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Biasing anomaly for old audio research amp

Posted by Leslie Loh (A) on April 19, 2007 at 08:38:52

Hi,

I have been using after-market Svetlana Winged-C 6550s (St.Petersburg production, the same as what ARC is using) for the past 12 months happily for my ARC Classic 120 monoblock. Only last week, I detected a slight deterioration to the sound and I was greatly suprised as the tubes have just passed their one year mark. I estimated the usage to be just 1,000 hours. I proceeded to check the power tubes. To my suprise, most of the 6550s have lost its mercury layer on top of the tubes. After I replaced with a new set of Svetlana Winged-C 6550s, the sound gets better.

I consulted my tube vendor. He attributed the short lifespan of the tubes to my bias setting, which is 65mA as recommended by ARC for Classic 120. He advised to set the bias to 50mA. His reasoning is that ARC Classic 120 was supposed to use NOS GE (General Electric) 6550 tubes during the early 90s. Modern production tubes like the Svetlana Winged-c 6550s can't take this kind of bias current hence the short lifespan.

IS there any grounds to this sort of reasoning? Are current production Svetlana that much more inferior to GE of yesteryears?

Thanks for the help.

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