bias Classic 60, Audio Research

Hifigeek, I recently purchased a Classic 60 and tried to bias 8 Philips 6550s over the weekend. It was supposedly already biased "perfectly" with Svetlana 6550s by a technician. However, when I put in my bias meters into the tube sockets, the two left tubes measured 60ma and the next two read 50ma current draw. The right channel read 55ma and 60ma. I assume the slave tube is behind the master? tube? The only bias pot (blue white center) I could find was in the back and it was turned fully counter clockwise so I could not turn the bias down, only up. The "small access" holes on the side?? do you use a very small philips screw driver?? that is what I see and not the red plastic bias screw driver. Anyway I was lucky and found 8 tubes out of 14 that would read between 55ma and 60ma. So I am confused, it seems you need a matched octet of 6550s instead of 4 matched pair?? as the manual states. Am I missing something? I sure like the D70MKII and D76a bias adjustment better. Any comments would be geatly appreciated.

The servo adjustment is through the small holes on either side of the amp. If you have run out of adjustment it either means the servo is not functioning properly, or the tubes are not matched properly. Also understand that the servo pulls in very, very slowly. It literally could take up to an hour for the servo to pull in. The servo should be adjusted to pull in closer than 5mV. If that is not happening, and you know the tubes are well matched, I would suspect both servos are not functioning properly and the amp requires service. I believe the trimmer pots require a slotted or small philips screwdriver. i.e. long and skinny. I usually shine a flashlight in there for proper orientation and keep it in there until the servo is properly set. It is a delicate adjustment and you should only turn the pot 90 degrees at a time and wait a bit to see which direction it moves. I measure across a master test point and the adjacent servo test point so I can see the difference voltage between the servo and master side. If you feel the trimmer pot click, you have run out of adjustment. Make sure the amp has been on at least 20 minutes so that the master bias is stable. I also adjust the bias to 65mV with a 120Vac line voltage. Hope that helps.

Very informative Hifigeek as usual. I have a pair of CL120's (modded by Steve Huntley in 2005) with balanced inputs. In addition to what was discussed thus far in this thread, the best place to start is to obtain tubes from ARC. That way you will know that they are well matched. Money well spend IMO. I have always taken a bias reading within the first 10 minutes. If you have a reading above 65mvdc (.065 VDC) you must drop it by turning the pot down. As the amp warm up the bias will increase and you do not want this to go to 75mv (or higher) as the amp warm up. Some techs will turn the pot to minimum before turning it on which is a good idea actually. Then you let the amp for at least an hour and you take a reading. Then you ramp up the bias to 65mvdc using the pots. Continue to observe your meter as the reading may go past 65mvdc ever so slightly. Bring it down to 65mvdc. ARC recommends doing a final read and setting after a full hour of thermal operation.

For the Servo, ARC suggests the amp to be left on 2-3hrs after the bias has been set so that it can reach full thermal operation. Resist the urge to fiddle with the Servo adjustment unless you are sure that it is out of range. You need to connect your positive probe to TP3 and negative one to TP2 (Classic 120) WARNING: HIGH VOLTAGE. You MUST use isolated alligator clips. The last thing you want is slip your probe onto chassi ground. You should get a reading of +/- .2mv or less. If it's above that you need to bring it down and as Hifigeek points out it can be a difficult task. Patience is of the essence here. Turn the pot ever so slightly (CW to make more positive and CCW to make more negative) and wait 5 minutes for the reading to stabilize. It's easy to over/under adjust so be gentle with the pot. It is normal for the reading to "float" somewhat as the Servo works. If you observe a reading that is within the range mentioned above when you connect your DVM then you need NOT touch the pot.
Hope this helps. Information above come from Leonard Gustafson and Chris Ossanna RE: Bias and Servo adjustment for a CL120's.
The best thing for you is to give Calvin a call and he will be more than happy to send/fax you the Bias/Servo adjustment procedure for your CL60. The procedure applies for both the CL60 and V70.
Good luck.

Hi arc160: Could you send me some photos and procedure so I can adjust my AR classic60? pikukt88@gmail.com
Thanks and saludos from Madrid
Juan Luis

I had a V70 and struggled getting the replacement tubes biased. I ended up sending it to ARC to get it done but it cost me $400. I went back to transistors to avoid the tube hassles. It was a nice sounding amp though.