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?


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.
Not really...Looking at it on a scope but it could also be a faulty filter cap as well. I would suggest having a tech you trust look at it. If that preamp has a tube rectifier in it, make sure the tube is functioning and if need be replace it.
Thanks. I gave up and took it to a "guy." He is finding all kinds of problems, mostly poor workmanship by prior repair attempts. The person who owned it before me spent $1,000 on shoddy work. And it was not local, so he paid shipping too. I befuddles me why he would not just have shipped it to c-j and had it done right (and probably for less). It is so modified now c-j told me they would not touch it. But my guy is good (did fantastic work on my McIntosh MR-67), so I have high confidence.