What Causes Radio Interference?

[blown240]

I just build a Dual Valve Caster overdrive pedal and when the gain is down it clearly picks up a Vietnamese radio station.  With the gain up it goes away.  Its the first gain knob that makes the difference.

What causes this?

Schematic can be seen here:
Scroll down a few pages for the Dual Valve Caster:

http://www.aronnelson.com/DIYFiles/up/12AU7-6111_Valve_Caster_Summary_Rev002.pdf

Its all point to point with most of the parts hanging off of the tube sockets.

[PRR]

Shorten the wires.

Close the box.

Add 33K grid-stoppers in series with grid caps.

Add 1000pFd from each plate to its cathode.

_IS_ there a Vietnamese radio station in town? If there's a 5KW two streets over, sure you are gonna get it (I grew up near a 50KW). If you are pulling it across the ocean, that's quite odd.

[Colas LeGrippa]

PRR, I think the 50 KW radio station has come in oscillation with your brain. That's probably why you irradiate so many informations in every post you answer. By saying this I surely do not mean you are a parasit !  I should maybe change the 500 W radio station I am working at for a stronger one hey hey !

Colas

[blown240]

PRR,  thanks for the info.  I will do that.

Yes there are several Vietnamese radio stations nearby.   I live about 3 miles from an area known as "Little Saigon."

[stingray_65]

Just to answer your original question.

The wire going to pin 2 is an antenna as well as an audio signal path.

Shortening the wire "mis-matches" the antenna and attenuates the radio signal, adding the grid stop resistor (directly to pin2 if possible) forms a low pass filter (audio frequency is low, radio frequency is high).

I'm unsure what the effect of the 1000pf cap does, but if PRR says use it, I would.

Oh, and closing up the box is just good shielding.

[phsyconoodler]

Get a grip Colas!

[Colas LeGrippa]

get a Grippa, Colas LeGrip !

[Colas LeGrippa]

Hey Blown, you saw the name of the pedal designer ? MAT SUMIN ! He's probably Vietnamese ! Build a Marshall pedal if you wanna pick up some England radio station ! ( and so on.i It's obvious to me ! )

[blown240]

Lol.   I should have realized that!

[billcreller]

A friend in Honolulu has radio stations booming in on his amps and recording pre-amps.  When I was there, I could see a couple radio towers from his 5th floor apartment window. They were like 1/2 miles away.  I told him he was really screwed  We tried some various types of shielding, some iron chokes on wires etc, but nothing came close to fixing the problem.

[Willabe]

I seem to remember Merlin has some info on this in one of his books. I don't know if it's posted on his web site.

Aiken Amplification has some info on this too, but his site is under construction.

IIRC, they run a small cap, (0.01?) from the ground on the input jack to the chassis keeping the caps leads as short as possible. The shield on the input cable acts as an antena and the cap bleeds this signal off to ground?


                   Brad      


Edit; Merlins site seems to be down also.