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Old 16th September 2005, 04:58 AM   #1
Trout is offline Trout  United States
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Default Guitar amp picks up radio??

Guitar amp picks up radio??


OK, I built this amp(3-4 weeks ago), Nice little PP EL84 job. Put a rather simple 12AX7 prestage. Tested it at 2 locations, Here at my house with a 80's Fender Strat with Fender lace sensors &
Tested it at my brothers about 40 miles away with His Carvin with humbuckers.

SWEET little jewel of an amp. Lovely responses and very sharp presence. Makes a Strat come close to The Old Rickembacker/Vox Combo of yesteryear.

Now the weird part,

Its at a new location several hundred miles away now, The claim is it picks up radio stations with his telecaster deluxe?
Schematic

Could this be a product of poor internal shielding on the axe? Or perhaps I need to add a grid stopper?
Gene
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Old 16th September 2005, 05:30 AM   #2
PRR is offline PRR  United States
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Try 10K to 47K, in series with the first grid, as CLOSE as possible to the socket. That is the main thing different from the old-reliable plans.

Use a shield on the first tube. If that is a pain, wrap it in tinfoil and use a jumper to ground it: if that cures the problem, get a proper tube shield.
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Old 16th September 2005, 01:53 PM   #3
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I wonder if the pickups in the guitar could be providing part of a tuned circuit?
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Old 16th September 2005, 05:28 PM   #4
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When I grew up there was a 10KW AM radio station about 1 block away from my house. Their elevator music came out of almost every audio device in the house, including the telephone. Guitar amps are a particular problem because you are plugging an antenna (the cord and the guitar) into the input.

Things to try:
1) A different cord. Any corrosion on the cord, jacks, or in the guitar could rectify an AM signal. See if the loudness of the radio music changes when the plug is twisted at the amp and at the guitar.

2) A different guitar. If the new owner can borrow a different guitar it would help isolate the source of the signal. Some guitars are shielded better than others. If it is guitar related the internal shielding may need improvement.

3) If amp modification is needed, a grid stopper would definitely be a good place to start. Check the input jacks for any sign of corrosion or rust. A small capacitor (about 500pF) from the grid to ground of the first tube may help. Make sure that all wiring to the input jacks is short. Shielded cable may be needed.

In my extreme case I had to put a parallel resonant circuit (known among old radio people as a wave trap) inside the amp in series with the input. It must be tuned to the ofending frequency.
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Old 16th September 2005, 07:30 PM   #5
Trout is offline Trout  United States
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Quote:
2) A different guitar. If the new owner can borrow a different guitar it would help isolate the source of the signal. Some guitars are shielded better than others. If it is guitar related the internal shielding may need improvement.
This I highly suspect.

Quote:
3) If amp modification is needed, a grid stopper would definitely be a good place to start. Check the input jacks for any sign of corrosion or rust. A small capacitor (about 500pF) from the grid to ground of the first tube may help. Make sure that all wiring to the input jacks is short. Shielded cable may be needed.
The distance from the input jack to the tube is 1/4 inch from each end of the cap inline to the tube. The Jacks are brand new so I kind of think seeing as other guitars never had this issue, I can most likely rule that out.
Dont think I could get much shorter leads than this

Click the image to open in full size.

Most Fenders are known for fairly poor shielding, Combine that with single coil pickups on a telecaster+ high gain factors from the amp, I guess its possible its just his location combined with this amp and his guitar.

Hopefully the grid stopper will fix her up (fingers crossed) I'll know later today .
Gene
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Old 17th September 2005, 06:29 PM   #6
amperex is offline amperex  United States
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Besides better shielding (important), a bad solder joint causes rectification which is a low level AM radio detector circuit.
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Old 19th September 2005, 11:36 PM   #7
Trout is offline Trout  United States
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Problem appartently has solved itself.
Well ok, Sort of.
Switched guitars at his location and no more radio.
So, as it turns out, this particular guitar has less than perfect shielding and a couple other issues, Bad pots, Of course The legendary fender wiggle to work switch and compound that with his home being rather close to a local tower. Go Figure.

I guess I should have known better than to think it was anything different.
Gene
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Old 7th July 2010, 09:00 AM   #8
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I have this problem with a small 5W guitar tube amp I built around 2 weeks ago. when you crank yhe gain up you can hear what seems to be a spanish Am station from my house and a french AM station from my friends house (It's funny because I live in Spain, 250 miles away from the french border, in a straight line). The amp does this even if the cable is unplugged, It actually sound the same with or without a cable. I tried to correct this by soldering a 47nF cap I had laying around from the input to ground and and giving the "input" shielded cable a turn around 2 ferrite ring I also had laying around. This did nothing to the radio signal (but it did tame most of the hum and the hiss in the amp). It apears to me that the signal is being picked up somewhere "inside" the amp. Is there anything I can do to get rid ofr it? Will a grid stopper resistor do the trick?

Thank you for the input.
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Old 7th July 2010, 11:45 AM   #9
Arnulf is offline Arnulf  Europe
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You shoudl definitely sue grid stoppers, you can also add additional capacitance to Cin + Cmiller, depending on what your amplifier looks like now (schematic ?). Be careful not to cut to deep with the LP filter these compose.
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Old 7th July 2010, 05:22 PM   #10
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Often a ferrite on the power cord and the input line will help. If it's solid state, on the speaker leads as well.

A small ceramic capacitor on the grid(s) of the output stage, and on the tone/volume controls may help.

If you know an old time ham radio operator, he may have some suggestions, as this was a common problem back in the day.
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