ARC REF 150 Buzzing noise

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

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.

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?

Update on the sad story of my Reference 150.  Took it to Audio Tube Supplies in Denver, they determined that one of the input tubes was out of spec, causing one of the KT150 tubes to become over-biased and over heat.  This in turn (educated guess) caused one of the rubber washers under the transformer to shrink, and the transformer began to hum -- after it gets warm.  In summary, the bad input tube caused the amp to get too hot. The shop placed a sliver of heat resistant neoprene under the washer, and the problem went away.

Lesson learned, check your tube bias regularly and often!  Especially if your tubes have some hours on them. 

It now has a new set of tubes.  I felt cautious and went back to KT120's, although the shop verified that there should not be a problem with using the larger KT150 tubes.  I don't think there is any permanent harm, the amp sounds as good as ever with the new tube set.  But, I am going to send back to ARC for complete check.  It has over 5000 hours.

@scottwsmith I'm having the very same problem right now. Buzz in about 30 minutes, sound gets leaner as the buzz is getting stronger. 

Good to know where to focus. 

KT150's should be alright, I checked with ARC some time ago but I'm still using KT120's as they still have lots of time left.