Anyone have any tales to tell?

I am getting mixed reports of a feedback problem. Some say it only affects valved amps or is much worse with them than transistors.
Humbuckers seem more immune than single coils (as one would expect?) but I have yet to get any solid information as to what the precise mechanism at work here might be.

The PA/ sound guys might also like to chip in?

Dave.

Well yes, proximity is key here in my experience. Moving something a foot further away can make a lot if difference.

Single coil guitar pickups are affected, monitors are, if you put them close enough. This is an issue in the local church near me , the induction loop was recently upgraded and rerouted. And the result has been distorted signals in personal monitors and all sorts of stuff.

Did look at routing the cable around part of the building, but that's against regulations. You apparently can't put up a sign that says 'Hearing loop over there' as people might be embarassed by having to sit in the deaf section.

I'm shortly going to attempt doing the obvious and screening a section of the wire and see if that imprives the situation. Can't screen the whole wire, bviously, but it might be enough to relieve the issue.

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I've never liked a solo violin, you need at least five for a proper fire.

An electrostatic screen won't help, and if you ground both ends of the screen then you effectively create a shorted turn which will screw up the loop performance elsewhere while probably still not helping enough.

Where is the loop pickup microphone and can you get the guitar amp further away from it or pointed away from it? Both things will help.
Sometimes the loop is turned up way higher then it needs to be, grab a (calibrated) loop tester and see if the input gain and loop current settings are correct (There is usually a compressor in play here).

The problem is that the thing is working as designed, its JOB is to create an audio frequency magnetic field in the room from the sound in the room, and when you put a coil in a varying magnetic field Faraday will not be denied. The answer is humbuckers and to simply turn down until the feedback goes away...

Valve/Transistor will make little difference except to note that valve amps are usually run further into distortion then transistor ones, so there will be effectively some compression there making the small signal gain larger in that case.

Regards, Dan.

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Audiophiles use phono leads because they are unbalanced people!

Thanks Dan,
Good to know that this is not a failing in the guitar ampliers (like for instance RFI susceptability would be) but merely the consequence of putting a coil of wire inside a magnetic AC field.
I am registered deaf and have a Siemens digital BTEar aid but it does not appear to have a "T" switch as my old analogue jobby did (that was handy, I could test TV remotes with that!). I do wonder however what folks are thinking with a loop on in room with a rock band playing? My aid limits very nastily at work at about 100dBSPL and I would expect the amps in it to do so even if picking up via a coil?

As a much younger man I installed a few loops in houses for old folks for their tellies (you never think when you are 22 that YOU will be there!)These were driven straight from the (about 2 watts) B&W tellie's output valve traff but we had to fit a second 2kV iso traff (Rad Spad)because all tellies then had chassis at 1/2 mains!

Dave.