Tube Preamp FM interference

jbldriven · 19th March 2007, 12:14 AM

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
Thanks

Geek · 19th March 2007, 01:43 AM

Hi,

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.

Cheers!

gofar99 · 19th March 2007, 02:48 AM

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
gofar99

jbldriven · 19th March 2007, 10:32 PM

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.

jbldriven · 20th March 2007, 01:21 AM

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

19th March 2007, 12:14 AM	#1
jbldriven diyAudio Member Join Date: Sep 2006	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 Thanks

19th March 2007, 10:32 PM	#4
jbldriven diyAudio Member Join Date: Sep 2006	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.

20th March 2007, 01:21 AM	#5
jbldriven diyAudio Member Join Date: Sep 2006	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
FA: Eico HF-85 Tube Preamp & HFT-90 Tube Tuner	joe-electro	Swap Meet	0	31st May 2009 04:14 PM
Tube Preamp Popping on Source Input Change (?) / 6DJ8 Preamp On Chip Amps	MartyM	Tubes / Valves	3	16th October 2008 04:52 AM
tube preamp using submini tube	unclejed613	Tubes / Valves	11	19th August 2007 07:14 PM
Is it possible to get "tube sound" with just a tube preamp and chipamp output?	Spasticteapot	Instruments and Amps	43	15th May 2006 05:28 PM
need diy tube preamp (line-stage) recommendation and general tube advice	Musiclover4	Tubes / Valves	15	24th November 2005 01:19 AM

19th March 2007, 01:43 AM	#2
Geek diyAudio Member Join Date: Sep 2004	Hi, 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. Cheers!

19th March 2007, 02:48 AM	#3
gofar99 diyAudio Member Join Date: Jan 2007 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 gofar99

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read