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Old 23rd August 2012, 03:30 PM   #1
ixe13 is offline ixe13  Canada
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Default Power and output transformers to ground.

Hi,

I have a general question: The power and output transformers on an amp are suppose to be grounded, right?

Now, it's done by attaching the transfo to the chassis using the bolts and nuts, so the "legs" of the transfo are then in contact with the chassis.

But what about the core, the laminations? is it suppose to be grounded as well?

I've been reading that the bells and the laminations are suppose to be isolate from each other. And the laminations are also isolated between themselves.

Then, theoretically, the laminations are not grounded through the bells.

I have 5 amps here, with new transfos, and on some of these transfos, the laminations have continuity with the chassis!!?

And they all sound fine.

Also, same questions about a power transfo sitting flat on the chassis like the Dynaco ST-70 that has the screws going through the laminations.

I have doubt that there is not a little spot where the screw make contact with the laminations...

Anyway,

I want to repaint some used transformers, but if I remove the bells, what would be the pittfall to watch for, concerning isolation between these different parts? and the best way to groud a transformer.

Well, it may not be a general question that much after all...


Thanx in advance for any infos.


Yves
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Old 23rd August 2012, 04:30 PM   #2
kevinkr is offline kevinkr  United States
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I've rarely run into transformers where the cores and bells were deliberately isolated from each other, from a safety perspective it is good when the whole thing can be grounded to the chassis which in turn is connected to mains safety ground.

Secondaries of transformers driving headphones or speakers should always be grounded for safety.
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Old 23rd August 2012, 06:09 PM   #3
DF96 is offline DF96  England
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There needs to be a highish resistance between laminations to prevent eddy currents. They are usually coated to provide this. The frame of the transformer should be grounded, and this will provide whatever grounding (or not) is needed by the laminations. In some cases, the laminations will all be connected together by a weld at one point on the edge - joining at one point does not support eddy current loops so OK.
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Old 24th August 2012, 01:19 PM   #4
palmas is offline palmas  Portugal
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E-I tramnsformers need to keep laminations insulated from eachother (usually be oxidating one side of each, also the bolt holding them together should be insulated in both ends and inside.
What you should have is a screen (copper sheet) between primary and secondary coils, and in the outside covering all coils, connected to ground.
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Old 24th August 2012, 06:59 PM   #5
12E1 is offline 12E1  United Kingdom
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Quote:
Originally Posted by palmas View Post
What you should have is a screen (copper sheet) between primary and secondary coils, and in the outside covering all coils, connected to ground.
"Should" is quite a strong word. Many transformers will not have a full metal screen between the primary and secondary windings. Even fewer will have a copper screen over all of the windings. In fact, of the 30 or 40 mains power transformers that I have only a couple with a grounded outer copper screen. They were designed for use in valve tape recorders where greater screening was a requirement.

If a transformer has clamp bolts holding the bell-end covers in place, it is good practice to ground these. However, in order to minimise eddy currents and loops, ideally only one of the bolts should contact the covers - the other three bolts would be insulated from them, typically with fibre washers.
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Old 24th August 2012, 07:21 PM   #6
palmas is offline palmas  Portugal
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Quote:
Originally Posted by 12E1 View Post
"Should" is quite a strong word. Many transformers will not have a full metal screen between the primary and secondary windings. Even fewer will have a copper screen over all of the windings. In fact, of the 30 or 40 mains power transformers that I have only a couple with a grounded outer copper screen. They were designed for use in valve tape recorders where greater screening was a requirement.

If a transformer has clamp bolts holding the bell-end covers in place, it is good practice to ground these. However, in order to minimise eddy currents and loops, ideally only one of the bolts should contact the covers - the other three bolts would be insulated from them, typically with fibre washers.
Shoul, could, might, ...
You're right, off course. I have my TR custom wound and I include copper shielding only when it is for sensitive stuff.
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Old 24th August 2012, 07:32 PM   #7
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Quote:
Originally Posted by ixe13 View Post
Hi,
...And the laminations are also isolated between themselves.

I want to repaint some used transformers, but if I remove the bells, what would be the pittfall to watch for, concerning isolation between these different parts?
Yves
I would avoid sanding/wire brushing/etc the lams for the reasons that you've stated above, as these activities can short them if they are isolated.
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Old 24th August 2012, 09:06 PM   #8
ixe13 is offline ixe13  Canada
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Thanx all for the infos and tips so far, it does "clear the fog" for me...

Quote:
I would avoid sanding/wire brushing/etc the lams for the reasons that you've stated above, as these activities can short them if they are isolated.
Do you think a shot of black spray paint on top of that type of finish on the lamination would hold?
Of course, if there is to much rust, it would have to go i suppose?

The two I have actually aren't so bad. What about a very slight sanding with a fine sandpaper just to clean the lamination a bit before spray paint? or maybe alcool, ...not the one you keep in the fridge, that for cleaning the brain
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Old 24th August 2012, 09:36 PM   #9
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The few that I've painted I've just sprayed over the lams with a spray can, surface rust and all. Still looking quite good after several years of steady use.

Auto body conversion primer, then paint........
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Old 24th August 2012, 09:53 PM   #10
ixe13 is offline ixe13  Canada
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Cool, thanx
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