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Mount rectifier above torroidal transformer?

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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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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
 
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.
 
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.
 
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.
 
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.

What matters is the separation and loop area. If the two signal wires form a big loop that intersects with the leakage field from the transformer, you'll get hum induced on the wire. By "loop" I mean that the signal forward follows a different path than the signal return conductor. The area of this loop determines the coupling between the signal pair and the interference field. The cure to this is to keep the signal forward and return wires as close together as possible. Twisted pair is even better.

With the low fields from the toroid you have little to worry about. Just take two lengths of wire and twist them tightly by putting one end in a vise and the other in the chuck of a power drill. Pull wires tight and twist until you have a good tightly twisted pair. It usually takes me a couple of tries to get the drill chuck to grip the wires, but after that it's smooth sailing.

Of course, if you can avoid running the wires near interference sources (output/power transformers, high voltage wires, etc), it's better. In my current design I plan to run the AC in the middle and signal wires along the sides of the chassis.

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.

That's commonly known as a belly band. You've got one of the newer AS-xxxx transformers then. There's also an electrostatic shield in those (purple wire I think). You need to ground that to chassis ground.

The belly band is to terminate, thus, reduce leakage field from the transformer. It's not very effective on toroids, but better than nothing. There's very little leakage flux from a toroid anyway.

The electrostatic shield shields the primary from the secondary. The intent is to reduce any capacitive coupling between primary and secondary. This should reduce RF coupling so any 'garbage' on the mains won't (theoretically) make it through to the secondary.

I'm also thinking of using grounded shielded cables to help screen the wires from the field.

The skin depth at 60 Hz is on the order of 5~6 mm. So unless the shield is upwards of an inch thick, it won't do you much good for shielding against 60 Hz signals. It will help with RF shielding - but so will a metal chassis. I see no point in using shielded cable inside audio equipment. Just use a tightly twisted pair as described above.

~Tom
 
Yep, I have one of those!
 

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Well, the amp is built! Sounds really good. Rectifier seems to work fine. I took a chance and ran the speaker wires and the signal input wires past the power transformer. There is a crackling sound which goes away when the input tube is removed. I was expecting a hum, not a crackle. I am going to try a different tube but if that doesn't work then I might move the RCA signal inputs. Note that the twisted speaker pairs seem to pick up no hum at all.
 
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