At the beginning of my first build.. a couple of Qs that relate to grounding

[BobRuth]

I am involved in what is my first amp build.  It is slow moving.... but I am moving at the best pace at which I can find time.  I don't know if the particular type of amp I am building is important or not but the closest one I can tie it to is a Fender tweed Pro 5E5-A.   I am still in the phases of getting my layout straight and my turret board prepped but my thoughts are turning to some grounding related issues and, if I may, I'd like to post a couple of questions I have and see where it goes....

(to set the stage, skills are not an issue.   I am NOT SAYING that I am an "expert and will not tolerate being questioned"... no SIR!!!!!   But I did attend tech school for 2 years and understand the essence of what goes on in these circuits.   That said, I do not have a wealth of hands on experience in this area and am dying to learn.  I would appreciate pretty much anything at all:   help, ideas, criticism, and in the right circumstances...  outright condemnation!!!!   :)    I just want you to know that I am ready to listen and learn!)


1)  Thinking of a main grounding bus: I have in mind placing several small terminal strips in the chassis between the board and the pots and running a piece of 12 or 14 gauge (ripped out of a piece of Romex wire) through the tabs all the way across the chassis and at the end closest to the PT, take a wire with a ring connector over to the PT and bolt it down with one of the PT bolts.  Then, along the board, or anywhere else that I need a tie to ground... just jump over to that copper buss and solder it on.   Would that be an appropriate way to do this or are there problems/pitfalls I don't see yet?  My thinking is based on "one and only one path to ground".

2)  If my input jacks (Switchcraft) are connected straight to the chassis.... do I STILL need to tie their grounds to the buss or is the direct chassis connection enough?   Actually....if they are mounted directly and I DID tie them to the buss is that even a BIGGER problem due to dual connection to ground for the inputs?

3)  I seem to remember in a guide text that I saw once (Perhaps Aiken, but I am not certain)  you don't want the audio OT speaker side common to tie to ground in the wrong place, like on the board, because the speaker side of the OT is perhaps the heaviest current path IN the whole amp and by driving it back to ground  through the wrong place you can CAUSE hum/noise problems.   That has me thinking... should I just take the OT wires, solder them to the output jacks...and then ISOLATE the speaker jacks?   Doing that, the speaker wiring never even touches the chassis.    Would that BE a good thing to do OR does the speaker circuit actually NEED to tie to ground?? IT seems to be simple to me but I would appreciate some opinions so that I am sure.

Thanks very much in advance for any thoughts you may have.
Bob

[Willabe]

Welcome Bob!   

More than 1 way to skin a cat but, here's a link on grounding that will be helpful; 

http://www.valvewizard.co.uk/Grounding.html


                Brad   

[sluckey]

Follow the link at the bottom of this page that leads to the "Tube Amp Library of information" page. There you will find a wealth of helpful info, including a well thought out grounding scheme that works very well.

For amps such as the 5E5A that have a NFB loop from the OT secondary, you MUST ground one side of the OT secondary (speaker).

[eleventeen]

You're definitely asking the right questions and seem to understand the ideas very well. I think if you read what Sluckey directed you to, you'll be fine.


The 14 ga grouding bar is something we often see in hi-fi amps of the era. It's a bit on the overkill side but it will not hurt. Many on this forum file a little patch of clean metal on the back of the pots and run an equivalent thing across the back of the pots. That way you get your ground buss *and* make sure your pots are well shielded. After all, those are in high-gain sections of the circuit.


As for the input jacks, many builders like to run shielded wire to the tube grids where they connect. Why? Not especially to ground them but to shield the connecting wires from internal fields. Now here is the place where we do not want to connect dual grounds. So it's a common practice to connect the shield to the input jack grounds, connect that shield to your ground buss, and leave the grid ends of those shields ungrounded. Now you have the input wires running through a pipe.


Again as Sluckey said, you need the ground connection on the speaker connection.

[labb]

This works:  http://el34world.com/charts/grounds.htm


I don't personally run a buss wire down the back of the pots. I just run a buss wire down the board and ground it on a lug at the input jacks.

[Fresh_Start]

You are asking important questions!

Hoffman's grounding scheme works. The only things I do differently are (1) mount preamp ground bus on board instead of back of pots, (2) separate and dedicated safety ground as close to power cord as possible and (3) separate ground for heater center tap because it's just easier with my layout approach and doesn't matter.

Soldering to the back of pots works OK but you must abrade the surface, use flux and hope you don't cook the pot before you get a good solder joint. Also, it makes changing a pot down the road more difficult. I put two eyelets at either end of the preamp and solder bus wire into those as shown below. Input jacks are isolated and I ground the bus as close to the input jacks as possible.

Merlin's article is good but overly complicated. The concept of "local star grounding" is worth paying attention to. I think it helps to have the filter caps as close to the circuit they supply as possible.  That's why you see them along the edge of the board instead of all together at the power amp end.

Cheers,
Chip