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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
2,206 posts
 
Hifigeek, happy to oblige!
pete279
1 posts
 
Could anyone run through the procedure for setting the bias up on my arc vs115
How often should this need doing
bifwynne OP
2,206 posts
 
Pete, I'll field the Q, pending a better response from Gary (Hifigeek). Adjusting bias on the VS-115 is simple, safe and fast.

First, buy a Radio Shack multi-meter with test probes.

Second, if you're biasing brand new tubes, turn the bias pots down to mimimum voltage. Drop the new tubes in and let them warm up for about 45 mins to an hour.

If your setting up new tubes, they should be matched pairs because only one tube of the pair is a set tube. The other is a slave.

Then click the multimeter to volts. Insert the probes into the set tube insert openings in the back of ther amp and carefully and slowly set the bias to 65 mVolts. USE THE PLASTIC SCREWDRIVER THINGY ARC provides. DO NOT USE A METAL SCREWDRIVER.

Then check the slave to make sure it's bias voltage is within spec, which is betwen 57 and 73 mVolts. Repeat the process for the other 3 sets and you are in business.

Hope this helps.
hifigeek1
770 posts
 
I normally run the tubes at 50mV after 20 mins. to warm them up. After 45 mins I set them to 65mV. I do this while the line voltage is adjusted to 120Vac with the use of a Variac. This step is somewhat important as line voltage fluctuations can raise or lower the bias voltage by a few mV's. Since the factory does that, I do too.
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