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Microphonic Feedback (Maybe...)
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UneducatedWeasel
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#1
8th December 2012
Old 8th December 2012
  #1
Gear Head
 
Joined: Jun 2008
Posts: 68

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UneducatedWeasel is offline
Microphonic Feedback (Maybe...)

Hey,

Been trying to describe an issue to my amp tech via email/phone, descriptions have pointed towards a microphonic valve, but wanted to get a few opinions from here, see there are any other causes that are easy to diagnose before I just give in and let my tech take a look.

Got a Selmer Treble and Bass 50 head, Treble side was gutted, new enclosure built from the chassis, pretty messy job, not much can be done without essentially a full rebuild. Once I get it up loud enough to compete with a kit, and push it with an overdrive pedal a little, getting a pretty horrible squeal from it, which alters in pitch as I move the guitar around the amp. Having heard a similar pitch when using horribly microphonic pickups before, I guessed it could be that, but had the same issue with a few different guitars, so guessed probably the amp.

The usual tapping the preamp valves test hasn't shown anything obvious when I did it, (though I'll try swapping out a couple spare ones, see what that does, just in case). I had the amp in the studio recently, head in the control room, cab in the live room, and was able to get the noise even with just the bleed through the walls, so it definitely seems to be related to how close I get the guitar to the head, rather than something acoustic from the speaker.

Any thoughts other than microphonic tubes?
#2
22nd December 2012
Old 22nd December 2012
  #2
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Joined: Dec 2012
Location: MD
Posts: 5

Brian Ingram is offline
This may be too late to help you, but you mentioned

"... Treble side was gutted, new enclosure built from the chassis, pretty messy job ...

That probably says it all.

If the wiring was done in a sloppy or haphazard manner, there could be unintended coupling between wires, especially from a later/high signal stage to an earlier/low signal stage.

This can result in feedback/oscillation, which can be difficult at times to track down because it might not consistently show up.

Best bet is a proper rebuild; rip everything out (that was done sloppy) and re-do it right.

Sorry that is an expensive answer (unless you know how to do it yourself), but your description sounds like an issue with the amp's wiring since you seem to have ruled out guitars and tubes.
UneducatedWeasel
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#3
23rd February 2013
Old 23rd February 2013
  #3
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Joined: Jun 2008
Posts: 68

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UneducatedWeasel is offline
I imagine that it may be my only option (although I do admit liking abusing the oscillation with feedback and a heavy dose of fuzz. Similar effect to playing around with some of the sillier fuzz factory settings).

My tech also builds amps, so I'll see if he can quote me a rough idea of a rebuild. There's also a guy I know who's done a few Selmer rebuilds, so I'll get in touch there.

It's not perfect, it's temperamental, but I still love the thing!
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