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?



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.

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?

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.
The only other thing it may be is the coupling caps in the C/J are leaking DC voltage causing it to destabilize the M100's. It's also possible a regulated power supply in the C/J or caps in the C/J are defective. How old is your C/J preamp??
Thank you for the reply.

The amps do not motorboat with the preamp turned off. Preamp is from the 80s, but in 2010, the tube sockets, all filter and coupling caps, all tubes, and the power supply (upgraded to Hexfred) were all replaced. With my previous c-j amps, there was no motorboating. I am beginning to wonder if it is an incompatibility.