Audio Research Classic 60 & 120 owners

Hello,
I've owned Classic 60 and 120 in the past. Very good amplifier, extremely dynamic and detailed. As far as i remember the power transformer is quite hot after some time, 1 or 2 hours and this is normal. I'm still owning a V35 which is also an excellent amplifier, same circuit design as the Classic 30 but with balanced input.
Laurent

I have worked on quite a few CL-30 and CL-60. Yes the transformer does get quite warm. I also remember the power transformer being close to the output tubes. They are designed to run at that temperature and if there was an issue with too much current being drawn through the transformer your line fuse would fail provided the fuse is the proper current rating. You might want to consider replacing your filter caps at some point. I've had to replace those too.

Hello, I am new to this site and hope some of your members here can assist me.

I have been using a pair of Audio Research Classic 120 for about 18 months now. This is my first valve amp and I am delighted with the superior sound quality it has over all my previous transistor amps I have owned over the years. My last amp was a Musical Fidelity A370.

The only problem I have is, one of the amp's line fuse keeps blowing! This is a slow blow, 250 VAC, 3 amp fuse. Can anyone tell me why? Anyone have similar experiences?

The engineer has given up hope in finding the fault! It happens happens intermittently. Sometimes, it will work for a few hours or days. Sometimes, it will blow immediately after switching on! This also happens on the engineer's test bench. He could not locate the exact problem as it works at times but not the rest of the time.

The engineer has replaced the rectifiers, all 12 caps (450v/ 800uF) and 16 x 6550 tubes on both amp.

I am using this power amp with Audio Research Pre-Amp, LS-2 and the Apogee Hybrid Ribbon Speakers, Centaur Major.

Your help and advice is most appreciated. Thanks

My guess is you have a faulty tube or tube socket. A lot of times a tech will miss either corrosion, if you happen to live near the coast, corrosion can occur under a tube socket, or you have some corrosion on the circuit board. It is also possible that you just have a bad tube socket, it's loosing it's dielectric strength and arcing to an adjacent pin. I have a D115mkII I'm working on right now with a similar issue. BTW it's not a rectifier issue as rectifiers do not become intermittent. They either work or they don't. Question, when you look at the blown fuse, is it hard to tell the fuse blew or is the metallic fuse element splattered all over the inside of the glass tube? That unit I believe uses a servo to match the bias from a pair of output tubes you set the bias on. It's possible and not at all unusual for the servo to not 'pull in' properly and so the servo'ed side of the output tubes are running away. I will look at a circuit diagram to confirm that but I believe that whole series, CL-30, CL-60, CL-120 used this servo bias arrangement.

Hi, I had the same issue a couple years ago with one amp. I called Chris O. at ARC and he asked me how old were the tubes. I said well over 2000hrs and the ran the bias at 0.7mv. His response was; the bias is too high so you actually have 3000hrs on those tubes and better replace them now before something worst than a blown fuse happens. Replaced the tubes with SED 6550C and biased the amps at .65mv and all was fine from that point forward.

And yes the servo should be checked out and be adjusted within spec as the Geek says. I have some documentation from ARC on how to adjust the servo but it's very tedious. Patience is key with servo adjustment because the pot is VERY sensitive when you turn it. You have to go in minute increments and wait for the voltage to settle (usually 5 minutes) and then make more if need be until you fall within spec. I believe you have to be within .2mv
In the literature, ARC says "RESIST THE URGE TO FIDDLE WITH THE SERVO". The servo should not under normal circumstances, require adjusting.