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Audio Research VS 115 bias Adjustments

Any A-goners familiar with setting bias on the ARC VS115 power amp??

Just checked and set the bias on my VS115 amp. For those who are familiar with the process on this amp, I checked and adjusted the 4 "Set" ports to 65mV, as specified. However, the bias readings from the other 4 "Check" ports ranges from 63-71mV. While technically in "spec" (i.e., 57-73mV for the "Check" ports), I would have thought the range of bias readings from the "Check" ports would not have been that much. As you know, the bias voltage for the "Check" power tubes cannot be adjusted.

As an aside, I only use ARC matched tubes.

Does anyone know whether the range of bias mV readings on the "Check" tubes is normal (whatever that means) or otherwise could result in a denigration of sound??
bifwynne
lostbears's avatar
lostbears
479 posts
 

Are you using 6550s, kt88s or the new kt120 tubes? But that is normal. I tend to move my tubes around so that they are as closely matched as possible. Are these new tubes that you just installed? If so the bias will drift a little as the tubes break in. It is best to check the bias often for the first couple of days.

BTW I like to set my bias around 63 mv on my ARC amps.
bifwynne OP
2,069 posts
 
Lostbears: SED 6550 "Wingeg C" matched tubes, purchased from ARC. Roughly 300 hours on the tubes. How critical is bias milli-voltage on sound? Any rule of thumb on how low you can set the bias??
lostbears's avatar
lostbears
479 posts
 

The lower the bias the longer the tubes will last. But the amp will not sound quite as good. I think the sweet spot is around 63 mv. I would not go much below 60 mv.

I believe the VS 115 is now shipping with the new kt120 tubes. I know that the Ref series amps are. According to ARC the kt120 tubes are a better sounding tube that will also increase the power roughly 20%. I have not tried them yet but plan to the next time I retube. They are not available seperately from ARC yet.
bifwynne OP
2,069 posts
 
Interesting. This is the first I heard about the kt120 tube. Who manufactures the tube, SED ("Winged C")? Will any mods be needed to accomodate the new tube type?
bifwynne OP
2,069 posts
 
FYI: The ARCD web site has some information. The URL is pasted below.

http://www.arcdb.ws/index.html

Before I pop the new KT 120 tube into my amp, I'll check with ARC. ARCD reports the new tube is made by the New Sensor factory in St. Petersburg, Russia. I believe that's the "Winged C" label. Per ARCD, the new KT 120 will bias at the same value as the 6550.

Interesting new development!
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