Metropoulos Forum

Hi guys,

I'm trying to reduce the hum my amp is making. It's a 100W Plexi Metro Kit, converted to a 4/5 gain stages with Master Volume (2203 + one more selectable gain stage before CF). So the preamp is heavily modified to be a high gain amp.

The thing is that as soon as I turn it ON (STDBY still OFF), the Power Trannie (Metroamp dual voltage PT 1203-80-MS) starts humming as you can hear in this mp3. The sound is captured with an Edirol R-09 directly on the back side of the amp, close to the Power Trannie. The amp is turned ON on sec. 5:

http://soundclick.com/share?songid=8422748" onclick="window.open(this.href);return false;

Then, once STDBY is ON, this same frequency-hum starts sounding thru the speakers even with volume on 0. Now the Edirol is in front of the cab.

With the gain on 0, and turning the volume up, there's a small part between 0-1 where the hum is reduced a little bit, but then again comes up with volume over 1 and keeps going up as I take the volume up to 10 (untill sec. 40). The hum increases with volume but not in the same way that a note would do... on second 25 aprox the amp is on 10, so imagine how loud is this. On this mp3 you can also hear the same volume travel (0-10-0) with the gain on 10. There was nothing connected on the input. Now there's more noise, specially on the higher frequencies, but I assume this is normal.

http://soundclick.com/share?songid=8422747" onclick="window.open(this.href);return false;

So, any ideas on how to reduce that hum? Could it be a faulty PT? Can a single triode added to the circuit be too much load for this PT?

Appart of this hum, the amp is sounding very good so I'm not hearing other bad things on it...

Thanks in advance!!!

I am afraid some do regardless of the state of tune

You can try loosening all the bolts to allow it to settle on the chassis and retighten or even fibre washers under mounting feet, just soe stand ups seem to do it.....

Thanks Neikeel. I think it's electromagnetic noise, because I don't feel any vibration on the PT, I mean, the screws are tight and putting my finger over the trannie I don't feel any vibration... but the humm is there.

Any other comments? Can it be a faulty trannie or maybe overloaded?

FWIW I've built a couple metros now and the PT hums on both of them. Just mechanical vibration. However, I do not get any kind of hum through the speakers.

Hey,

I'm going through kind of the same thing right now. I also have a lot of hiss in the amp. Is that like yours? I've just build it stock but with a MV in the back. Have you found the problem yet? I'm up the creek on this one. Also my amp is running at about half power.

Thanks,

J-C

The Metro 100w PT in my build hums considerably compared to the laydown PT in my Metro JTM45 which is wisper quiet. I don't get that hum with other PTs I tried out in the 100w build. It's probably in the nature of this particular beast...

I to just finished a 100w plexi build and I am getting that same "HUM" you described. Apparently this is do to bad lamination on the coils most often. I found that you can pot the tranny. This is the process of pouring in various liquids the squeeze out the air between the windings. This liquid then hardens and stops the coils from moving around and makes your tranny silent again. I generally prefer to call it a vibration rather then a hum because it can confuse people with the actual problem. It reminds me of a pager or cell phone on a table set to vibrate. I can actually feel the housing vibrating. Sounds like a good reason to upgrade the tranny

mrxistential wrote:I found that you can pot the tranny. This is the process of pouring in various liquids the squeeze out the air between the windings. This liquid then hardens and stops the coils from moving around and makes your tranny silent again.

This isn't the best of ideas. You COULD take the bulk of the transformer apart and try and run some polyester impregnating varnish into it, but too much and you run the risk of overheating I fear. I doubt that you could really get anywhere near the centre of the coil this way. Not something I would try myself, and I used to do a lot of rebuilding coils in starters and alternators, and have quite a bit of experience with re-lacquering these. But to try to do it on a coil around a core just has bad news written all over it. I tried several times to rebuild coils around cores in many irreplaceable vintage magnetos, and never had any joy. Unreliability was just too big an issue. And when given that these were for aircraft use, I just wouldn't allow it.
I try to use the same principles I learnt in my amps, because;
a) They CAN kill people just as easily as a plane falling out of the sky
b) We care a lot about our tone and getting things just right
c) We really don't want amps failing mid-gig
d) I'm incredibly anal!

They all do it. Mine does it. Chances are the one that Metro cloned did it too.

Unfortunately in the world of cloning you end up copying mistakes, inefficiencies, and the like. The original Marshalls weren't perfect yet everyone claims to want them...even down to the earlier circuits that weren't tight, sagged a lot and had low end flub which I never understood. Then they build it and complain that it's flubby and not tight, then come on here and ask how they can tighten things up...some wanna do it without changing anything and the only answer to that is "build the later circuit".

I know some say "Well these earlier amps are the amps that made my favorite artists' albums" but the thing you don't know is whether the one they had was modded/tweaked in the same way that the later circuits ended up being or not, nor were you actually there for the recording session so you have no clue as to what they REALLY used other than what was stated by said artists in interviews, which some of them lied about shit all the time (*cough* *cough* EVH *cough*).

When you clone shit you have to accept the flaws of what you're cloning as well as the "perks" (age old saying...you can't take the good without the bad).