Audio Research M100 Thumping

I just picked up a pair of ARC M100 mono blocks. When I turn them on, all is good. They operate fine for about 15-20 minutes, then the output cuts on one, and it begins a three-beat thump which sounds like a loud heartbeat. This occurs even if the volume is all the way down on the preamp. It also bleeds over to the other channel, but if I pull the speaker wires from the offending channel, the other channel is clear.

Ideas? Should I simply try replacing some of the output 6550s?


You should have them checked pronto. They have some issues and you don't want them to damage themselves or your speakers.
This is called motor-boating and it means the amp is oscillating. M100's run very hot. Make sure the driver tubes are okay. I believe (from memory) they are either 7119's or 7044's. Those could be problematic. I would suggest you find a place to get both your amps checked out. I would not run the damaged amp.
I exchanged emails with Kalvin. He suggested switching out the output tubes and all of the fuses and running them for a while. I did that, and about five hours in, they really sound fantastic and seem to have no issues. I think these amps sound better than my old ones. Much more bass. Much cleaner high end. Sharper sound. And the others were "better" amps - conrad johnson Premier 8As with factory C1 upgrades.

Thanks, all.


I am very glad that Kal set you right. Having said that, according to the ARCDB website, your mono blocks were introduced in 1984.

Not knowing the service history of your amps, if never serviced, the components could be upwards of 30 years old. I think it's worth letting Kal know the amp is operating ok, but ask him if he thinks it would be a good idea to do some preventative maintenance.

As an aside, my son owns an SP-9 preamp, which is of similar vintage. Kal sent me a kit of parts that he recommended replacing. The local ARC service tech did the job. All in, less than $100.

Btw, per the ARCDB site, your tube compliment is:

(2) 6DJ8, (5) 7044, (2 matched pairs) 6550, (1) 6550, (1) 12AT7

If you check back with Kal, ask him if any other tubes should be replaced.

It has been my experience that my ARC gear sounds best with fresh tubes. I follow ARC recommendtions and have never looked back. Didn't have to. ARC gear is great stuff.

Let us know what you decide to do.