ferrite core placement

[JasonG]

Ok
I am rebuilding a small tube amp and I am adding a 3 prong ac jack so I can switch out power cords. One jack I have has a ferrite ring on it. I know this is not a big deal but still would like to know how the transformer would interact with the ferrite ring.
In other words how far should the ferrite ring be from the transformer?
Thanks!

[jrfrond]

There should be no issues whatsoever.

[Enzo]

Far enough away that it doesn;t get in the way. Electrically it will be invisible.

[stingray_65]

I have to ask,

I've seen those rings before (iirc on digital stuff) but have always wondered just how effective can they be and just how are they supposed to work and what do they hope to acomplish?

if it makes a sort of choke doesn't that supress ac passage and wouldnt you want full power on your mains?


Ray

[defaced]

Here's my simple understanding of the ferrite bead. An inductor has an impedance of L*w where L is the inductance and w is the angular frequency. So, if the inductor is in series with an AC source (like a ferrite bead), and the frequency is low, like the 60 Hz of US line voltage, and the inductance is low, like a tiny ferrite bead, the inductor is basically invisible because the series impedance is small. Now, if you've got some nasty switching circuit working in the 100KHz range, then the frequency is large and even with a small inductor the impedance is enough to keep noise from entering the mains.

[Steve Conner]

Correct. The ferrite rings are there to stop digital and switchmode gear from spewing out RF, and they shouldn't have any effect on line frequency current.

EMC regulations limit the amount of RF that equipment is allowed to emit, and also mandate an ability to withstand a certain amount of RF from outside without acting up. So these little ferrite rings are now a multi-million dollar business.

In the case of a tube amp, there's not much inside that will generate interference, apart from the diodes in the power supply. So any filters would just be to stop RF getting in from outside. And the impedances of tube circuits are so high that I don't think ferrite rings are really effective, compared to, say, the 68k grid stopper resistor on the input, capacitors across the diodes, shields on the preamp tubes, tinfoil on the inside of the cabinet, and a Faraday screen in the transformer. Unless you have a radio ham living next door, or something.

[Robert M. Martinelli]

+1.

They're also used on VFD ( Variable Frequency Drives ) in the industry, to keep the drives' carrier frequency and its harmonics from going around on an unauthorized trip .

Cheers

Bob

[JasonG]

Ok, that's enough info to get me where I need to go. I have seen these beads on some audiphile tube amps but the way audiophiles are that doesn't say much. I will give it a shot on this project and add it to some others that I am sure could benefit from them.
In response to the idea about using aluminum foil to block out RF , I thought the you were supposed to use something that had magnetic properties like some screen used in screen doors? I thought that was also to block out stray magnetic noise.

[Enzo]

But most noise is not magnetic. The shielding on your guitar cord for example is not there to block magnetic fields, it is there to block electrical fields. And when you are talking RF, it is not probably coming from a magnetic field. Most any magnetic coupling in a guitar amp will be at 60Hz.

[Steve Conner]

Yes, when I say the impedances in tube amps are high, I mean that the internal circuitry is more sensitive to electric fields than magnetic ones.

Something like a microphone is the other way round, down at 200 ohms magnetic shielding becomes more important than electric. SM57s and 58s have bodies made of steel, I guess to try and keep magnetic fields away from the little transformer in there.

Tip: If you have trouble with interference from that radio ham next door, ask him for ferrites, he may well hand them out free for a quiet life :-)