Grounding - alternative grounding schemes - any difference?

[bulatovic]

Hi there guys, i'm about to get a JTM45 kit - and as i'm really into ampbuilding process, i really like planning everything before actually doing it.

So one of the questions is the grounding of the JTM45. I see that the regular "original" wiring would be that the entire board is grounded to the bus which is soldered to the back of the potnentiometers. And as i understand the electronics, that causes ground loop. My question is - if you do ground wiring like on those SDM's layouts (where the bus is not soldered to the potentiometers and where the entire preamp is grounded to the ground point of the preamp filter cap, the presence pot to the PI cap grounding point and bias to the main filtering ground) what do you really gain?

How would you do such a thing on a JTM45 - would the bus be grounded to a random point on the chassis (since it doesn't have a can cap for preamp filtering) ie. choosing any place near the preamp section and grounding it there, while PI and bias would be grounded at the PI and Main's grounding point?

Or should i just stick to the original - back of the potentiometer - ground scheme?

I'd like to hear your opinions on this!

Thanks guys!!!

Regards

Luka

[Roe]

try larry style grounding if you'd like to minimize noise

[novosibir]

try larry style grounding if you'd like to minimize noise

http://forum.metroamp.com/viewtopic.php?f=4&t=25351" onclick="window.open(this.href);return false;

Larry

[bulatovic]

Thanks Roe!!!

I've been reading through the topics, and have a question
- since screen and main filtering are one one capacitor with common ground connection, should i use it to ground both points 4 and 5?
The ground scheme would look like this:

1 - Inputs, V1 cathode, Heater CT
2 - Volume potentiometers, Middle potentiometer, V2 cathode, preamp filter cap
3 - Presence potentiometer, PI filter cap, output jack (OT common)
4 - Screen filter, Bias supply, Main filter, HT CT (via fuse), V4 and V5 cathodes
5 - power cord earth

or should i connect bias supply to some other point and the scheme would look like:

1 - Inputs, V1 cathode, Heater CT
2 - Volume potentiometers, Middle potentiometer, V2 cathode, preamp filter cap
3 - Presence potentiometer, PI filter cap, output jack (OT common)
4 - Bias Supply
5 - Screen filter, Main filter, HT CT (via fuse), V4 and V5 cathodes
6 - power cord earth

I'd drill 3 holes in a line to ground the first 3 points, for 4th point i'd use the 32/32 cap mounting point, and power cord to the other mounting screw.

Something like this:




EDIT: slow typing... Hey Larry, great to see you here could you check if this is OK or/and give some suggestions! Thanks!!!

EDIT2: Better layout further on in the thread

[novosibir]

EDIT: slow typing... Hey Larry, great to see you here could you check if this is OK or/and give some suggestions! Thanks!!!

Read my advices once again, to find the bugs in your description!

Middle pot is wrong grounded!

Try this:

1 - Inputs, V1 cathode, Heater CT
2 - Volume potentiometers, V2 cathode, preamp filter cap
3 - Presence potentiometer (involves V3 cathodes!), PI filter cap, output jack (OT common), Middle potentiometer
4 - Bias Supply
5 - Screen filter, Main filter, HT CT (via fuse), V4 and V5 cathodes
6 - power cord earth

Larry

[bulatovic]

Again, slow typing

Larry, i've sent you a PM

Thanks for the answer!

[novosibir]

Larry, i've sent you a PM

Post the contend of your PM, your questions here, so that also other users can benefit from my replies!
Believe me - there are some more 'silent' forumites except you with the same questions... so no reason to hesitate or for being embarrassed!
I don't like it too much, to reply individually to tech quests via PM and are replying only by exceptional circumstances

Larry

[bulatovic]

oooh - sorry Larry!
Will do!

[bulatovic]

Would the differences between wiring it this way and doing it the Marshall "original" way be just the lack of hiss and hum, or the overall sonic response would be improved?

And yeah, i'll do it as you said:

1) Inputs, V1 Cathode, Heater CT
2) Volume Pots, Preamp Filter
3) Middle Pot, Output Jack (OT Common), PI Filter, Presence Pot
4) Bias Supply
5) Screens Filter, Main Filter, HT CT (via fuse), V4 and V5 Cathodes
6) Power Cord

Now i've read your posts on grounding, and i do get the concept of it, to look at it in circles - where each circle between the points is between the new voltage feed and the ground. The questions that i have are - is there any sort of way of telling how far away should these points 1,2 and 3 be apart for the amp to run at its best?
As i said, i was thinking of drilling holes to grund these points, i'm not stressed about the authentic looks and stuff. So i thought to do it like this:

http://i39.tinypic.com/6pyl45.jpg" onclick="window.open(this.href);return false;

But where to put point 4? should i put it in the middle of the 3-5 line - somewhere near the HT CT fuse, or in line with 123?

