Low Biasing on an ARC Ref75

Hi Audiogoners,

I need help with a question concerning biasing an ARC Ref75 amp.

I recently purchased the amp from an Audiogon member who promptly shipped the unit to me. Unfortunately, in the process of packing and shipping one of us lost the biasing tool. ARC is in the process of getting one to me but it will take some time.

The problem is that I have been running the bias too low (due to not being able to make any change) and I have just finished reading that it causes the tubes to run hotter. Intuitively I thought the opposite to be true (meaning high bias equates to low tempts and visa versa but I have learned the opposite is true).

The biasing adjustment is suppose to be at 65mA but is currently set at approximately 52mA.

My question is, of course, is the bias setting of 52mA too low and producing excessive heat and therefore I should leave the amp off until I receive the biasing tool? Or is such a low bias okay to maintain until I can make the proper adjustments? I am not so concerned with the tubes as I am damaging a tube socket or the main board. My plans are to replace the tubes in the near future just to be safe.

Any correct answers or advise is certainly appreciated.

Showing 1 response by ljs_fin

My understanding is that 65ma adjustment for ARC amps relates to the idle current, not the bias voltage.  So if you reduce the idle current, you are not reducing the bias, and the amp will run cooler, not hotter.  I don't think this damages the amp though the sound may not be optimal.

It makes sense that if you reduce the bias voltage, the amp will run hotter.  But the 65ma measurement is not the same as the bias measurement.

That's my understanding, but I'm far from an expert and would like someone to clarify.  Thanks.