Audio Research M100 Thumping

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.

Brian.

Brian,

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.

BIF

After considerable effort on the amps, including all tubes and caps, I think the problem is actually my conrad-johnson PV11 preamp. Once it gets warm, after about one to two hours, the heartbeat begins, now in both channels, volume independent (and it kills most of whatever is playing). If I kill the preamp, the thumping stops. When I turn back on the preamp, all is good for a few to ten or fifteen minutes, depending on how long it has been off, then the thumping comes back.

Tubes have all been tested. It was supposedly newly recapped when I bought it here a year or two ago. It did use to have more breathing room - could it just be something getting hot? What?

Thanks.

Run the amp without turning the C/J on, but leave it connected to the amp, and see if it motorboats. If after the 15-20 mins. the amp remains fine, it's not the amp.