Audio Research M100 Thumping

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.

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.

You may have a bad Hexfred in the power supply.