bias Classic 60, Audio Research

Hi George,
It simple to do, i did several times when i had this amplifier. First of all you need matched tubes (Matched Octet tubes).
Switch Off the amplifier and unplug the 110V
Then remove the tubes cage and place the trimer in Med position
Then place the amplifier to be lying on one side
Remove the top back cover to access to the circuit board
100 Ohms resistor are located near the Power and output transformer, they are 2 W and are in black color (you should be able to find pins near this resistor for TP1, TP2, TP3 and TP4
So use Alligator clip between 2 resistors and place the DVM on Continuous Voltage, for measuring at least 200mV
Then Plug the 110V and Switch on the Amp
The DVM should start to display voltage for 10 - 20 mV at the beginning and increase at least for 30min then it should be stable (during this phase if the 65-70 mv is reached turn a the trimer a little bit to read lower than 65mv
So when it is stable, adjust the value to 65mv
Verify the other resistor they should between 5mv in general if your tubes are matched
If you provide me a email address, i should be able to send you some pictures, i did some when i used to run the procedure
I hope this helps
Laurent

could send picture to email ngockhanhpl@yahoo.com

I will throw my 2 cents in. You need long hook clip probes to safely measure the bias. There is an issue biasing this amp because it uses an opto-coupler and an IC to servo its mating output tube. Once you adjust the biasable output tube the slave tube is supposed to meet it. The problem is it takes forever for the servo to work. On each side of the amp is a small access hole to adjust the slave tube bias. A very small adjustment can make a large difference and again you have to wait for the adjustment to work. It takes me over an hour to properly set up a CL-30/60/120. I would suggest letting a competent service provider do that for you. I have seen many instances where the servo does not function properly and when these amps are purchased used there's always a chance. If the servo malfunctions it's possible for the tube to run away.

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.