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ARC tube biasing

 I noticed that in the manual of the ref75se, it explains that when biasing the amplifier, for best results, voltage should be at 117.5. The voltage in my home fluctuates between 118-119.5.  Would it be best to peg the the needle to about ever so slightly onto the positive side of the window past the bias target line to slightly compensate for the higher voltage, therefore theorically hitting the proper bias point when and if the voltage were to be 117.5? Or should I bias to hit the target line even though my voltage is higher than 117.5, and in actuality 118-119.5?   Splitting hairs? 
audiolover718
6 responses Add your response
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jea48
4,446 posts
 




Output Tube Bias Adjustment As shipped from the factory, the output ‘bias’ adjustments are set for a nominal 65mA per KT120 tube. Under these idle conditions the tubes are each dissipating approximately 27 watts of their 60 watt rating. This point of operation provides ‘enriched ’Class AB1, and will satisfy the most critical listener.For best results, operate and adjust the Reference75at 117.5 VAC(230 VAC). Adjustment must be made under zero-signal conditions after at least 15-20 minutes of uninterrupted stabilization time.
No doubt ARC would like the home owner’s main voltage to be 117.5 volts. Arc needs to retool for the 21 first century. I would bet most homes today have a mains voltage in the 120Vac to 122Vac range.

You want to set the bias at 65mA per KT120 tube per ARC specs using the AC mains voltage at your house.


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atmasphere's avatar
atmasphere
11,841 posts
 
^^No.
Would it be best to peg the the needle to about ever so slightly onto the positive side of the window past the bias target line to slightly compensate for the higher voltage, therefore theorically hitting the proper bias point when and if the voltage were to be 117.5? Or should I bias to hit the target line even though my voltage is higher than 117.5, and in actuality 118-119.5?
If anything you might set it ever so slightly low so as to have the same dissipation. The problem isn't at idle- its at full power. Since your B+ voltage will be higher, the plate dissipation will be higher too. At full power you won't have as much margin left.
audiolover718 OP
132 posts
 
Thanks for your replies. Set it to target line, and will check on it every month or so. The system sounds incredible and I’m very pleased. My gear is still breaking in hasn’t achieved its final voice, but it’s obvious this gear means business. With a ref 6 and Ref75se going through the motions, it’s hard to tell what piece is doing what. All I know is that it’s the finest sound I’ve ever owned and they will probably stay with me until Somethig substantially better is created, but I’m just peachy at the moment
xti16
1,525 posts
 
I I agree with atmosphere and slightly under bias the tubes. Andreas Hoffman from Octave recommended a slight under bias. He said it help extend the tube life. That's what I did when I had one and didn't notice any difference in sound.

enjoy the music

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