MT15 gain and Hum / Buzz on lead - what is normal expected operation ?

[RyanCarr]

Hi

I just picked up a used MT15 as my first tube amp and am wondering if it's working as expected.

On "clean" it is completely quiet regardless of volume setting unless I'm playing something - which seems good.

on "lead" there are two things I'm not sure about.

first, if the gain is above the very lowest setting it is EXTREMELY loud. even with the power switch at half. Is that normal ?

second, as I increase the volume there is a consistent hum (not really a buzz.. just a low bass background sound) on the lead channel even with no guitar plugged in.

Are both of those normal ?

They don't bother me as I am playing it at lower volumes. I'm just wondering if it's normal or not.

thanks for any help !!

 

[RyanCarr]

Plugged the amp directly in the wall instead of into an extension cord - no real difference

swapped out cables for new ones and it's a little better

oddly enough, if the sound is bad, I switch the power off and back on twice and it's gone.

 

[Rider1260]

Guessing you have a bad tube or soon to be bad tube.
I have only played a MT15 for a short time there was no hum on it but it was VERY loud for a 15 watt amp

 

[RyanCarr]

Guessing you have a bad tube or soon to be bad tube.
I have only played a MT15 for a short time there was no hum on it but it was VERY loud for a 15 watt amp

Oh. Interesting. Thanks !

the amp was shipped pretty sketchy. in a cardboard box barely bigger than the amp with one layer of bubble wrap and like 50 packing peanuts, so I could see a tube being bad.

If I turn the power off and back on a couple of times the hum/crackle sound seems to dissipate

I'll look for extra power tubes and try those out. I already have a few extra 12ax7 tubes sitting around from my phono amp. not the best, but good to test and see where the crackle is coming from.

 

[Toolmaster Of Brainerd]

Just to check, when you say "if the gain is above the lowest setting, it is EXTREMELY loud", do you mean gain or master volume?

 

[andy474x]

The MT15, especially some of the older ones, occasionally had issues with ground hum, which could be the low pitched noise you’re describing. The lead channel does hiss just due to all the gain. It can be a little deceiving, because even with the lead channel master at “only” 1/4, there will be a good bit of hiss, but it’s partly due to that hair-trigger MV control. That’s really about 2/3 the max volume of the amp. If you have it adjusted for home-level play and it’s still really loud, you may have a separate issue.

Both the ground hum and the lead channel hiss should be well below the output level of the amp and not audible when playing.

 

[László]

There are a bunch of possibilities:

Bad tubes will hum.

Ground loops from connected gear will hum.

Sometimes being near an amp's output transformer will induce hum into the pickups.

Some cables are poorly shielded and hum.

Sometimes the AC in a house isn't clean, and will hum.

And sometimes it's the amp itself. It can be a real PITA to troubleshoot hum, but I'd start with a fresh set of tubes and go from there.

 

[RyanCarr]

Just to check, when you say "if the gain is above the lowest setting, it is EXTREMELY loud", do you mean gain or master volume?

really either. the low buzz/hum sound got extremely ear splittingly loud with the master on like 5% and gain up at maybe 50%

I've been able to make it whisper quiet when there's no signal by turning it off then back on a couple times, and it stays that way for the 30 or 45 minutes I play through it.

 

[RyanCarr]

There are a bunch of possibilities:

Bad tubes will hum.

Ground loops from connected gear will hum.

Sometimes being near an amp's output transformer will induce hum into the pickups.

Some cables are poorly shielded and hum.

Sometimes the AC in a house isn't clean, and will hum.

And sometimes it's the amp itself. It can be a real PITA to troubleshoot hum, but I'd start with a fresh set of tubes and go from there.

my local guitar shop in Raleigh (Harry's) has a guy who is pretty experienced with these amps (not a tech just an experienced player who has used them)

he offered to test it out and at least see if it's normal.

I bought new cables and put a small patch cable in the FX loop and that made the amp quieter in general. it wasn't this issue but my cables were like 35 years old from when I tried to learn electric bass back in the late 1980s.

 

[László]

my local guitar shop in Raleigh (Harry's) has a guy who is pretty experienced with these amps (not a tech just an experienced player who has used them)

he offered to test it out and at least see if it's normal.

I bought new cables and put a small patch cable in the FX loop and that made the amp quieter in general. it wasn't this issue but my cables were like 35 years old from when I tried to learn electric bass back in the late 1980s.

Hopefully it all works out!

 

[Goran Lorencin]

Hi

I just picked up a used MT15 as my first tube amp and am wondering if it's working as expected.

On "clean" it is completely quiet regardless of volume setting unless I'm playing something - which seems good.

on "lead" there are two things I'm not sure about.

first, if the gain is above the very lowest setting it is EXTREMELY loud. even with the power switch at half. Is that normal ?

second, as I increase the volume there is a consistent hum (not really a buzz.. just a low bass background sound) on the lead channel even with no guitar plugged in.

Are both of those normal ?

They don't bother me as I am playing it at lower volumes. I'm just wondering if it's normal or not.

thanks for any help !!

All good points before me, just one question, did you bias the tubes to the correct values?