100W Plexi Grounding

[basschops1528]

Hey guys,

I'm really starting to get into the major wiring of my amp, but I'm having trouble figuring out where the input stage(s) caps are to be grounded. If you look at the general grounding scheme below, it says to ground them to the "preamp grounds on board", but if you look at the power supply diagram, it's clear that the "preamp" filter caps are grounded with the main filter caps for the B+ and screens. And the layout diagram doesn't help either. How should I ground them so that the input jacks and pots aren't subject to ground loops?
I hope you guys can understand what I mean.

[Willabe]

I'm having trouble figuring out where the input stage(s) caps are to be grounded. If you look at the general grounding scheme below, it says to ground them to the "preamp grounds on board"

Do like the general 'grounding scheme'.   

The input stage caps are grounded through the buss bar on the back of the pots, along with the preamp board grounds to the input jack. The input (switchcraft type) jacks body/threaded sleeve makes contact with the chassis  when it's bolted in place, that's the ground connection.
 

[basschops1528]

I'm using Cliff style jacks but I bought some of those grounding washers. Will everything ground through there? I feel like the caps will be floating if not connected with the B+/screen caps.

[sluckey]

Drill a small hole in the chassis near the input jacks and put a ground lug there. Connect a wire from ALL the Cliff jacks sleeve lug to that newly installed ground lug.

[basschops1528]

Awesome. Thanks.

[sluckey]

The buss bar still needs to be connected to either the input jacks or that new ground lug near the input jacks.

[basschops1528]

As in the pot buss bar?

[Willabe]

As in the pot buss bar?

Yes.

[EL34]

Hey guys,

I'm really starting to get into the major wiring of my amp, but I'm having trouble figuring out where the input stage(s) caps are to be grounded. If you look at the general grounding scheme below, it says to ground them to the "preamp grounds on board", but if you look at the power supply diagram, it's clear that the "preamp" filter caps are grounded with the main filter caps for the B+ and screens. And the layout diagram doesn't help either. How should I ground them so that the input jacks and pots aren't subject to ground loops?
I hope you guys can understand what I mean.

Do it like the top image in your post
The others are correct also, they are doing the same exact thing

The 100 watt power supply image is just an image of the schematic.
The wire connecting all al the grounds on the caps just represents the chassis

[MFowler]

Is the buss wire floating or soldered to back of pots?  If it is soldered to back of pots doesn't that create a ground loop through each pot bolted to the chassis. I'm positive the buss wire soldered to back of a Marshall was done to alleviate pot anti rotation problems not for grounding.


The floating buss wire, like I use, is only tied to terminal #1 of the pot and ground wire from buss goes to same single ground point near input jack as shown in diagram #1 above.


Mark

[Willabe]

Is the buss wire floating or soldered to back of pots?  If it is soldered to back of pots doesn't that create a ground loop through each pot bolted to the chassis.

Technically, yes it does, but it's a small loop with low current and it's separated from the high current power amp chassis ground connection. Our host Doug has been doing his amps like this for years with no noise problems and so have many guys here.

I'm positive the buss wire soldered to back of a Marshall was done to alleviate pot anti rotation problems not for grounding.

Yeah that could be, it does work for that. IIRC Doug has said he likes that too.

The floating buss wire, like I use, is only tied to terminal #1 of the pot and ground wire from buss goes to same single ground point near input jack as shown in diagram #1 above.

Sluckey changed over to the floating buss as it's easier to change out a pot if needed. I just run a wire back to the board from each pot where needed.

You could do both, solder a wire across the back of the pots for anti rotation and then run a floating buss for ground connected to the input jack ground, more work but........  might be worth it?   

[EL34]

It's easy to change a pot with a bus soldered to the back
Just snip the bus wire right at the solder blob
remove the pot
install a new pot
solder the two leads back to the pot

[MFowler]

Thanks for the reply back guys always great to read what works for others.

[Ed_Chambley]

Different builds I do different ways.  On Marshall builds I like to bend the pots ground lug up and twist it 90 degrees.  I make a loop at end I am planning to attach to the input jacks (Cliff or Switchcraft).  I then thread my buss wire through the lugs I bent 90 degrees.  To me it looks very neat, and I have plenty of places to add ground wires if needed.

I agree that it is not difficult to replace a pot that has a soldered buss to it provided your buss wire is not 11ga. and all you have is plastic scissors to cut it.

I don't really understand how soldering to the pots makes ground loops.  If it does, then doesn't a buss wire connected to 2 pots make a ground loop as well?

[shooter]

I don't really understand how soldering to the pots makes ground loops

Me either, since the back of the pot is actually *thee chassis" (assuming non-isolated).  If I'm an electron, I'm gonna like the copper road way better than the aluminum road.

[EL34]

I don't really understand how soldering to the pots makes ground loops.

It's technically a ground loop because the path back to the main ground has multiple paths
The multiple ground paths go from the board, to the pot bus, through each one of the pots to the chassis


It's nothing to worry about, it works just fine soldering to a buss wire down the back of the pots
The paths are all so short from the board to the chassis it's not a big deal

Now say you had a long ground bus on your board and you ran a wire from each end over to the power transformer
That's a big ground loop

[basschops1528]

Hey guys,

A bit off topic but would you see a problem with using 20 gauge 600v rated wire for the mains side of the PT; from IEC to fuse to on/off switch? I really only have that type of wire to do the whole amp with.

[EL34]

20 ga is fine

[basschops1528]

Thanks Doug,  can't wait to finish my amp and post it.