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Huh?

Posted by KurtP (A) on February 25, 2007 at 07:28:26

In Reply to: Re: Heater wiring sequence: please advise posted by Ron Oehlert on February 24, 2007 at 18:40:01:

Maybe I misunderstand you, but semantics are important.

Twisting the wires does not null hum pickup.

Twisting the wires keeps them closely spaced together, which will permit the magnetic field from each (the two currents are 180 degrees apart) to effectively cancel, or at least be at a minimum. You will obtain very similar results with speaker wire. No doubt some will claim twisting is superior, but the foundation is the same, to minimize magnetic fields by cancelation.

The result of this is that nearby circuits will not have currents induced into them by this magnetic field. So the proper description is "twisting wires minimizes the radiated magnetic fields, reducing inductive pickup in other circuits"

With regards to shielding, you are dealing with a totally different animal. Traditional foil or mesh shielding creates a static shield around the conductors, forcing the electric field intensity to zero at the shield (relative to the inner conductors). So any electric fields generated by the conductors cannot radiate into the surroundings, as the shield makes the wire "look" like it's all at ground potential.

Totally different from the magnetic issue above. Magnetic fields will pass right through a copper or foil shield; electric fields will not.

So with the statement, "shielded wire will cancel that effect", I disagree. It is rather unrelated, and will only benefit. The twisting deals with the magnetic fields, which is generated as a result of the current magnitude. The shielding deals with the electric fields, which is generated as a result of the voltage magnitude. They are separate, and I encourage the use of both shielded and twisted.

We won't address the fact that magnetic and electric fields propogate each other, as this just adds confusion. As usual, the statements made above are simplifications that at least permit us to understand the basics of what is going on.

Last point is that the greater culprit in tube amps is the electric field strength; with the exception of heater circuits, the currents we are dealing with are quite low. The voltages we deal with, however are significant. It is for this reason that shielding is very effective in keeping an amp quiet.

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