Audio Research VS 115 bias Adjustments

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.

Thanks Hifigeek1: I spoke with a tech guy at ARC yesterday about the KT120. He confirmed what you are reporting above, although he did not know for sure who the manufacturer is. I did some on-line checking and noticed that the only label for KT120 tubes is Tung Sol.

As far as compatibility is concerned, the ARC guy said that the VS 115 could physically and "should" (that's my quote, see below) technically handle the new tube. Because the VS 115 has an open topology, its ability to physical accomodate the KT120 is a no-brainer. Most significantly, as to the all important question of sound quality, he advised that ARC believes the new tube should make the ARC VS115 sound better. There's also the added plus of a slight boost of output power.

On the other hand, when I pressed on the "technical" issue re electronic compatibility and power transformer over-heating, I sensed just a smidge of tentativeness because he did not have tech sheets in front of him. FWIW, out of an abundance of caution, I will wait until I see something in writing from ARC about the tube change. As a fellow A-goner mentenioned in a post, I do not want to chance turning my power transformer into an expensive ship anchor.

The ARC tech guy also mentioned that as long as the "Set" tubes are adjusted as prescribed (65mV), the "Check" tubes could vary as long as they were within the 57-73mV spec. Out of an abundance of caution, Hifigeek1, I tend to agree with you about tapping down the "Set" pots to bring the "CHeck" tube bias current down.

Happy New Year!! BIF

Bifwynne,

I'm on the phone with ARC 2-3 times a week lol. Remember the KT-120 is a new tube so as info. becomes available it will be disseminated. If I hear anything, I will let you know. Happy New Year to you as well.

geek

Where are you guys getting this info about new tubes in ARC amps? Their web site does not mention them anywhere and the amp specs still call for 6550c? Why would they make this sudden change after using the same tube for almost ever? What will this do to the supply of 6550?

Hi 6550c: Initially, I picked this up on a forum thread. The information is also reported on ARCD.com (see News tab). Finally, I called ARC.