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IEC jack signal/chassis ground 
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Hello! I'm not building a guitar amplifier yet, but I've built a few guitar pedals before and would like to get started with some of the theory behind amplifiers. Since I've only worked with unipolar DC power where you only have to worry about 2 poles (where chassis and signal ground are combined), I have a question regarding 3 pole power from an IEC jack.

From what I've understood, the earth/ground connection on the IEC jack is mainly used for chassis grounding. Does that mean the neutral connection (on the secondary side of the transformer) acts as the signal/power ground? This is how I've understood it, but when I check continuity between the ground connections on the 1/4" jacks and the earth pin on the IEC jack, it turns out they're actually connected while the jack ground isn't connected to neutral. This seems like such a simple thing to grasp, but I'm really having trouble finding any information on it. Please help!

Thank you!


Wed Jul 18, 2018 2:28 pm
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Welcome to the fold brother! This is the best guitar/amp forum on the interwebznet!!! :welcome:

There are a few guys on the forum who are very knowledgeable about amp design and repair.
I'm sure they will chime in, over the next day or so.

I started studying amp repair about 2 years ago and I've yet to grasp the ins-and-outs of grounding schemes.
Grounding design is strange alchemy.

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Wed Jul 18, 2018 6:51 pm
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This explains pretty much everything you'd want to know about grounding: http://valvewizard.co.uk/Grounding.pdf

The outlet ground is safety ground. This should be made securely on the chassis as close to the IEC socket as possible on it's own screw, and not as part of some other assembly, i.e., the transformer bolt as seen on some old amps. This is there to act as ground path in the event of a catastrophic failure, such as the B+ or an AC winding shorting on the chassis, stopping the chassis from becoming live. Fun fact; before anyone gave a shit it was perfectly legal to sell radios and the such with a hot chassis and shocks were common.

The neutral is part of the AC along with the live. The important thing to grasp is that the AC goes through a transformer to step the voltage up and then by means of diodes or a tube, is then rectified to DC. This DC voltage is then referenced to some other DC voltage. Usually 0V though another ground connection, usually close to the signal input.

The pdf I linked to above does a much better job of explaining this and there are many ways of doing it. I follow the ground plan outlined here and usually end up with pretty quiet and safe builds.


Thu Jul 19, 2018 2:23 am
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ajaxlepinski wrote:
Welcome to the fold brother! This is the best guitar/amp forum on the interwebznet!!! :welcome:

There are a few guys on the forum who are very knowledgeable about amp design and repair.
I'm sure they will chime in, over the next day or so.

I started studying amp repair about 2 years ago and I've yet to grasp the ins-and-outs of grounding schemes.
Grounding design is strange alchemy.


Thank you! :wave: You're right. Grounding is an artform in its own right.

Zozobra wrote:
This explains pretty much everything you'd want to know about grounding: http://valvewizard.co.uk/Grounding.pdf

etc


Thank you for the link and explanation. I've read parts of that pdf before, but I'll take a look at it again.

I continued checking the continuity in one of my amplifiers and the image below showcases how the ground seems to be connected. Sorry for the rough sketch. I don't have access to a computer atm.

Image

As you can see, the earth pin on the IEC jack and the neutral on the secondary side of the transformer are both connected to the chassis and circuit ground. Is this a good way to do it? The amp is a discontinued, but still pretty modern transistor amplifier.


Thu Jul 19, 2018 6:39 am
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Ok, so to simplify the question a little bit, do you use the neutral connection from the transformer secondary as the circuit ground?


Mon Jul 23, 2018 1:37 pm
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No. It's not especially useful to think of the secondary as having a live and neutral. You need to ground the DC reference, which then acts as the ground reference for the amp.

You may find this better for your needs as it's specifically about solid state amps:
http://www.thatraymond.com/downloads/so ... a_v1.0.pdf

It's a bit slow moving but this is a good forum on SS design/repair:
https://www.ssguitar.com/index.php


Wed Jul 25, 2018 2:01 am
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Zozobra wrote:
No. It's not especially useful to think of the secondary as having a live and neutral. You need to ground the DC reference, which then acts as the ground reference for the amp.

You may find this better for your needs as it's specifically about solid state amps:
http://www.thatraymond.com/downloads/so ... a_v1.0.pdf

It's a bit slow moving but this is a good forum on SS design/repair:
https://www.ssguitar.com/index.php


Thank you for the links! I'm not entirely sure what you mean by DC reference, but I'll read at least the power supply chapter of the PDF soon and see if I can figure it out. Unfortunately I'm in the middle of a move right now, so I don't really have time for it now.


Sat Jul 28, 2018 10:44 am
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