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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
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!
hifigeek1
770 posts
 
First off, KT-120's are not available yet through ARC for re-tubing. They are only available for new production. Hopefully this will change in the next few months. If you are concerned about some of your output tubes getting up into the 70mV range you can always turn the main bias pot down to 61-63 instead of 65mV. That wont hurt anything, and should not effect tonality or distortion.

The KT-120's I saw from ARC did not have the Winged "C" logo on the tube. Instead, they were branded Tung Sol. They bias the same as the 6550. However, this tube does pull more heater current. That shouldn't be a problem for ARC amps unless the cage wont fit because of the tubes. That can be a problem with early ARC product. Caution may need to be taken if using this tube in a Dynaco MKlll, for example as it could make the transformer run very hot. I have not tried them in that amp however. In addition, those ARC amps that use a 6550 regulator in the power supply, that tube must remain. At this point KT-120's are not being used for the regulator tube.
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