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it hums, it squels, its microphonic....help!


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2/13/2001 2:14 PM
MikeZ
it hums, it squels, its microphonic....help!
my continuing saga with my plexi circuit...  
 
 
 
im going through star grounding to cure hum and it is working as i put each stage on star ground. the point at which it squels is going down too, hopefully ill get somewhere there. now im paying more attention to a problem thats been there for awhile...  
 
 
 
if i tap on the chassis with a screwdriver, i can hear it loud and clear through the speaker. this goe for anywhere on the chassis, and does this with many tubes, so im fairly sure the tubes arent microphonic. what oher parts in an amp go microphonic like this?? could it be as simple as something loose? bad socket?  
 
 
 
thanks so much for all the help!  
 
 
 
mike z
 
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2/13/2001 2:30 PM
R.G.
Ceramic capacitors are notoriously microphonic - remember "ceramic phono cartridges"? Same stuff as many ceramic caps.  
 
 
 
Some resistors can be microphonic, but it's rare.  
 
 
 
Get yourself a 12" length of wooden dowel, and start tapping individual parts. You'll hear each part you tap faintly, but when you tap on the the offender, it will be much louder.  
 
 
 
You don't mention whether the microphonics get worse as the volume control is turned up. If they do, the offending part is *before* the volume control. If not, the offender is *after* the volume control.  
 
 
 
A signal wire with DC voltage on it (like a wire coming off a plate) can generate signal simply by moving toward and away from ground - the movement modulates the capacitance from the wire to ground. It might even be a loose, flappy wire. This is a very small effect, but in a high gain amp, it could happen, I guess.  
 
 
 
All tubes are microphonic to some degree. Some older amps soft-mount the tube sockets to keep chassis vibrations from being coupled into the tubes. Some approaches are to put rubber O-rings between the socket and the chassis/mounting screws, to mount the socket in a sheet of dense rubber, and so on. Only go to such lengths if you've already done the tapping on every part without identifying one worst offender.
 

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