2. amps-preamps
  3. 182920-19964

AR Classic 60 Bias Confusion

Purchased my Classic 60 new in 1991. Loved the sound for many years. Had it retubed in 2013 but little time to listen with a toddler running around. All I know is that the amp has never sounded the same since being retubed.

The new tubes were old style Winged C 6550s.

I thought I should check the bias to be sure the tech who installed the new tubes had it right. It’s running at 36mv on the left and 33mv on the right. These numbers are pretty close to the 32.5mv bias recommended in my manual that came with the amp.

My problem is that every bias setting I have seen online for the Classic 60, including the pdf manual downloadable from AR says the bias should be 65mv.

Does anyone know whether AR had two versions of the amp? Is my manual a misprint at 32.5mv? 65ma is mentioned in my manual as the nominal current per tjbe but no mention of 65mv as the correct buas).

I’ve sent the question in to AR service but no doubt they are busy. Not sure when they will be able to respond.

Any insight would be greatly appreciated,

Perry

perryg

Showing 1 response by decooney

decooney
1,908 posts
 

@perryg reading over the manual a bit, appears to be written more for a tech who's familiar with the basic procedure. Check this too - here is an older post on Audiogon worth browsing. There are varying references to different measurement values of 65ma, 65mv, and .65vdc used interchangeably - pay close attention to that, use of alligator clips, and a few other comments about how to do this. Some of the replies down on the first page are a bit more informative and helpful fwiw. Something else for reference, be sure to note the 2-3hr waiting time to let the amp reach full operation on your very last bias check before closing up.

 

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