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Post subject: The twin 1991 black knob
Posted: Fri Jul 16, 2010 5:26 am
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Hey guys
I bought the twin amp 4 years ago and it worked like a clock until a month ago. A weird ringing noice is coming out of the amp especially in the second channel which i use most of the time, and that noise is maximised by adding reverb from the amp. When i slightly hit the amp yoy can hear the ringing. Iam also experiencing volume drops and swells . Any thoughts on that. Thanks.


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Posted: Fri Jul 16, 2010 7:22 am
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You may have a tube that needs replacing.


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Posted: Fri Jul 16, 2010 8:39 am
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Agreed.

One or more tubes heading south for the "final round-up".

Arjay

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Posted: Fri Jul 16, 2010 10:07 am
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The tubes are old but they are still working. I heard something about an ic??? Any thoughts on that?


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Posted: Fri Jul 16, 2010 11:16 am
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Do you have a spare preamp tube? Would be an easy diagnosis.


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Posted: Fri Jul 16, 2010 11:29 am
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I can find one. So should i try changing a preamp tube 12ax7 one by one ??


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Posted: Fri Jul 16, 2010 11:41 am
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Yes, try swapping them one at a time until the ringing goes away.


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Posted: Fri Jul 16, 2010 11:52 am
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One more thing . Because there are 11 preamp tubes ....should i turn off the amp every time i swap the tubes? Can i do it while the amp is on? Thanks.


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Posted: Fri Jul 16, 2010 12:05 pm
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Do not have the amp on with a missing tube.


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Posted: Fri Jul 16, 2010 7:08 pm
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Right Presvis, there are eleven tubes total, but four are power tubes. More than likely, it's one or more of the smaller(pre amp or phase inverter) tubes that have worn out. A handy way of isolating the problem is usually to shut off & unplug the amp( no guitar plugged in). Remove the rear panel and metal tube shields. You'll need a chopstick or any plastic item with no metal parts, to gently tap each tube, one at a time. With the controls turned up about half-way, plug in and turn the amp on and into play mode. Gently tap each tube( including the power tubes)and take note of the reaction of each one. It's that ringing that you're listening for(all the tubes will react), a dull thud is best, put keep track of what you hear. The culprit should be very apparent. Shut the amp down and unplug, let the tube cool and make a reference mark on the glass and base, to aid in the replacement of the new tube. I suppose an IC could go microphonic, but that thought scares me. And yeah the amp is always shut down & unplugged for tube swaps. Welcome to the forum and be safe! Art

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