bias Classic 60, Audio Research

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.

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.

Probably had a servo issue as stated above....