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Old 24th June 2009, 10:49 PM   #1
dogfish is offline dogfish  United States
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Default Simple SE grounding -- metal spacers/standoffs?

I feel like if I thought about this hard enough I could figure it out...

I am starting my first tube build, a Simple SE. I'm going to attach the PCB to an aluminum top plate (in a wood chassis). I've read that I should use insulated RCA jacks (to prevent ground loops).

Question #1:

I was going to use conductive standoffs between the PCB and aluminum plate. Should I instead be isolating the PCB from the plate? Are the holes in the PCB (where the standoffs pass through) connected to ground, and if so, does it matter?

Question #2:

Should I insulate the transformers from the aluminum plate (via non-conductive washers)? Or is it OK that the transformer bodies contact the plate?

- Dogfish
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Old 24th June 2009, 11:04 PM   #2
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I chose to use rubber washers to support the circuit board. They were less expensive than metal standoffs, easy to adjust for length, and offer some vibration isolation from the power transformer.

Click the image to open in full size.

You may isolate the transformers if you choose, but the transformers themselves must be attached to the safety ground somehow. If there is a short between winding and core, there could be high voltage on an exposed transformer surface.
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Old 25th June 2009, 12:04 AM   #3
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Quote:
Are the holes in the PCB (where the standoffs pass through) connected to ground, and if so, does it matter?
The holes in the PCB are not connected to anything, so metal mounting screws can be used.

Quote:
Should I insulate the transformers from the aluminum plate (via non-conductive washers)? Or is it OK that the transformer bodies contact the plate?
The transformer bodies must be connected to the safety ground (the third prong on the power cord). This can save your life if a transformer develops an internal short (not common but it does happen).

Quote:
I've read that I should use insulated RCA jacks (to prevent ground loops).
In most cases I do exactly the opposite. There must be a connection from the amplifiers circuit ground to the safety ground. To avoid a ground loop, there should be only one such connection. I have found that the input connector is the best place for that connection if a metal chassis is used.

The wiring from the board to the input connector (including the volume control if used) makes the circuit ground connection to the chassis. I run a wire from the third pin on the power connector to the ground pin on one of the input connector. This makes the safety ground connection. If the transformers are bolted to the grounded metal chassis, they will get grounded.
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Old 25th June 2009, 03:32 AM   #4
rknize is offline rknize  United States
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I made standoffs by cutting long aluminum bushings into the lengths I needed. The holes on the board are sized for 10-32 screws.

Click the image to open in full size.

Insulating the inputs does give you the choice of where to make the chassis ground connection, if your chassis somehow works better that way.
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Old 26th June 2009, 04:26 PM   #5
dogfish is offline dogfish  United States
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Thanks for the help.

I've got everything I need (except the output transformers, waiting for Edcor). Now all I need is some free time!
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