AR Classic 60 Bias Confusion

@perryg reading over the manual a bit, appears to be written more for a tech who's familiar with the basic procedure. Check this too - here is an older post on Audiogon worth browsing. There are varying references to different measurement values of 65ma, 65mv, and .65vdc used interchangeably - pay close attention to that, use of alligator clips, and a few other comments about how to do this. Some of the replies down on the first page are a bit more informative and helpful fwiw. Something else for reference, be sure to note the 2-3hr waiting time to let the amp reach full operation on your very last bias check before closing up.

Adjust the "bias" for an average reading of 65 mVDC (0.0650 volt DC) between TP1 - TP2 and between TP3 - TP4.

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

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.