Proper Grounding of a Tube Pedal?

[Twhjelmgren28]

Disclaimer: I also posted this on related amp forum so apologies for those who have seen this twice.

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.

[amptramp]

You will in all likelihood want to use a three prong plug and safety ground in the power amplifier.  Another safety ground connection in a pedal would probably be the granddaddy of all ground loops if you had the same thing on a pedal.  If you add a safety ground to the pedal, it should not be connected to the internal signal ground.  If you do have a connection between safety ground and signal ground in the pedal, you may find you have to disconnect safety ground and signal ground in the amplifier.

Remember at a gig in North America, you may be connecting the amplifier and pedal to different power lines and in fact, they could end up on opposite sides of the 120 - 0 - 120 supply.  Since you are on 240 VAC, you would not have this problem but you have twice the voltage.  It could be one of those cases where the pedal (or amplifier) ground gets connected via a parallel R-C combination - enough connection to prevent floating but enough isolation to avoid dangerous shocks.

[Twhjelmgren28]

You will in all likelihood want to use a three prong plug and safety ground in the power amplifier.  Another safety ground connection in a pedal would probably be the granddaddy of all ground loops if you had the same thing on a pedal.  If you add a safety ground to the pedal, it should not be connected to the internal signal ground.  If you do have a connection between safety ground and signal ground in the pedal, you may find you have to disconnect safety ground and signal ground in the amplifier.

Remember at a gig in North America, you may be connecting the amplifier and pedal to different power lines and in fact, they could end up on opposite sides of the 120 - 0 - 120 supply.  Since you are on 240 VAC, you would not have this problem but you have twice the voltage.  It could be one of those cases where the pedal (or amplifier) ground gets connected via a parallel R-C combination - enough connection to prevent floating but enough isolation to avoid dangerous shocks.

Thanks for the response!  A couple of clarifying questions and remarks.

I'm in the U.S. so we have 120 mains voltage.  Also, the 240+ volts I mentioned is the DC that is going to the plates - it's not mains.  Totally understand how confusing I was above, so apologies.

It sounds like you're saying that in an amp we have the mains ground to chassis AND we can get away with a power supply and preamp ground connection to chassis without worrying about loops.  However in a pedal, it may only make sense to have the mains ground to chassis ONLY.  Correct?

So:
1. Mains ground to chassis in amp for safety and you can still avoid ground loops by attaching power amp and preamp sections to their own chassis connection  for safety (which is what I've done in all my builds)
2. If mains were going into a pedal, I would also run a safety ground to the chassis BUT I would be unable to run a separate safety ground for preamp / power amp due to potential massive ground loop.  In this instance, the preamp / power amp grounds and signal grounds would be "floating" inside the chassis.  I'm guessing this is less safe but quieter.

I failed to give you all the pieces of the puzzle.  I'm bringing 9vDC into the pedal from a wall wart into a Nixie power supply.  It's this specific one:



I'm assuming this may change things. 

[PRR]

> I'm bringing 9vDC into the pedal from a wall wart into a Nixie power supply.

Then you do not have wall power in your pedal. Be sure your 9V wart is good quality and UL approved, and there is very little danger.

[iainpunk]

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?

yes, about 80% correct.
when there is any current from either B+ or mains to ground, the fuse should blow due to the current being nearly unimpeded.

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.

B+ ground IS signal ground, so every ground connection should go top a single ground point!
that's generally the No.1 rule of grounding.

a lot of nixie supplies seem to switch at audio rates; either use one that goes way above that, or use plenty of power filtering.

cheers

[Twhjelmgren28]

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?

yes, about 80% correct.
when there is any current from either B+ or mains to ground, the fuse should blow due to the current being nearly unimpeded.

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.

B+ ground IS signal ground, so every ground connection should go top a single ground point!
that's generally the No.1 rule of grounding.

a lot of nixie supplies seem to switch at audio rates; either use one that goes way above that, or use plenty of power filtering.

cheers

Okay - Think I got it! 

So really the main concern for safety is if there is mains coming in - that would need to be grounded to chassis for safety - I guess grounded to earth is a better phrase.  In my pedal above there is no mains coming in so no concern there.

All other grounds in a circuit are signal grounds (including B+ / HT - in my case, +240V).  So, I'm grounding everything to chassis to act as a Faraday.  There is no need to connect to chassis for safety in my case.

BUT, sounds like if I had brought mains in and grounded it to the chassis for safety, then it would be better to float all signal grounds (including B+ / HT grounds) to avoid ground loop hum?  Would I be sacrificing safety for low noise in that case?

Apologies, this is more about me grasping the idea of grounding for safety (vs. grounding for function and low noise) rather than solely for the pedal above.  I'd like to understand.

[iainpunk]

all ground is ground, just make sure all grounds terminate at the same point, both for noise and safety, make a 'star ground'

BUT, sounds like if I had brought mains in and grounded it to the chassis for safety, then it would be better to float all signal grounds (including B+ / HT grounds) to avoid ground loop hum?  Would I be sacrificing safety for low noise in that case?

B+ ground and mains ground have to be terminated at the same place on the chassis, both for safety and noise.
bringing in mains doesn't mean you should float the other grounds, all grounds have to be connected, full stop, no exceptions.
ground loops exist if there are multiple paths for the ground current to flow, but the chance of making a ground loop is really small in a tube amp/preamp, especially if you keep to rule No.1 of grounding:
ALWAYS TERMINATE ALL GROUNDS AT A SINGLE POINT. NO EXCEPTIONS

cheers

[Twhjelmgren28]

Done - thanks for the tough love!  I've read up on star grounds a bit - mostly through Merlin Blencowe.  Looks like he grounds mains directly to chassis where the mains comes in; everything else is still connected to chassis via the main star point - but I know Hoffman has a semi-star ground layout in which the mains is grounded with B+.   

Anyway - thanks again everyone!