Audio Research VS 115 bias Adjustments

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?

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!

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??

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

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.

Hey Hifigeek1: I believe the KT120 is branded by Tung Sol. I think one or two tube outlets on the web advertise the KT120. If the tube is matched, do you think it's worth trying out. The price is about $90/pair. ARC will charge twice that amount, when the tube becomes available from ARC in the Spring.

Hifigeek1: have you heard the new KT120 in ARC gear yet? If yes, any comments about sonics, e.g., compared to 6550 "Winged C"?

G; Thanks for the comment. I realize my next question is like pondering angels on pin heads, but I wonder how a VS115 amp (which is the type I own) would sound compared to a current REF 110 withthe old 6550 Winged "C" type. Care to speculate??

Tom: I believe Tube Depot also sells the KT120 for the same price as HiFiConnexion, $89.95 for a matched pair. Any reason why one should prefer one vendor over another?

I know Hifigeek has a strong bias for buying from ARC, and for good reason. However, ARC will not be selling the KT120 for replacment until March/April or later. Also, I understand that ARC will charge more than twice the price than Tube Depot and HiFiConnexion, $100 per tube. For two quads -- that's a lot! Thanks. BIF

Hifigeek, happy to oblige!

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.