Input Ferrite Beads

[GainFreak]

Hi,

In some amp instead of the usual 68k input grid resistor ferrite beads are used. It's known that the 68k resistor takes care of the RF but it also generates a lot of noise. I noticed that Marshall have those beads almost in every amp. I've seen similar in Mesa amps as well.
It looks like they're using this type of beads:

Your Search Results | Farnell United Kingdom

Other companies make them as well. Several types are available - with one turn, 2 turns etc. My question is what to look for when buying those - impedance, MHz, DC resistance?
Thanks.

[dynaleadamps]

I based mine on impedance. I previously used 10k resistors for my input stopper, so I bought the 10k beads. They work perfectly ... no noise at all! I think I found mine on eBay, though.

[Steve Conner]

To have the same RF attenuation as the 68k resistor did, you'll need a bead that has an impedance of 68k in the AM broadcast band. That would be an enormous lump of ferrite with dozens of turns.

That's why I prefer to just use the resistor. I don't think the Johnson noise from it is significant. I don't know what the source resistance of a guitar is as far as Johnson noise is concerned, but it's got to be somewhere between the DCR of the pickups and the resistance of the volume pot, probably nearer the latter at high frequencies.

[GainFreak]

I previously used 10k resistors for my input stopper, so I bought the 10k beads.

I've seen comments about using 10k resistor instead of 68k because it generates less noise but none of those beads from the link has such high impedance.