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??

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.
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??

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.
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.

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!
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

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.

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 (see News tab). Finally, I called ARC.
6550, I service ARC equip.
Yikes! Looks like the new tubes come with a little price increase too. I wonder if this is why I see a few dealer floor samples for sale too?
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.

The problem is, all ARC tubes are matched for current draw so that all tubes draw the same current. Also the ARC tubes that I use are run for 24 hrs. at bias to cull out the bad ones and to make sure the tube won't majorly drift over time after you install them. One more very important point, if an ARC tube ever goes bad within the first 60 or 90 days, I don't remember off the top of my head, the damage is covered by ARC. If you buy your tubes elsewhere you are on your own. I would wait till ARC makes them available. You can contact me if you wish. I don't want to make this an advertisement for my business as I don't have that type of account. Hopefully the admin won't mind a private email. G
Hifigeek1: have you heard the new KT120 in ARC gear yet? If yes, any comments about sonics, e.g., compared to 6550 "Winged C"?
No I haven't personally however I have received feedback from people I trust. It's great! It should be understood that ARC listens to the tubes for a long time as well as life tests the tubes before they give their ok. If the tube sounded bad, they wouldn't be using it in their products. Your going to get a bit more slam and impact with the KT-120 and not in a bad way. It has an effortless quality. Very musical. The funny thing is I remember my old Dyna MK III's with the tung sol 6550 Coke Bottle tubes in them, and they were awesome. More slam, more impact w/o sacrificing any detail. By more slam I mean better transient impact. I think it's going to be a win win but only time will tell. G
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??
Well..see my comments above. Those sonic changes were due to the KT-120. I can only assume it will sound the same in your amp. G
I Purchased a matched octet set of Tung-Sol KT120's from parts Connexion for my ARC VS115. Good people to deal with. Tubes were well matched and arrived promptly.I put them in the amp biased them ,same as 6550 biasing, and as is well. Listened briefly and what was apparent immediately was: could see into the music better,better dynamics and micro dynamics,better mid-bass and overall better resolution.
Tomj2617. Awesome. they will take approx. 20 hrs. to break in so they should only get better over time. No need to run the amp non stop for 20hrs. however. Enjoy
Good to know Thanks Hifigeek1
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
Bifynne...I have a strong bias for buying from ARC? Awesome pun! LOL
Hifigeek, happy to oblige!
Could anyone run through the procedure for setting the bias up on my arc vs115
How often should this need doing
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.
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.