Audio Research Classic 60 & 120 owners

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.

There is also a way to 'speed up' the servo so it pulls in sooner but that is something a qualified tech should do with parts and documentation from ARC.

Many thanks for sharing your thoughts and advice.

I will check the tubes as suggested by Hifigeek1 and will also check the tubes are operating with a bais of .65mv as suggested by Smoffatt.

Hifigeek1 could be right regarding a faulty tube (despite the tubes are new with less than 100 hours usage). Next, I will check on the socket as suggested.

Last week, ARC technician did suggest I should swap the tubes from the working Amp to the Amp that I am having problem with the blown fuse. After swapping and if the fuse still blows, then I should try swapping the caps from the good amp to the bad amp. I hope this will narrow down the uncertainty of tubes or caps being faulty.

I will report back with my progress. Wish me luck and keep your suggestion coming!

Happy hunting!