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Old 19th March 2007, 12:14 AM   #1
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Default Tube Preamp FM interference

Has anyone experienced local FM radio stations interferring with you tube preamp. I tied a friends preamp and as soon as i powered up I got the local radio station bright and clear throught the preamp. There was no tuner hooked up just to make thing clear. I wanted to purchase or build a tube preamp and wonder if this problem will go away with other designs or will i have a problem due to the close proximity of the local transmitter tower. I think it is about 3 moles away. Any thoughts or suggestions
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Old 19th March 2007, 01:43 AM   #2
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Not FM, but an AM station.

I added a 100uH inductor in series with the grid snubber and played with some low value ceramics to ground until it went away.

Increasing the grid snubber resistor can help too and let Miller C form the RF killing filter.

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Old 19th March 2007, 02:48 AM   #3
gofar99 is offline gofar99  United States
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Location: Arizona USA
Greetings, I have not had this happen recently, but the advice is sound. Another possibility is that it is getting through the power cord. A snap on ferrite will generall fix that. BTW, the process involved when an amp picks up the RF signal is ofter referred to as grid rectification. The grid and cathode can act much like a diode and "detect" and amplify the rf. Usually it is AM but stong FM and TV stations can cause the problem too. It became common many moons ago when the stations became more numerous and powerful and the amp designers didn't keep up. SS equipment can also do this, but perhaps because of design changes over the years, it doesn't seem as common.

Good listening
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Old 19th March 2007, 10:32 PM   #4
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Default PReamp FM interfrences

Thanks for the information. I was hoping there was an easier solution. I would hate to buy a manufactured unit and find i have to disect it.
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Old 20th March 2007, 01:21 AM   #5
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Default WHere is the interference coming from

Can someone tell me where the interfence is induceded into a preamp. Is it directly into the drive tube or some other place such as cables. I need someone to identify where this is coming from and why. THanks
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