!

I'm going to go try out the list you've suggested. I received some tubes today to try out too. Will report back :)
 
Apologies for the double post lol.

Over the weekend I did a bunch of stuff. I pulled all the tubes and put them in one by one and came to the conclusion that the hum appears when the FX loop tube is installed. I put a 5751 tube in the FX loop position and I do notice a drop in hum, it's almost bearable. The 5751 in other positions made no difference to the hum.

I've also opened up the amp and had a look inside. Nothing really stands out for things like bad solder joints or bad caps. All connections and ground points appear solid.

I came across a facebook post in the PRS MT15 group where someone took theirs to an amp guy to fix a FX loop noise issue (images below). The ribbon cable was replaced with a shielded cable and they bridged 2 points on the FX loop board. I've got some shielded cable but I'm not sure if this would solve my hum issue since the hum issue isn't caused by something plugged into the FX loop. I can't imagine the non-shielded ribbon cable from the FX loop board picking up noise causing the hum?

161362740_10208195297356785_225392265248693332_n.jpg
161412392_10208195297836797_2074144252338260397_n.jpg
161606882_10208195297476788_3920273444289789324_n.jpg
 
Apologies for the double post lol.

Over the weekend I did a bunch of stuff. I pulled all the tubes and put them in one by one and came to the conclusion that the hum appears when the FX loop tube is installed. I put a 5751 tube in the FX loop position and I do notice a drop in hum, it's almost bearable. The 5751 in other positions made no difference to the hum.

I've also opened up the amp and had a look inside. Nothing really stands out for things like bad solder joints or bad caps. All connections and ground points appear solid.

I came across a facebook post in the PRS MT15 group where someone took theirs to an amp guy to fix a FX loop noise issue (images below). The ribbon cable was replaced with a shielded cable and they bridged 2 points on the FX loop board. I've got some shielded cable but I'm not sure if this would solve my hum issue since the hum issue isn't caused by something plugged into the FX loop. I can't imagine the non-shielded ribbon cable from the FX loop board picking up noise causing the hum?

161362740_10208195297356785_225392265248693332_n.jpg
161412392_10208195297836797_2074144252338260397_n.jpg
161606882_10208195297476788_3920273444289789324_n.jpg
Great info, mine had the hum whrn nothing was plugged in the FX loop and was ok when FX loop bridged.
 
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