AM Radio Interference-How to cure?

[Cygnus X1]

Many of my amp builds have a problem with picking up AM radio signals. It isn't just in this house-it happens elsewhere
as well.

Is there a quick fix for this problem without drastically altering the tone of the amps?

[Structo]

Boy, I don't know.

Try to shield them as best you can (cover plate, preamp tube shields, grid resistors, shield your guitar cavities).

Do you have an AM transmitter nearby?

Seems like at one house I lived in as a kid, we were near a transmitter.
If I remember correctly, they had to give people that lived nearby some kind of filter that plugged into the wall then you plugged your radio or tv into that.

I could be mistaken but I think that is what my dad did.
But thinking about it now, the RFI is in the air, not just the house wiring.

Good luck on finding a fix, that has to be irritating as hell.

[dobbhill]

Grid resistors are used as a low pass filter (with the Miller capacitance of the tube) to filter out RF.
Shielding of some sort may be needed. Do you know which stage of the amp is the culprit?
D

[Andy Le Blanc]

Double check for ground loops, and the size and length of your power cord
can create a thing called the "standing on one leg" effect, where the whole thing
becomes an antenna. And review your house wiring and grounding.
You can put a small value cap and ferrite bead at the input if its required.

[Phil_S]

First thing, I'd try a different guitar cable. I've got one amp is a radio with one high quality cable I've got, but has no problem with another cheap one, go figure.

Next, I'd try a small value cap (10-100 pf) from the input jack sleeve to the chassis.

[Structo]

Heheheh, the cheap cable has obviously higher pf/ft.

That would be a bummer to have that crap infiltrating your home.

[Cygnus X1]

Tried several different cables.
The three amps that I have taken to other folk's places
do it, so I'm counting out the house wiring.
The power cables I use are typically less than 4 ft. long.
(I recycle IEC chords from other worth equipment).

I thought I saw somewhere-that a cap on the inputs would help. Not sure.

[jjman]

Does a guitar cable need to be plugged into the input to allow for the radio sound?

Does the amp's volume knob change the volume of the radio sound?

[paulster]

Are you using insulated input jacks, e.g. Cliff jacks?

If so then solder a 0.01uF cap between the ground connection (not the signal connection, despite what some may mistakenly say, or you'll lose all highs) and the chassis right at the jack, keeping the leads as short as possible.

Or add a grid stopper on V1a if you don't already have one.

[stubbyfex]

Hello!
I'm a ham radio operator and a musician. Perhaps you should pick up a copy of the Radio Amateur's Handbook which is put out by the ARRL. I go to "hamfests" several times a year, where I buy tubes, books, and all kinds of tube gear. In the handbook there are chapters on basic electronics, design of power supplies, and one on RFI that gives many ways of filtering out the RF both on the input side of things in an audio amp, and in the power supply too. You could also contact any ham radio operator in your area. Most of us would be more than happy to help you with your amp.

[Cygnus X1]

Many questioners...no real simple answers except for one which I might try.
It actually is worse with nothing plugged in to the input.

It might help that the amps I build are typically cathode biased power tubed...some high gain on the preamps, some not. I like a fairly raw signal chain.

When I say "help", it more than likely isn't doing any good.

[thousandshirts]

Yep. One of us Radio folks would probably help out.

You can build a jammer that will block a range of frequencies, for example:

http://home.telkomsa.net/jeanward/fm_jammer.html

Or a jammer that you tune to the specific problem area. Make sure to keep
the power of your jammer small. It only needs to block the area around
your amp. Test this field by moving your jammer near and away from a
radio.

[Structo]

If you are using a shorting jack for the input I don't understand how it would be picking up the radio signals?

That should short the grid of the first tube to ground.

[stubbyfex]

Hello Again,
I remember when I was a kid, (1880??) we used to make crystal radio sets. They were what amounted to a diode (galena) and a cat's whisker (wire). To pick up strong nearby stations in an amp, it would only take a bad solder joint, that would "detect" or pick-up the station. The bad solder joint would act as a diode of sorts. Just a thought.

[ampdoc1]

You might try something along the lines of the input filter cap on this Peavey Classic 20. I remember Hartley Peavey telling me they were having troubles with RF when the first prototypes were being tested.
The amp has plenty of "sparkle" and no problems with radio frequencies.

But, it may be the location, or just the combined circuitry of your amp. I was called to a church years ago that had just installed a new EV mixer. RF from a local AM station showed up on each and every one of the mic inputs. I tried adding RF caps as specified by the manufacturer, but it did no good.
Plugged in their old Peavey mixer, and nary a whisper of RF was introduced.

ampdoc1