Radio noises through an Amp?

[murraymurray]

Just curious, was at prac last night and my other guitarist turned his amp on and we started hearing atomic kitten playing through it? what would cause this? is it a problem or just a random occurance
it was a newish JVM410, so i was guessing marshall had put in an fm radio mode, there's so many other modes in there i wouldnt be too surprised.  :lol:

[Jonny]

It happens. It's not broken or anything. Just think of it as waves of various kinds and you have a speaker. So you would in a sense pick up pretty much anything that's close.

[Twaddlefish]

http://www.youtube.com/watch?v=I8JiQCIzCZA

We all know what's coming in this clip, rock up to 2.40 for relevance!

[marantz1300]

I sometimes get it through my Boogies.Never had it through my Hiwatt. I just move or play over it.

[murraymurray]

yeah didnt think it was a problem, just never heard it before. just reminded me of spinal tap at the airforce base, but he wasnt using a wireless.
is that what the youtube clip is of? cant check it at work.

[Alex]

The amps preamp amplifies everything that comes in. If you have an antenna feeding it radio signals it will make them audible.

So where is this antenna? It is logically one of the cables, and that means than one of the cables isn't shielded properly (or the shielding is broken).
Find the bad cable!

[murraymurray]

true, i was assuming it may have been something to do with the guitar, it was one of the bottom of the range Epi LPs, thought some dodgy wiring may have been picking up the radio signal

[Simon D]

I get this sometimes when I boot in my BB Preamp in front of my amp's drive channel. Picks up French radio, which is odd cos I'm in Wales.

[Dmoney]

i get it sometimes in a certain place using my stuff. and never anywhere else.
i think it is the cables, which has been said already.
we should all start using shielded twisted pair guitar cables.

[Bob Johnson]

The amp will pick up any radio signals that are around; possibly from the guitar but usually through the lead. It's not necessarily a grounding issue. Coiled leads are more prone to this problem since they can act as a variable condenser just like the tuner in a radio. I'd suggest getting broadcast quality leads like Van Damme or Klotz as they less susceptible to RFI than a lot of trendy stuff sold from guitar shops etc at exorbitant prices

[the_bleeding]

check cables. Then check your guitar. It has to happen before the preamp for you to hear it.

[Johnny Mac]

Its cos you have Nigel Tufnell Pickups.

[jpfamps]

Radio break through is caused by the combination of capacitance and inductance in the guitar lead/ pickup forming a resonant circuit which happens to be tuned to a radio station frequency. This is then amplified and demodulated by your amplifier.

If the amp is designed correctly there should be no radio break through.

Generally, assuming that the amp has a sensible grounding strategy, RF interference can be eliminated using the following strategies:

1) Ensuring that the first stage grid stopper is wired directly to the valve socket. I like to use 47k for 12AX7s. Larger valves will reduce the risk of RFI, although really high valves will start to attenuate treble frequencies.

2) Shielding the wiring to the first gain stage (if not already done).

3) Grounding the socket end of the first stage shielding to the chassis via capacitor (eg 0.01 µF).

Usually step 1) is enough to kill any radio break through in most amps.