You are here

Microphonic Feedback (possibly)

For all things relating to guitars, basses, amps, pedals & accessories.

Microphonic Feedback (possibly)

Postby UneducatedWeasl » Fri Jan 18, 2013 7:55 pm

Hi all.

I'm getting a high pitched squeal from my amp whenever I get close to the head. I've had the issue occur in the studio, when the cab is in the live room, amp in the control room, and the feedback seems to be due to proximity to the head, irrelevant to the cab. I've even had it squeal in this setup with nothing coming through the monitors, the amp is loud enough that I can just about hear it do this through the wall. It's a Selmer Treble and Bass, though the treble side has been gutted, fairly messily. I've tried tapping the preamp tubes, and they don't seem to make a sound, so I don't know if they've gone microphonic, but it sounds like that kind of feedback.

The fact that it doesn't seem to be volume related, and is proximity related, is what is throwing me. Any thoughts, before I drop it off with a tech? any diagnosis I can do is bound to make his job easier
UneducatedWeasl
Regular
Posts: 65
Joined: Sat Feb 24, 2007 12:00 am

Re: Microphonic Feedback (possibly)

Postby Hugh Robjohns » Fri Jan 18, 2013 8:13 pm

Can you control or vary it by changing the angle of the guitar relative to the amp? It could be magnetic coupling from the amp's output transformer to the pickups.

H
User avatar
Hugh Robjohns
Moderator
Posts: 17071
Joined: Thu Jul 24, 2003 11:00 pm
Location: Worcestershire, UK

Technical Editor, Sound On Sound


Re: Microphonic Feedback (possibly)

Postby UneducatedWeasl » Fri Jan 18, 2013 10:42 pm

Tends to vary in pitch as I move around. Initially it was amusing, rising in pitch as I ducked if I was near the amp. Now it's just annoying!
UneducatedWeasl
Regular
Posts: 65
Joined: Sat Feb 24, 2007 12:00 am

Re: Microphonic Feedback (possibly)

Postby Nico 3313 » Sat Jan 19, 2013 9:00 am

Did anyone think of the possibility of inductive coupling of the Output Transformer and the pickup coils?
I remember the phenomenon, but not with what guitar, bass or amp.
Nico 3313
Poster
Posts: 35
Joined: Tue Jul 11, 2006 11:00 pm

Preamps are Class A by default


Re: Microphonic Feedback (possibly)

Postby UneducatedWeasl » Sat Jan 19, 2013 12:06 pm

Based on the little bit of research that I've done on the subject (hard to find much, with a quick google, annoyingly) that does seem a likely culprit. Anything that I can do to resolve the problem, or is it time to bite the bullet and call my tech?

FWIW, when I got the amp I took it to my tech for a checkup, it being from the late 60s, and a bit of a homebrew (it's just the chassis, in a handbuilt enclosure) and he said that inside is a mess, hence why it's a noisy thing. Luckily I play loud, fuzzed up rock, so a bit of noise becomes fairly irrelevant in a hurry. At some point I might be able to afford to get it pretty much rebuilt, but we'll see.
UneducatedWeasl
Regular
Posts: 65
Joined: Sat Feb 24, 2007 12:00 am

Re: Microphonic Feedback (possibly)

Postby Goddard » Mon Jan 21, 2013 2:21 am

UneducatedWeasl wrote:Tends to vary in pitch as I move around. Initially it was amusing, rising in pitch as I ducked if I was near the amp. Now it's just annoying!

Ah, that describes "Moog-ophonic" not "microphonic" feedback.

Does this pitch variation also occur if you approach the amp without a guitar?

And, does this amp have a "tremelo" or "vibrato" section by any chance?
User avatar
Goddard
Frequent Poster
Posts: 997
Joined: Tue Apr 03, 2012 11:00 pm

 



Who is online

Users browsing this forum: No registered users and 1 guest