RF (Radio Noise)

[tjauernig]

I've been searching around, and there are several posts on grounding methods to reduce amp noise, within the amp itself. Has anyone had any problems with picking up radio stations on the input of the amp. I've tried about 6 or 7 grounding schemes. Some worked better than others for noises inside the amp, but none so far have worked for rf interference. I've tried almost everything I can think of, and tried a bunch of stuff from the various forums, Weber books, Torres books, etc. Still no avail. Any suggestions would be greatly appreciated.

[doctord02]

What specifically have you tried? Otherwise we are guessing blindly... Do you have any pics? Is your lead to the preamp tube shielded? Do you have a grid stopper resistor soldered directly to the tube socket pin? Say anywhere from 68K to 10K?

If those things didnt help, have you tried putting a ferrite bead over the input wire?

[FUCHSAUDIO]

Try adding an insulated wire from the plate of that input tube, and wrap it a few turns around the grid wire/resistor. Besides the ferrite bead, the wrap reduces bandwidth by increasing the Miller effect of the tube. Too many wraps could kill the high end, a few could kill enough to knock out the RF.

I assume you have a tube shield in place, and the cabinet has shielding foil inside, grounded to the chassis ?

RF problems are no picnic to fix, but can be eventually resolved with time, tweaks, and patience.

[tjauernig]

Thanks for the tips. I'm working on an amp right now. I'm planning on a 2 point star gound for this one. I'll give the ferite bead, and/or the wire wrap a try. It is frustrating to be an amp that sounds perfect in your house, and take it out to the gig, and it's an FM reciever.

Thanks for the help from Fuchsaudio, and Doctord02

[Darkbluemurder]

You can try a 47pf cap from plate to cathode. A 47pf cap from plate to grid also works but I don't like the tone - it kills highs and harmonics.

Good luck!
Stephan

[MarkB]

Are you sure it's at the input? A bad solder joint can pick up RF. Unplug the guitar cord and see if it's still there. If it goes away with the cord unplugged, then it's probably getting in throuh the shield of the cord. If you have an insulated input jack, put a .01uF cap from the input jack ground to the chassis, as close as possible to the jack. That will dump RF riding on the shield straight to ground.

[FUCHSAUDIO]

Shoot, I forgot that. We do it all the time too.

Make sure it's a ceramic .01 disc, as they reject RF better than a mylar or other non ceramic type. This is especially true on a star grounded amp, and grounding the ground of the jack to the chassis (at the jack) can kill RF on that "leg" of the star.

[tjauernig]

Thanks for the extra tips. I re-wired the grounding, and checked all the solder joints throughout the entire amp. It's almost gone, except when the volume knob on the guitar is rolled all the way down. I'll give the caps a try. I've seen 500pf caps from plate to cathode in a few amps. Is this also an attempt to block rf noise?

Thanks again

[FUCHSAUDIO]

I would not suggest that plate/cathode cap. it;s for frequency contouring, and not RF, and would wipe out alot of the high end....

[tjauernig]

Thanks FUCHSAUDIO, I appreciate the help.

[drz400]

Thanks for the extra tips. I re-wired the grounding, and checked all the solder joints throughout the entire amp. It's almost gone, except when the volume knob on the guitar is rolled all the way down. I'll give the caps a try. I've seen 500pf caps from plate to cathode in a few amps. Is this also an attempt to block rf noise?

Thanks again

dont put a bandaide on it, fix your grounding
Do a ground buss with individual local stars to the filter cap ground for that stage the filter cap ground for that stage goes to the ground buss, the OT ground goes to the speaker jack ground, run the speaker jack ground to the phase inverter ground if there is a feedback loop. This works better IME than grounding to multiple places at chassis, You have complete control, ground the ground buss in the middle of the buss to the chassis and run that .01 from the input jack ground to chassis or ground at the jack to chassis. Power third pin ground keep far away from other chassis ground. If you ground the jack to the chassis it will go away I promise. Preferable to use a nice bolt into the chassis or solder direct to chassis with a bastard iron

[tjauernig]

Thanks for the tips Drz400,

This has been a very frustrating problem. It's only recently that is has been an issue, and only with the amps I've built. My fleet of z's are whisper quiet for the most part, but my 18 watt clone-ma-bobs have been the issue. As you say, the band aids have cured 99% of it, but I'll try your suggestions, and let you know what transpires.

[drz400]

Thanks for the tips Drz400,

This has been a very frustrating problem. It's only recently that is has been an issue, and only with the amps I've built. My fleet of z's are whisper quiet for the most part, but my 18 watt clone-ma-bobs have been the issue. As you say, the band aids have cured 99% of it, but I'll try your suggestions, and let you know what transpires.

In case I wasnt clear make sure you only have one ground to chassis.
If at the input jack you wont have RF
Pickup the London Power book that has the section on grounds, it is pretty much right on, this is also covered by Aiken on his site.