Also, how much of a difference does it make if i ground 1R resistors from the cathodes just on the sockets, compared to having them grounded to the 5th ground point?


Oh, and yeah - i'm really hoping this amp to be a killer!

this is what i've had in mind:
Metro kit but with the following upgrades -

PEC Potentiometers,
Allen Bradley CC Resistors,
NOS Mustard signal caps,
NOS 12ax7 for V1 and V2 (not sure which i'll get) and NOS Telefunken 12ax7 ribbed plate for V3
Metal case diode for rectifier

Hopefully with your grounding scheme guidance it will be a KILLER!

Thanks again!

[novosibir]

Would the differences between wiring it this way and doing it the Marshall "original" way be just the lack of hiss and hum, or the overall sonic response would be improved?

My groundig scheme primarily helps to reduce/avoid hum, buzzing & ghosting
I can't confirm, that it's also reducing hiss - maybe a secondary effect.
The sonic benefits are a second effect, because the intermodulations between signal and noise are missing.

Now i've read your posts on grounding, and i do get the concept of it, to look at it in circles - where each circle between the points is between the new voltage feed and the ground. The questions that i have are - is there any sort of way of telling how far away should these points 1,2 and 3 be apart for the amp to run at its best?

1, 2 & 3 should be in a straight line along the front edge of the chassis and far away from 5 & 6
5 & 6 can be along the rear chassis' edge, but 6 must be the farthest away from 1
Distances between the spots depend on the chassis' material, where aluminum allows closer positions in general due to better conduction, but any distance from 1" to 5" should be ok, though it sometimes depends on the particular amp, what then requires some experimenting for the quietest spots.

But where to put point 4? should i put it in the middle of the 3-5 line - somewhere near the HT CT fuse, or in line with 123?

4 can be placed anywhere between 3 & 5, it's not a critical spot, as long as it's not too close located to 5 (HT CT) and not a PT mounting bolt

Also, how much of a difference does it make if i ground 1R resistors from the cathodes just on the sockets, compared to having them grounded to the 5th ground point?

Usually no problem, but in this case spot 5 has to be located anywhere along the rear chassis' edge - not along the front edge.

Larry

[bulatovic]

So, something like this:

EDIT: IMAGE REMOVED

Larry?

[novosibir]

Looks good!
But I'd move #4 to one of the choke mounting bolts, no matter which one of both, so that it is no longer so close to the PT

Larry

[thousandshirts]

So, we can say that Larry is "well grounded."

In theory and in practice!

[bulatovic]

Thanks for the answer!

So bias should be in line with other 3 points, or just somewhere far from the PT, but it's not essential to be the chassis "quietest" spot such as point 1? In other words, it won't "hurt" if the bias point is in the "way" of the first 3 ground points?

[bulatovic]

Right, SO!

is this better?

http://i42.tinypic.com/282n7v7.jpg" onclick="window.open(this.href);return false;



Or should i move point 4 more to the "north-west" of the chassis (oriented by this photo)?

The other question is on potentiometer case grounding. I'm planning on going with PEC pots, but since they have long shafts, i have to put spacers - i'm not gonna install plastic spacers, but use additional nuts. Is that going to provide proper case grounding or should i try to connect a bus along the back of the pots and ground it?

Is there any rule on which cables should or should not be close to each other along the chassis (not counting on cables from the board to the sockets)? Grid leads as short as possible and to have nice distance between them and the plate wires - well just as shown in the George's manual.

To put it like this - i was thinking of twisting all AC wires together (220V live wire - going from the power cord to the power switch, 2 HV wires and 110V tap that i will use for the lamp that i'm going to rout around the PT with other AC wires), and run them next to the 2 DC wires which will be twisted together. Is it OK to run them two sets together or should you separate them a bit?

What about the Choke wires and OT CT. I was thinking of twisting all 3 of them?

Any suggestions on the layout of the wire going to the potentiometers (apart from the presence pot wire going as far away as possible from the other tone stack wires)?

I don't exactly know how much of a difference it might be between "theoretically ideal" wiring and the "regular" like the one in the manual, but i reckon it's no sin if i ask Well that's why this place exists, right?

Thanks guys!!!


EDIT: OH MAN, WILL I HAVE TO TREAT YOU WITH SOME HIGH QUALITY CLIPS WHEN I GET MY AMP DONE - AS A THANK YOU FOR BEARING ALL OF MY BORING QUESTIONS AND PROVIDING HELP