Classic Rock in my Amp

[JHow]

So I moved to to Fruit Cove FL, about a mile or two from WSOS 94.1 FM Classic Rock station's antenna (I found that out later). Yesterday, I brought out my "18-watt lite ii" (marshall circuit on a pdb) and now I can hear the station in the amp. It's a bit like when Spinal Tap played the gig at the air force base. What a hoot.

[Enzo]

Usually I find it is the AM radio stations that get picked up.. Does that station have an AM channel too?


Do not plug any instrument into the input. Still get radio? Radio is usually something involving the input, mostly the grounding of the input.

[JHow]

Usually I find it is the AM radio stations that get picked up.. Does that station have an AM channel too?


Do not plug any instrument into the input. Still get radio? Radio is usually something involving the input, mostly the grounding of the input.

I got the station id during the commercial break - thats how I heard the announcer say "94.1 wsos" I don't know if they have an AM repeater, but here is the station:

Classic Rock 94.1 // Are you ready to rock?

When I remove the cord it won't play the radio, but it will if I have just a cord plugged in and no guitar. I tried three cords - they all do it. Also, I can hear the radio with the volume all the way down. I have the schematic somewhere but it Z Stingray's "baby will" board, with just one volume and tone control:

18 Watt LiteIIb "Baby Will"

[drewl]

Hey check out my thread "Ampeg AM Radio"....I was able to get rid of the radio interference by running a wire from the input jack ground to the first preamp tube cathode ground.


At least you don't have a bible thumper show coming in like I did.....although the harp music was nice!

[R.G.]

Often there is something a bit funny about input grounding. One thing that might work is to remove the input jack nuts, clean the jacks, nuts and chassis, and ideally put a big toothed/star washer under the jack from the back. This makes a solid connection to the shielded chassis (... uh, it's a metal/metalized enclosure, right?) and the RF would rather be on the conductive exterior than the inside.

If that's not possible for some reason like plastic-nosed jacks, put a big ring terminal under the inside of the jack and a star washer to bite into the chassis metal, and use a 0.001uF *ceramic* cap from jack ground to the ring terminal, again to seduce the RF to being on the chassis, not inside.

Another step is to do the above, but to put a 100R resistor in series with the signal line from the input jack to the first board input, and to put a 100pF cap to ground - ideally first cathode ground if it's set up like that - right where the input wire comes onto the board. The resistor is important. Using just a cap can actually help tune in stations. The resistor eats up energy and damps the tuning of the parasitic Ls and Cs of the input wiring.

In hard cases, take both input signal and jack ground at the input jack to chassis with a 100pf-1000pF ceramic cap, then put 100R in series with them to the board, then use a 100pF from signal to ground at the board.

At some point, Faraday's law and the resistor dissipation will eat enough RF that it won't be audible.

[Chuck H]

*NOTE... The OP hasn't submitted a schematic for the home built amp in question. We don't have any idea what the input circuitry is. The amp may not even have an input grid stopper at all for all we know.