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 3 responses by jea48

jea48
4,733 posts
 

Adjust the "bias" for an average reading of 65 mVDC (0.0650 volt DC) between TP1 - TP2 and between TP3 - TP4.

Do not make any other adjustments of pots in the amp.

 

For power tube bias adjustments go to pages 7 and 8.

https://www.arcdb.ws/Database/CL60/ARC_CL60_manual.pdf

 

jea48
4,733 posts
 

@perryg,

I found this video for the ARC Classic 60.

For the bias adjustment go to time marker 12:20.

Note, when the tech first starts taking about where he has the meter probes attached on the left side of the amp he say points 2 & 4. I pretty sure he meant to say 3 & 4. When he moves to the right side of the amp he says points 1 & 2.

 

jea48
4,733 posts
 

@perryg

I hope you have at least a pair of something like these.

 

 

You’ll need something insulated to connect the DMM two probe leads to the two bias points on the circuit board for channel of the amp. You’ll need the meter around the other side so you will be able the read the display as you adjust the blue trim pot.

I thought my ARC VT50 amps where a PITA to bias the power tubes. The Classic 60 looks worse. Older ARC amps were not user friendly when it came to biasing the power tubes.

At least you don’t have to bias the input and driver tubes for each channel like I do on the VT50. That can hours to do...

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