RFI filter for guitar inputs

[Mars Amp Repair]

Years ago, we had a really simple RC filter to eliminate RFI interference from the inputs of receivers.
As I recall, it only consisted of a coil in series & a cap to ground (or possibly 2 caps to ground to form a pi filter).
I have an amp that is used relatively close to out Lookout Mountain in Golden, CO where all the transmitters are.
I used to have but have since lost all my info on filters...does anyone know of a good aftermarket in-line filter or the values of said filter to install in line between the input jack & input amp? I suppose it might be as easy as calculating RC for a given frequency you want the input to cut off at.

thanx, g

[dai h.]

there was a really simple one described on the Bartolini site which was just a cap from hot to ground of ea. pickup IIRC. Just enough to reduce the RF and no more. The one you're describing I think is shown in the Darr book. And there is one on the Aiken amps site as well. Maybe it would be good to combine the 1st and 3rd and divert the RF to the chassis.

[Enzo]

You want in the amp? Or in the guitar?

ferrite beads around the input wire or other small inductances are common, and a small cap can help. just a few pfs is all. If the jack is plastic, run a small cap to chassis from the sleeve contact. Ground the cap right there, don't run wires.

Make sure the innards are shielded. A metal plate over the input stages would help. Most amps have foil or metal screening or sheet metal in the cab to cover the open side of the chassis when it is installed. The bolts hold them together for electrical contact. Make sure the amps have that intact. SOme amps like PV use a conductive paint inside the cab. Looks like black paint but it is a shield.

If the amp is like a Fender with the jacks in front and a wire running all the way to the rear of the chassis to get into a tube, change that wire to shielded.

Get some schematics of recent digital music stuff like a synth or some DSP unit. Yamaha has been doing this stuff for decades. look at how they filter and isolate the audio ins and outs. Steal the approach.

Is this preemptive, or are you actually having RFI?

Contact the agency who broadcasts and ask if there are common countermeasures for potential RFI to audio that they can share.

[Mars Amp Repair]

Well Enzo,
As with years ago, this only happens in Golden where the guy lives & plays out sometimes.

I like the approach of better shieding & small cap. I believe I have some ferrite beads in junkers to use, too.

The issue is the same as years ago, we never know if we've got the issue fixed, until the guy takes it home where the RFI field is high.

As usual the amp is detecting an AM signal. I was going to focus on the amp, but I suppose it could be shielding in his guitar (actually it's a harp...yeah a real harp) or perhaps it could even be a cheapl cable with minimal shielding like so many....I think he tried a better one.

Thanx as usual for the ideas....g

[Bruce / Mission Amps]

Since the amp is not a radio... I'd bet you have some RF rectifiction happening somewhere too.
Crappy cables, dirty input jacks (push a 22 caliber brass cleaning brush in and out of the input jack) oxidized solder joints and bad/dirty tube sockets can do this.

FB77 J series beads seem to work OK if you get the small diameter ones.
Otherwise a FT (toroid) work OK with the lead weaved in and out of the loop.

http://www.amidoncorp.com

By the way.. when I was still in the 2-way land moblie commercial radio biz, I used to be one of the monkeys who climbed those giant towers by B. Bill's Grave up there and installed that high power radio equipment and hardware!!
Talk about RFI!!!! That was a serious challange.

[Mars Amp Repair]

Thanx for the info Bruce & the link for the ferrite beads. I'm glad I don't run across too many issues like this.

This amp is a little Flat somethingorother battery powered tiny portable thing. Frankly, it looks like an old portable radio. It is a new unit. Fairly inexpensive looking thing, tho.

I really haven't had the chance to open it yet. It could very well be just poorly placed or made grounding as you-all have mentioned.

I'm affraid climbing those towers would have had me going through undergarments at an alarming rate...I've seen those things. I didn't mind teetering on roofs back in the 70's when I used to install TV antennas on homes or climbing trees...but towers...No thanx.

I knew of a guy who used to replace the flashing warning lights on the am & fm & tv towers. He'd take a running start at the tower & then leap through the air & grab the tower (which actually is the antenna)....he was ok as long as he didn't get between the tower & the ground!! I guess he wasn't interested in offspring...g

[Bruce / Mission Amps]

The high current node of the TX tower is closer to the bottom then the top.
Feedpoint Zed is pretty low on those so there isn't much voltage even with a 10Kw-50Kw transmitter.
The high voltage node of the TX tower is up near the top where voltages can be astronomical if not tuned correctly.

In my youth, my highest free tower climb was 270 feet.
I just took my 22 year old 70 foot HAM tower down last summer when we sold the house in Thornton and it scared all hell out of me and I know what I'm doing!!