Tube Pedal Grounding

[Pick-n-Strum]

I've decided to venture into building a tube pedal and have some questions on grounding - mostly related to safety and how it compares to amp grounding.  For reference, I've built a few amps and 9-18v pedals.  Here is the layout:



So I currently have all signal grounds and the power ground going to a buss and the buss grounded to the chassis in one spot.  Note:  I'm using isolation washers on the jack to prevent loop.  Am I correct in thinking that, in addition to a voltage reference, this would act as a Faraday cage for signal ground and a safety ground in case the 240 VDC somehow shorts to the chassis? 

Here is my thought process / confusion:

In an amp, we attach mains ground as soon as it comes in.  My understanding is the reason we do this is if the chassis becomes live (with mains voltage or B+?), current would rather go through that mains ground than through your body to ground.  Correct?  In the instance of a pedal only bringing in 9v, we only really need it to connect for ground reference and not necessarily safety.

So in my pedal, power ground would be similar to B+ CT and should be grounded to the chassis for reference voltage and safety in case B+ shorts to chassis.  Correct?

I know many amps connect power ground to a single point on the chassis and preamp ground to a different point on the chassis.  Since this pedal is basically just the preamp, it should be okay to connect all grounds to the buss and then connect the buss to the chassis at one point, like I did. But it does make me wonder if I should connect all signal grounds to the buss and then to the chassis in a separate place (to act as a Faraday cage) than where I connect the power ground (to act as voltage reference and safety).   

Am I thinking through this correctly?  In other words, the only reason to connect signal ground to chassis is to act as a Faraday, otherwise, the buss alone would be suitable and you don't need to connect to chassis.  But, B+ ground would always need to connect to chassis for safety and voltage reference.

EDIT:  Or is the only ground truly needed for safety when talking higher voltages the mains ground to keep a live chassis from going through your body?  Maybe someone could clarify that...In this case, the only way to truly get shocked by B+ would be to literally grab B+ AND power ground; which would be unlikely since I'm not putting my hands in a live chassis. 

[PRR]

If you are just taking low voltage DC, grounding is much less important.

[Pick-n-Strum]

If you are just taking low voltage DC, grounding is much less important.

I assume you mean from a safety standpoint, low voltage grounding is less important.

I realize I got real pedantic above.  I guess my biggest concern is the ground for the +240v going to the preamp plate.  Does connecting that ground to the chassis do anything for safety?

[Willabe]

I realize I got real pedantic above.  I guess my biggest concern is the ground for the +240v going to the preamp plate.  Does connecting that ground to the chassis do anything for safety?

Read this, all of it.

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

[d95err]

The safety ground is mainly relevant for the mains voltage. Your grounding looks fine to me.

However, your layout doesn’t include the power supply, so I can’t tell if there are potential safety concerns there.

[pdf64]

Schematic?

[Pick-n-Strum]

I realize I got real pedantic above.  I guess my biggest concern is the ground for the +240v going to the preamp plate.  Does connecting that ground to the chassis do anything for safety?

Read this, all of it.

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

Thanks - Yeah, I have the book and I've just gone back over the grounding chapter.  The 2 main points for safety that I understand are:

1.  The mains ground needs to be near where mains enters and on its own dedicated screw connected to chassis.  I totally get that.  In my case, it's a 9vDC wall wart that is going to be bringing voltage up to 240 DC for the plates.  I'm guessing that the ground for the 9v wall wart input doesn't need to go to the chassis for safety, necessarily.

2. Once you have the ground scheme created for your circuit, you choose one point to connect it to chassis for safety and shielding.  Blencowe states:  "connect circuit ground to the chassis (which is bonded to earth) at one point only.  This connection is purely for safety/shielding purposes. "  Obviously he is talking about just preamp in this instance because I know that often there is a separate connection directly to the chassis for the power amp section.  The part I'm getting hung up on with this point is that my pedal chassis technically is NOT connected to mains since it's a 9v DC supply coming in...

The safety ground is mainly relevant for the mains voltage. Your grounding looks fine to me.

However, your layout doesn’t include the power supply, so I can’t tell if there are potential safety concerns there.

Okay - that's what I was wondering, if it's mainly a concern of mains voltage for safety.  According to Blencowe, there is also a safety concern for a grounding point tied to the chassis for the preamp and power amp respectively.  Like noted above, I'm concerned because, as noted above, my chassis isn't grounded to mains and it sounds like that is what is making the B+ safe???

Schematic?

Yeah, I didn't post the schematic b/c it's from another forum.  I'll attach the pedal schematic but the other issue is I decided to buy a premade Nixie power board for ease.  I'll attach a photo of a picture with info but I don't have a schematic for that.

Thanks for the help everyone!

[sluckey]

KISS. There is no need to connect any part of your pedal circuit to chassis. It's OK to connect to chassis for convenience and/or shielding. The shielded cable between your amp and this pedal will also provide an earth ground.

[Pick-n-Strum]

KISS. There is no need to connect any part of your pedal circuit to chassis. It's OK to connect to chassis for convenience and/or shielding. The shielded cable between your amp and this pedal will also provide an earth ground.

Awesome!  That's what I like to hear.  I'm very pro KISS (heavy on the second "S")

Not to beat a dead horse, but in what instance would I need to be concerned from a safety standpoint?  If I'm understanding all the responses correctly, the biggest issue would be if I was bringing in full mains and grounding it to the chassis - Would I then keep all grounds (including the +240) floating and NOT connected to chassis?  Are there any other instances that I need to be concerned from a safety standpoint about whether or not to ground certain points to chassis?

[sluckey]

You're still beating the horse.   

[Pick-n-Strum]

You're still beating the horse.   

I know :-(  What I'm trying to understand is if the only point of safety for grounding in an amp or tube pedal, is the mains ground - i.e. earth.  (Obviously I don't have mains to worry about in the pedal above but I want to be sure I understand the concept)

Otherwise, there is no reason to necessarily connect B+ / HT to chassis from a safety standpoint....it may only make sense to connect B+ / HT and all other signal grounds to chassis in order to lower noise. 

I guess this is what I'm understanding from this thread.  Let me know if I'm wrong, otherwise, this is what I'm going with!

[PRR]

[d95err]

Like noted above, I'm concerned because, as noted above, my chassis isn't grounded to mains and it sounds like that is what is making the B+ safe???

The current your B+ supply can deliver is measured in mA. The current mains voltage can deliver is measured in A. That's the difference.

If your body accidentally makes a connection between B+ and ground, it will sting, but it is highly unlikely to cause injury or worse. I'm sure most people here has done that many times.  (yes, in very unlucky circumstances it could still be dangerous).

Getting your body between mains voltage and ground/earth though...

[Pick-n-Strum]

Like noted above, I'm concerned because, as noted above, my chassis isn't grounded to mains and it sounds like that is what is making the B+ safe???

The current your B+ supply can deliver is measured in mA. The current mains voltage can deliver is measured in A. That's the difference.

If your body accidentally makes a connection between B+ and ground, it will sting, but it is highly unlikely to cause injury or worse. I'm sure most people here has done that many times.  (yes, in very unlucky circumstances it could still be dangerous).

Getting your body between mains voltage and ground/earth though...

Perfect!  Thanks for the explanation - feel like I get it now.

I'm guessing the silence from PRR is a good sign that my above post is accurate - which is confirmed by d95err.  Thanks guys!

[sluckey]

PRR did have something to say. He posted a pic but I can't see it. Don't know why. So, I'll post it a different way.