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
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
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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. |
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.
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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.
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. |
