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ARC REF 150 Buzzing noise

I have an ARC REF 150 amp with over 4000 hours on it, and recently it has developed a buzzing noise.  This appears after about 30 minutes of use, and seems to get steadily louder.  I thought it might be a tube.   I had KT150 power tubes in it and I swapped those out  for KT120's and re-biased.  No change.  I then swapped out the 6H30 input tubes and re-biased again.  Buzzing is still there.  It is very difficult to locate the sound on the boards.    I have also had several blown fuses, this has been in the midst of a move and may or may not be reIated to the buzzing.  I have to assume this needs a repair, but I am wondering if anyone has a guess or even better, experience with the problem.  I am in the Denver area and if anyone knows of a local repair opportunity, I would appreciate knowing that too.  I do not look forward to the expense and delay of sending it back to ARC repair.  Thanks.  Scott
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spinaker01
491 posts
 
Agree that this sounds like transformer hum. Hopefully from the external sources as this would be fatally expensive to replace any of the 3 xformers in your amp. Try to eliminate other sources as mentioned above.
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scottwsmith OP
60 posts
 
Should have mentioned that the amplifier is plugged into a Bryston BIT-20 power unit, an isolation transformer.  I am in a new house, but only for a few weeks.  I am unsure if I heard the buzzing in the old place.  Perhaps so.  I will try plugging the amp directly into the wall and see if that fixes the problem.  I would have to believe that a failure of one of the three transformers would be fairly unusual?  Thanks for the replies, I will update the post as soon as I have more results.
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scottwsmith OP
60 posts
 
The speakers are 89 dB.  Up to a few months ago, I didn't hear buzzing at all.  I did remove the amp from the Bryston BIT-20 and the buzzing didn't go away.  I will try turning off other circuits in house next.

It takes an hour or so for the buzzing to appear, as if it is usage related or heat related.
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