Metropoulos Forum

Posted: **Wed Oct 20, 2010 10:27 pm**

So, in the instructions for the 100 Watt Plexi Kit (and some of the other kits) it says to solder the ground bus wire to the back of all the pots. However, I've been doing some research and a number of people have mentioned that a better way to do it is to only route the bus wire through the necessary terminals on each respective pot. I've also seen a number of builds where it's been done this way (mostly in 50 watters and JCM's, though).

For example, here's what I'm talking about:

Do not solder the ground buss to the back of the pots. Instead, lift the grounded legs of the Midrange pot, Presence pot and both Volume pots, then run the buss wire through those, ground all input jack and preamp grounds on the same solder joint on the ground buss (right at the grounded leg of one of the volume pots is most convenient for this), then run 1 wire from that solder joint to the (-) terminal of the preamp filter cap and ground the (-) of the preamp filter cap to a chassis ground lug fastened by one of its clamp mounting screws. Everything else can be grounded the way they show to in the instructions.

So, as an example (not a Marshall, I know) this: http://www.c3amps.com/Photos/SLOBuildLo ... roPots.jpg" onclick="window.open(this.href);return false;
Or this: http://i255.photobucket.com/albums/hh15 ... C00469.jpg" onclick="window.open(this.href);return false;

Not this: http://www.cpamps.com/images/000_0618.JPG" onclick="window.open(this.href);return false;

My main question is what's the result of doing it either way? Less noise? Are there any potential issues if one doesn't solder the bus to the back of every pot?

Posted: **Thu Oct 21, 2010 5:30 am**

http://home.comcast.net/~jbjdav26/1959/ ... ubbers.JPG" onclick="window.open(this.href);return false;

the idea is to avoid ground loops, so less noise. Even better, you do Larry's ground scheme, and

schematic is here:
http://forum.metroamp.com/viewtopic.php?f=45&p=319620" onclick="window.open(this.href);return false;

and it should be done like:
http://i42.tinypic.com/2jaztxt.jpg" onclick="window.open(this.href);return false;

even tho this is JTM45, you get the idea how to ground those points:

novosibir wrote:As for the bias wires it doesn't matter, which one goes to ground!
Your grounding is nearly the diametral opposite of perfect!
There are some basical rules:

Think your ground points along a line across the chassis.
1-st come the inputs & V1 cathodes
2-nd come volume & V2 grounds
anywhere between 1 & 2 the preamp filter ground
3-th comes tone stack, V3 (presence), PI filter & speaker jack grounds
The OT's secondary common has to be grounded to the speaker jacks !!!
4-th comes the bias & screen filter's ground
5-th comes the rectifier ground (or HT CT), 1-st filter ground & output cathodes
6-th is the earth from the wall supply ONLY
Best (quitest) place for the heater CT would be 1-st ground spot, although a long way there. May put to 4-th, but NEVER 5-th

I've showed the 'ideal' above!
Grounding a filter cap, a output cathode or the speaker jacks to the closest bolt available or the preamp grounds to a buss wire along the pot's backs is a simplification, to save time, lugs and wires - might work properly, but also might hum.

Try to think in closed circles! After each R in the HV supply rail you have a new voltage supply node. Therefore all what's fed from this node between the previous coupling cap until to the next coupling cap in the layout has to be grounded to one and the same spot - including the supply node's filter cap.

Do it simpler and have the risk of hum!

Larry

and for the placement of the ground points:
http://forum.metroamp.com/viewtopic.php ... 26#p250697" onclick="window.open(this.href);return false;

Posted: **Thu Oct 21, 2010 8:55 am**

Awesome, thanks!

Aren't the cases of the pots grounded to each other anyway just by being mounted in the chassis? Grounding them all to each other with a bus wire seems redundant and I can see how that would create a ground loop.

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