Grounding points location

[tepsamps]

Hello everybody, i ask this question to those who build amps from time to time.
I don't use turret boards, but slug boards, which are almost the same, 2 slugs and 1 eyelett, instead of a turret. And on the board, a ground bus, a 2,5 mm2 thick copper solid wire all along the board. So each component has a different point to ground, like on a schematic.

The problems come when i have to ground the gain and volume pots, the question is where to solder them?
I noticed that if i put the ground like in the schematic, some issues may happen, unwanted buzz.
So on this one, the clean channel is one the left, the lead on the right. The clean'got a TB tone stack, the lead a TMB TS. The clean is a fender supersonic (twin with additional gain stage) and a master volume, the lead is the EVH 3 lead channel, with it's 6 gain stages+ CF.
The amp is sounding very good, except that if i put the clean master volume to 0, i get a loud buzz (100Hz) that disapear when i put the FX send volume to the max.
I tried other locations, but it's getting worse.
I had already this kind of problem (not exactly but king of), and i always find a cure, by changing the location of the pots ground point onto the main ground bus.
But in this case, i have not found yet, and i ask you the question :

Did you have already this kind of problem, do you know it, are you used to it ?

I can post the schematic, it would be long for me to do it, but my question is more general than solving this amp issue.

[xtian]

It looks like your buss bar is grounded to the chassis near the input jack. That is good. But what about the other end of buss bar? I can't see it in the photo. Are your filter caps connected to the buss? In general, we want to split that ground buss into two parts, preamp (grounded near input jack as you've done) and power amp (separate from preamp grounds, grounded near PT center tap).

[tepsamps]

It looks like your buss bar is grounded to the chassis near the input jack. That is good. But what about the other end of buss bar? I can't see it in the photo. Are your filter caps connected to the buss? In general, we want to split that ground buss into two parts, preamp (grounded near input jack as you've done) and power amp (separate from preamp grounds, grounded near PT center tap).

Generally i use PT without center tap winding for HV, like 0-300 VAC and a diode bridge, and this time i used a PT with 300-0-300 VAC winding and i put a full wave rectifier with 2 diodes. And i did not know where to put the CT, so i put it near the filter caps ground, but it's connected to the main (preamp) ground bar via a flexible wire.
Ok so the mains outlet ground is connected to the chassis at the opposite location of the input jack with a screw and bolt. So at this place, i connect the PSU ground bar (small board) with the PT CT on it, that's right ? And i remove the wire of course between the 2 boards.

But can you explain this ground loop noise while turning of a volume pot, or middle range pot also, it can happen, whilst there is no ground loop? It's not progressive, it's sudden
I dont anderstand...
Anyway thanks for your reply, i will try it soon.

[tepsamps]

Well, it did not fix the problem, anyway i like your flamed orange amp shell. Very unique!!!

[xtian]

Lots going on in that chassis! My basic point was, the reservoir (first stage) filter cap, rectifier and screen filter node draw the most current, and are the "noisiest" ground returns. So you want to return that current to a point on the chassis close to the PT, and far away from the input side of the chassis.

Another lead dress issue is that ground return paths should be as short as possible, because each inch of wire adds resistance (and possible noise) to the circuit. I put the ground buss close to the pots. You have a lot of flying ground wires. Not sure it that's causing your noise, but it's certainly not helping.

[tepsamps]

I have already tried a ground chain connecting all the pots that need ground (gain, MV, mid range) from pot to pot arriving close to the input jack, and it's really worse, it's unplayable, buzz in many combinations of pots settings.
I would like to anderstand some symptoms : when i get the buzz (clean MV to 0), if i touch with my hand the shielded wire that goes from clean MV wiper to the relay, it stops the buzz. What does it mean ? What do i add when touching this wire ( shield connected to ground or not )?
Thanks

[xtian]

Just a guess: When using co-axial cable, you know the shield should be grounded on one end only, and the other end of the shield left unconnected?

[tepsamps]

Just a guess: When using co-axial cable, you know the shield should be grounded on one end only, and the other end of the shield left unconnected?

Yes !

[Helmholtz]

I would like to anderstand some symptoms : when i get the buzz (clean MV to 0), if i touch with my hand the shielded wire that goes from clean MV wiper to the relay, it stops the buzz. What does it mean ? What do i add when touching this wire ( shield connected to ground or not )?

Are you sure the shield is well grounded? Touching a grounded shield should have the same effect as touching the chassis.

[tepsamps]

I've found the error! The shield of the wire from fx loop output on the board and send volume pot was not grounded yet. It solved everything...
Thanks for participating...