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Interference in an amp

PostPosted: Fri Apr 24, 2009 9:57 pm
by leftyguitarman
My buddy who lives in Washington has some huge radio towers across the valley from his house on top of a hill and every time he turns his Marshall half stack onto the distortion channel he can hear music (not his own :lol: ) and voices. He thinks it's coming from some sort of interference with the radio towers. Is there a way to stop that?

Re: Interference in an amp

PostPosted: Fri Apr 24, 2009 11:27 pm
by ricochet
It's a common experience and a FAQ on every board where amps are discussed.

Some basic ideas: Try reorienting the amp or moving it to a different location. Plug it into a different outlet. Plug the power cord in through an RFI filter, which has filter chokes in both lines. (RFI often comes in through the power lines.) The amp may need modding with something like a ferrite bead around the input lead to the first stage tube grid, or it may need a resistor added there if there's not one already. It needs to be as close to the solder lug on the tube socket as possible.

Re: Interference in an amp

PostPosted: Sat Apr 25, 2009 12:16 am
by Ed Boyd
When I was 13 I took 2 - 15" speakers out of a Gretch Nashville amp and got 2 more 15" speaker and I built these 2 huge 2x15 cabinets. For front grille I covered them with steel hardware cloth. I set them up in my upstairs bedroom, hooked them up to an old Kustom head and played guitar through them.

Holy crap I could recieve ham radio, airplane traffic, police band, AM/FM and probably some secret communications from Russian Communist spies. :lol:

Re: Interference in an amp

PostPosted: Sat Apr 25, 2009 6:13 am
by jaybee
are you sure it's the amp? no effects in line? I used to have a 60's VOX wah, that I called "Radio KGB" since every time I turned it on there was a lot of noise that I didn't make (for a change) but never quite tuned to a station we could really make anything out of... 8)

Re: Interference in an amp

PostPosted: Sat Apr 25, 2009 7:16 am
by leftyguitarman
Well it isn't my amp so I can't be exactly what's happening. My buddy was just asking me if I knew anything about what was happening to it. It is a Marshall solid state head into a 4x12 cab. He plays in 1960 LP reissue through some Boss pedal and then into the amp. Jaybee, I'm (he's) sure it's the amp because it only happens when he goes to the second channel, the distortion channel.

Rico, that thing you were talking about; could that be a problem on a solid state amp too? I noticed you mentioned tube in your post. Also, I'll let him know about the RFI filter.

Re: Interference in an amp

PostPosted: Sat Apr 25, 2009 5:48 pm
by maxx england
If you ever have to play in a place with a hearing aid induction lop built in, keep the amps away from the walls. Better still, get the loop switched off, or you'll get deaf people talking over the top of what you play.

Re: Interference in an amp

PostPosted: Sat Apr 25, 2009 8:35 pm
by ricochet
When I hear "Amp," I always think of tubes, but similar principles apply to solid state equipment. I have little practical working knowledge of SS stuff, though. Solid state's a fad, it'll pass.

Re: Interference in an amp

PostPosted: Mon Apr 27, 2009 6:40 am
by leftyguitarman
So if the AFI filter doesn't work, what else could fix the problem in a SS amp?

Re: Interference in an amp

PostPosted: Mon Apr 27, 2009 4:53 pm
by LesFromChicago
EDITED: I erased my post here because I got a bad habit of telling stupid jokes in serious discussions.

Re: Interference in an amp

PostPosted: Mon Apr 27, 2009 5:32 pm
by jeffl
leftyguitarman wrote:So if the AFI filter doesn't work, what else could fix the problem in a SS amp?
The earlier remark about trying it plugged into a different outlet is the place to start. I have an outlet in my house that sends me messages from above. I've redone the ground in it twice and it doesn't make any difference...I think it's just the geometry of that particular household wiring circuit. It can be something else that's plugged into or run off the same circuit too.. like a ceiling fan or table lamp. I'd fool around with the household circuit first.

Re: Interference in an amp

PostPosted: Tue Apr 28, 2009 11:32 am
by tz1238
Do you have a computer running in the same room? I found out that when I turn on my computer I get a ton of interference. It's either from the computer or the monitor.

Re: Interference in an amp

PostPosted: Tue Apr 28, 2009 2:24 pm
by leftyguitarman
tz1238 wrote:Do you have a computer running in the same room? I found out that when I turn on my computer I get a ton of interference. It's either from the computer or the monitor.


He doesn't have a computer in there anymore. If you have a CRT monitor, I bet that's where your interference is coming from.

Thanks so far for the help guys. I told my buddy about some of this stuff. They've been having problems in their house with the cable too. The HD signal has vertical lines in it so they called Comcast and had a tech come out yesterday. The guy was gonna bring one of those AFI filters, supposedly. So we'll see if it works. I'll call my buddy tonight.