AR Classic 60 Bias Confusion

The tech most likely got it right with a bias of 32.5ma.

Based on the manual, there is a plate voltage of 420V and a bias of -46 V. If the output transformer is between 4 and 6k ohms, then 65 ma is the correct bias for the 6550 tube as it operates at 65% dissipation in Class A/B (or "enriched A/B" as ARC calls it). But that is a 42 watt tube.

If using the 6550C, which is rated at 35 watts max dissipation, a bias of 65ma will take you into Class A operation (80% dissipation) territory and definitely not a recommended thing to do unless the amplifier is designed from the ground up to operate in Class A/A1. By using a bias of 32.5ma at 420V plate and -46V bias, that is "cold" Class A/B operation instead of "enriched" Class A/B the CL-60 was designed for.

Don’t crank up the bias until you hear from ARC

Many thanks for all of the input.

I will look at the old forum post. I’m comfortable and careful working around high voltages but sadly my knowledge of amplifier/tube function is essentially nil. I know enough to know where to test and how, but beyond that I’m a bit lost. I have enjoyed listening to it which I can do:)

It doesn’t seem as though there should be rocket science here but the bias in my manual is clearly different than the one online.

The amp has two pairs of matched quads. I can’t recall if the Classic 30 has two matched pairs. Maybe my adjustment page should be in a Classic 30 manual?

I just know that the Classic 60 manual in the ARDB says to set the bias to 65mv.

I’m curious to see what the reply from AR service will say.

In the mean time I’ll leave the bias as it is and read back in the forum.

Thanks again for all of the info.

@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.

Hi jea48,

I know that same video and have been in touch with Mitch who was the tech who made that youtube post.

He does say 65mv in that clip. I also noticed the labeling of the test points but the locations of the points for each side were really what I wanted to see and he illustrated that well. I figured out the TP numbers on my own.

I suppose I could have checked the Classic 30 manual on the ARCDB website earlier. It lists a bias setting of 32.5mv. 

My guess is the amp was biased to 65mv when it came from the factory. Sounded great for many years. When I changed the tubes the bias was set to 32.5 as per page 8 in my manual but I think my page 8 actually was from the Classic 30 manual. The amp hadn’t sounded very good since the retube.

I have to leave town for 5 days but will set the bias to 65 when I get home. Looking forward to hearing how it sounds.

Will keep you guys posted.

@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...