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Old 17th February 2011, 01:33 AM   #1
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Default Mount rectifier above torroidal transformer?

I am planning my amp layout: I have a torroidal transformer inside the chassis, and can't find anywhere to mount the rectifier tube except directly above the centre of the torroid.

I am worried that the electro-magnetic fields of the torroid underneath the chassis will affect the performance of the rectifier tube directly above? i.e. that with each 60hz cycle it will interfere with what the electrons are trying to do inside the rectifier?

Related to this is running the wires to the rectifier - they would be in close proximity to the above-side of the torroid - so again I am wondering if the torroid's 60hz alternating current is going to interfere with the filament and power wires running to the rectifier?

Advice appreciated.
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Old 17th February 2011, 02:00 AM   #2
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I'm not an expert in such things; however in your situation I would get a piece of MuMetal and make a 'shield' over the torrid. Mount it on brass standoffs so it grounds to your chassis.

I don't know if the rectifier tube will be affected by the field, assuming it is a great enough field to affect it. But, a shield can't hurt!
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Old 17th February 2011, 06:34 AM   #3
tomchr is offline tomchr  United States
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You're running 60 Hz at whatever secondary voltage into the rectifier. Several hundred volts of 60 Hz AC most certainly will swamp out any itty-bitty AC component injected by the leakage field from the transformer. The electric field set up in the tube by the incoming AC is certainly higher than whatever the leakage field can set up.

Recall in a toroid the magnetic field is contained almost entirely in the magnetic core due to its doughnut shape. In an EI core or other shapes with sharp bends and corners, some field will leak out at the corners. Around the toroid (including the center), there's a small leakage field as some field escapes due to the space between windings, but it's really tiny -- even in the center of the toroid.

I wouldn't worry about it. Just watch out for mechanical interference between the mounting hardware for the transformer and the pins on the tube socket. You don't want any sparky action there...

~Tom
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Old 17th February 2011, 07:08 AM   #4
Alkis is offline Alkis  Greece
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Leave as much space you can between the trafo and the chassis using some diy platform-made of cork perhaps, or putting plastic rings together(one above the other) in order to maintain some hight.
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Old 17th February 2011, 09:50 AM   #5
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What tomchr says: it's not an issue. Don't worry about it. Moreover, the rectifier is before any filtering, so any (theoretical) 60Hz ripple injected at that point will be reduced by the PS filtering circuit.
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Old 17th February 2011, 09:59 AM   #6
DF96 is offline DF96  England
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Nothing to worry about. Toroidal transformers are quite good at holding on to their magnetic field. A vacuum rectifier will not be much affected by a magentic field. The only issue you should worry about is heat.
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Old 17th February 2011, 10:15 AM   #7
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+1

Last edited by SemperFi; 17th February 2011 at 10:16 AM. Reason: ?
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Old 17th February 2011, 06:10 PM   #8
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Thanks for the clear answers...makes sense.

On to a related question: What about lower voltage signal and speaker wire proximity? I would like to place the RCA inputs and Speaker outputs on the back of the chassis, which requires that the signal and speaker wires run parallel past the one side of the torroid. There will be about an inch of space.

I've noticed on this torroid (Antek) that it has a metal shielding strip around the outside face, which might attenuate some of the field. I'm also thinking of using grounded shielded cables to help screen the wires from the field.

Am I taking a bit of a chance on hum?

Thanks again.
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Old 17th February 2011, 06:30 PM   #9
DF96 is offline DF96  England
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Keep signal wires well away from the power supply. I would not even run an output within an inch of a power transformer. Screening won't stop a magnetic field. However, screening or twisting will ensure that both wires get the same voltage induced so careful grounding can then help the hum balance out.

Leave a bit of space between inputs and outputs too.
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Old 17th February 2011, 06:40 PM   #10
wa2ise is offline wa2ise  United States
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Back in the vacuum tube TV set era, they often used power transformers that had, on the top bell, the rectifier tube. A power transformer with a rectifier tube sticking out of it on top. Saved chassis real estate, and inside a TV set cabinet, vertical space was plentiful. Also it placed a heat source up and away from other parts of the TV chassis. So what you are thinking of doing, placing your rectifier above the toroid transformer, should work fine. Just give consideration to heat and insulation. Stray 60Hz magnetic fields wont matter to a rectifier.
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