Audio Research Classic 60 & 120 owners

For those who own one of those, does the power transformer (left one on the 120 and the center one on the 60) get quite warm to the touch after a few hours of operation? Mine get very warm. So much so that i can hardly keep my hand on it for very long. The bias setting is dead on with SED 6550C. Mine were upgraded by GNSC back in 2004.

This is not a new phenomenon. They have been warm like this for as long as i can remember. I had meant to ask other owners but never got to it until today.
Any feedback much appreciated.
The power transformer on any amp will get hot. If the unit has output transformers, (Tube amps) those would stay pretty cool, compared to the power transformer. Hope this allays your worries.
Sounds normal to me for those amps.
HI, I'm glad to hear there another person enjoying these amps. I've had the ARC 120's in my rotation since the early 90's. The temp situation you describe is exactly how mine behave. They do get hot after a while even with the fans and if I remember correctly they are drawing 300 watts each at idle. I typically use them only in the winter months here in Michigan. If you can manage the time, I would appreciate it you would share any feedback on the GNSC upgrades. I've been thinking about this path for a while. Cheers,
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.
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.
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!
I currently own a pair of 120; just wonder if the fans have to be constantly on, ie. the cage have to be on; how can you access or adjust the bias port which is located inside the cage?
Yes, the fans must be on in order to properly cool the tubes and the surrounding components.
Hey Hifigeek1, thank you for your reply!
So from your experience, please advise how to adjust the bias port as it is located inside the cage!?
I suggest you find a qualified Audio Research Service provider.
Thanks, I will give ARC a call tomorrow.
Hello Pioneer48,
I used to own a pair of CL120 in the past, i i have done the biasing myself. The best solution is to remove the plate which is under the amp, and also the tubes cage. Then place the CL120 onto 2 pieces of wood (around 10cm * 10cm), then with that solution you are able to grip the measurement points to the multimeter (That is really preferable to grip the measurement points when the amplifier is not switch on, otherwise it can be extremely dangerous).
Then it is also important to let the amp connected to an old speaker or resistor of 4 or 8 Ohms. Then switch on the amp and follow the evolution of the voltage. Then when the voltage is quite stable (after 20-30 min), adjust the bias the 65mv DC. If during the stabilization phase the bias goes over 65mv, reduce the bias via the appropriated pot.
You can operate with no tube cage, just for the bias setting, but it is not recommended to run the amplifier, without the tube cage. When you change the measurement point, please switch off the amp, but please be extremely care, the capacitors are dangerous even when the amp is not switch on. If you want some pictures, i do still have some from my V35 amp, basically is the same operation, so please send me your email @
At last, I managed to identify the cause of the Line fuse that keeps blowing on one of the amp.

Hifigeek1 guess was correct. It is due to a faulty tube! I do not know how many were faulty as I replace all six tubes with new ones.

These replaced tubes were less than 100 hours of use. It just shows new tubes can be faulty at times.

I do not have a tube tester. Can anyone suggest how I can find out which one is faulty from the six I have taken out?

Sebastianloke, where are you located???
Many apologies for the delay in replying. I have not logon here for awhile. I live in London, UK.
Can anyone send me a picture of the test points (TP1 & TP2 and TP3 & TP4) from the bottom of the circuit board? I would greatly appreciate it! I know which resitors are to be tested from the top, however there is not enough room to test them from the top.

Thank you,

Todd Hadican