Grounding, Shielding and noise - Observations

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Haze13
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Grounding, Shielding and noise - Observations

Post by Haze13 » Fri Jan 04, 2013 6:05 am

Some time ago I built a jcm 800 2204 clone on jtm45 chassis. I had some big problems with noise, but after shielding the first stage it's gone. BUT. Some time later I realised that on higher volumes my amp is picking a radio. During the building prosses I was picking the information from the internet if I had any quastions or just wanted to be shure that what I doing is right. On youtube there is a TubeDepot channel, there is a video about how to build a jtm45 kit. The guy solders a buss wire on the back of the pots. Same thing I saw on other pictures in different amps. I used a CTS pots and they are metal from every side... I knew that if I'll solder the buss wire like that, I'll have many ground loops, but I belived more to the pictures and the guy from TubeDepot (he used Alpha pots and they are more or less like CTS). The Radio was gone when I removed that buss wire from the backs of the pots.
No radio but tiny bit of noise is left.
Than I had to put a shilded wire on the first half of V2 to see if it's helps. It's working, but not enough...
The last thing. A Sheet of metal on the bottom of a had cab! Strange that this thing is not there when you buy a new head cab...
It's like a cry baby pedal. When it's opened it picks every noise that you have, but when it closed - dead quite.

May be this will help to some one who is going to build and amp.

madamp
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Re: Grounding, Shielding and noise - Observations

Post by madamp » Sat Mar 30, 2013 2:35 pm

Thank you for haring your experience.

fretwizard
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Re: Grounding, Shielding and noise - Observations

Post by fretwizard » Tue Apr 09, 2013 7:57 pm

I was also checking the internet while building mine...

and while I was at it, someone suggested to incorporate Star/Larry grounding, and I did. I removed the buss from the Ceriatone board and I used 6 grounding points...

I've followed this thread. Make sure you check the SECOND diagram at the bottom of the page! Plus read the posts, their contain complementary info
http://forum.metroamp.com/viewtopic.php?f=45&t=31328" onclick="window.open(this.href);return false;

I also had squeals and noise without a cabinet. I still don't have a cabinet but I keep the chassis laying on a piece of cardboard covered in kitchen aluminium foil and it works very well for the moment and the hum level is very low, and inexistent when using an attenuator... :thumbsup: Remember to use the metal side of the foil, not the plastic side, it must make contact with the chassis. Of course a proper head cabinet will have the foil too.

Apart from the foil shielding I also noticed that the wire going to V2 pin2 is very sensitive, so when I made my first mod (RR 1 wire mod) I shielded that wire too...
My only problem after the mod was parasitic oscillation when the bright volume was over 8 (no sound), but I only needed to move the input wire away from the remaining mixing resistor and pots to fix it.
Now I can crank both volumes to 10 when cascaded with no issues and I'm about to perform the SMAK mod :twisted: .

I don't know if the larry grounding is making all the difference in this case, but everybody was recommending it so you might wanna try that in your next build.

Cheers :rockon:

R

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