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There isn't a schematic on the Audio Research database.
When checking the bias voltages on the back of the unit there's nothing for V2 and about 10 mV on V4. If I interchange all of the tubes with the other channel the problem doesn't move, still nothing on V2 and very low on V4.
I did open the unit and visually everything seems fine. I may have to send it out for repair, but was wondering if anyone had any suggestions for something I haven't tried yet.
Follow Ups:
Amplifier back home today, and what a relief. In my desperation I went to all of the pawn shops in town and came home with a Denon $150 SS amplifier while the AR was being repaired.It was apparently an easy fix, replacing the 1 and 100 ohm resistors for the plate and screen supplies.
Edit: My wife was joking - "I'm not sure if I hear a difference".
Edits: 08/18/14
Hi,
I owned a VS115 for a few years and the amp uses resistors as 'fuses'. So if a tube starts to fail and a high current flows a resistor near the valve base fails and the power to the tube goes to zero or becomes very low. What it needs is the resistor replaced (ARC can supply them or you can locally buy the type specified by them) and then everything should be OK, although there may be a tube about to fail or it has already failed. People have complained and asked for real fuses but ARC say it affects sound quality.
I bought a VS115 a couple of years ago brand new from an authorized dealer. It came with stock VT120 tubes. In the first week that I had the VS115, I noticed a hum and the transformer was getting extremely hot. Then a resistor popped. Actually I think 2 resistors blew. I found pieces of the resistor on the table below the amp. When it happened, it scared the shit out of me. I was driving my then brand new Harbeth M40.1s. I called my dealer and they actually offered to get me a new amp but I decided I don't want to go through that again. My dealer took the amp in exchange for a McIntosh MC452. I really think Audio Research needs to figure out a way to use a fuse or a breaker rather than a blowing a resistor. Fuses can be replaced and breakers can be reset by anyone without needing to break out the soldering iron. Anyway, the VS115 is a nice sounding amp. I've been happy for the last 2 yrs with my Mac amp and I have no desire to upgrade. It's nice not to worry about tube biasing, getting old and finding replacements.
George,Thanks for taking the time to respond, that does sound like the problem I'm having. I bought the amplifier used and had no idea how many hours the tubes might have had.
I was thinking recently that it might be time for a new set. Then, with a night listening session where I could see the tube glow I noticed that two of them were dead. I'm reluctant to continue operating the amplifier because with (presumably) all tubes about the same age more could go any time.
I'll give AR a call in the morning. Changing out resistors isn't a big deal as long as I know the positions and the values.
Thanks again.
Edits: 07/27/14
Ah......and you may want to look at upgrading to KT120s. They sound better and are cheaper than 6550Cs.
They agree that a screen or plate resistor is likely gone due to an arcing fault in a tube.
And yes, I think I will replace all of the power tubes - I bought the amp used and don't know how many hours are on them. Upgrading to the KT120's definitely seems like the way to go.
Did you notice that the bad tubes were not 'glowing', or something else. Not glowing would indicate a filament issue.
That's what started the whole thing, when I decided to check the tube bias the next day.
It's a second hand amp and I had no idea how many hours were on the tubes. As it turns out, a few too many.
I'm not familiar with the VS-115 specifically but....
Just curious.... what happens if you unplug the amp, unplug the power tubes, let it sit for maybe 1/2 hour.
Now instead measuring bias voltage (mV) try measuring resistance between each of the bias test points for each power tube.
What is the resistance for the 'good' locations vs the one or two that are 'bad'?
Is there possibly an open resistor in one or more locations?
Again, I'm not familiar with this amp but one thing you can try is to characterize component values around the good tube locations by measruing, then see if any of the components around the bad tubes are 'out of whack'.
V2 and V7 measure at 1.16, all the others from 1.21 to 1.22 ohms.
I'm guessing you were thinking a bad tube might have opened a bias resistor?
I don't have an issue with sending it out for repair. I am nervous of continuing to use the unit now that I know there's a problem. The outputs are with parallel tubes for each side, eight total.
With one side gone for one channel the power output will drop in half, but the last think I need is to blow up the amp or my speakers.
Yes, I was thinking possible open bias resistor but the value that you measured isn't that far off. Might be best to call ARC or an authorized dealer with good tech.
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