2. amps-preamps
  3. 182920-19964

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

perryg

Showing 1 response by gs5556

gs5556
1,245 posts
 

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

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