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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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scottwsmith OP
26 posts
 
Moved the power cord to another circuit, quite close to the power panel.  The amp was quiet for about an hour, which got my hopes up, but then started the nasty buzzing again.  It MIGHT be one of the three transformers, but the exact source is hard to locate.  It is obnoxious enough to be a problem, for sure.  Some of you have mentioned DC content in the power, but wouldn't the Bryston BP-20, which is a large isolation transformer (I think) eliminate that if it were present?  BTW, my most recent tests have not had the BP-20 in the power feed.  The Bryston does not seem to be the problem, as the amp acts the same with or without it.

I am going to call ARC service today, will report back on what I hear from them.
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scottwsmith OP
26 posts
 
Update after last post, there is no time dependency.  No matter when I turn on the unit, it will start to buzz after about an hour.  Anyone know what those transformers cost?
minorl
740 posts
 
An educated guess but I think you may have damaged the transformer by using KT150 tubes.if this t
is the non SE REF 150 then according to Audio Research to upgrade to SE and use KT150’s some major upgrades needed to be done,including replacing the transformers.

i would put the KT120 tubes back in it and send it to Audio Research for evaluation, inspection and repair.  Also,find out how much to repair and how much to upgrade to SE.  it might be the same price if you have to change the same transformer
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scottwsmith OP
26 posts
 
Called Evan at Audio Research, seemed very knowledgeable.  He thought the problem was a transformer, the good news is there is a service center near me that can repair it.  I think my cost will be about $ 800 including labor if it turns out to be a transformer.  Given the great sound that this amp makes, I don't mind too much.  I asked him what the official stance from ARC on the use of KT-150's in this amp, his answer was "highly recommended, it makes the amp sound better and the tubes last longer."  So there you are.  I will post updates as this moves along.

bdp24's avatar
bdp24
4,908 posts
 
I’ll bet minorl is right. The KT150 is much more demanding of transformers than is the KT120. Verify that the tech is replacing the one in your amp with one rated for the KT150, otherwise you may end up right back where you are. Maybe consider getting the complete SE upgrade?
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