[CButterworth]

I am in the middle of building an Aikido preamp using 6sn7's and the stereo board from John Broskie.  The chassis is built and the sheet metal parts are securely earthed to the earth-pin of the IEC plug.  The transformers and chokes are enclosed Hammonds, but I have also solidly connected these to the metal chassis using star washers.  So, should human touchable metal parts become live, they should be grounded to earth.

Now, the PSU will not be connected directly to Earth (the IEC earth pin).  It uses a bridge rectifier, a network of Amperite tube time delay and regular realy to charge the heaters prior to main power-up.  B+ joins the Aikido circuit board, and so does -ve (ground).

The circuit board has a traced star-earth and a pad to connect the circuit to the chassis.

So, I have read that the best designs connect the ground circuitry to the chassis then to Earth (via the chassis).  Although the board does give the option of not doing this, and also allows the ground to be floated by connection to the chassis via a capacitor.

My first question is:  Is my plan to tie the chassis to Earth, with the PSU/circuit-board being earthed via the Earth-pad/Chassis SAFE??????

My second question may be real dumb, and indicative of a brain-fart, but...... if the ground of the PSU/circuit-board is tied to the chassis, won't the chassis be hot and cause shock and possible death upon contact?

Regards,
Charlie

[JoshK]

My first question is:  Is my plan to tie the chassis to Earth, with the PSU/circuit-board being earthed via the Earth-pad/Chassis SAFE??????

My second question may be real dumb, and indicative of a brain-fart, but...... if the ground of the PSU/circuit-board is tied to the chassis, won't the chassis be hot and cause shock and possible death upon contact?

Regards,
Charlie

No need to feel embarrassed.  Grounding in audio is a difficult subject and can be very confusing.  It is something that I am just getting the hang of recently, so what I am about to say could be wrong but I think I have it sorted out right.  I hope others will chime in to verify.

The way you describe the above is safe.  You are in essence referencing your audio/psu circuit ground to earth ground.  However, may I suggest that you reference them all at one single point on the chassis.  To be more clear, don't tie the third prong to your chassis at one point on the chassis and your PSU/audio circuit ground to another.  This can cause currents to flow to your chassis from the one ground to the other....not a good thing.

If you don't tie your PSU/Audio grounds to the earth ground at the chassis, you have a floating power supply.  Then you have to take extra precaution that you are very well insulated from the chassis and somewhere in your system there is a path to earth (safety).  It is probably safest to not use this approach for high voltage components such as tube preamps where the insulation has to maintain high voltage potential differences.

[CButterworth]

Josh,

Yes, I was planning on connecting the PSU/audio to the chassis at the bolt that ties the chassis to Earth.  What I have actually done is to make a wooden box with a metal top-plate (the chassis) and a back metal plate for the connections (IEC, fuse, switch, inputs and output).

The back metal plate is tied to the top plate (chassis at one point), which also joins to the IEC Earth.  I suppose that this is a star Earth.  The circuit-board Earth will tie to the chassis at this same point.

I think that I am making sense.

Anyway, thanks for the reply.

Charlie

[JoshK]

What I have actually done is to make a wooden box with a metal top-plate (the chassis) and a back metal plate for the connections (IEC, fuse, switch, inputs and output).

What I am doing too.

I suppose that this is a star Earth. 

Exactly. 

[CButterworth]

Josh,

What a coincidence.  I decided that a wooden box would look great with a metal top chassis with transformers, chokes and tubes.  My initial amp will only use a simple pine box, although I half-blind dovetailed the sides and put on a 1/4 inch round along the top edge and side edges.  The sheet-steel chassis is painted black.  My future goal is to remake the wooden part with a really good wood such as Walnut, or even a quilted Maple.

I'll let you know how things go.

Thanks,
Charlie