Audio Research M100 Thumping

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.

Is there an easy way to test for a bad Hexfred?

conrad-johnson pv11 pre-amp
PSB Imagine T Towers
McIntosh MR67 Tuner
Dual CS508

The problem followed the amp.

I did notice the output tubes were a mixture of 6550s and KT88s. I replaced them all with a different set of 6550s which I knew worked and re-biased. It ran about an hour without problem. I will try again today to see if it was a cure.

Thanks!

Thanks for the good advice. Anyone know of an authorized ARC tech in Portland, OR?

This morning I went to test if the tube switch had worked, and it blew a fuse. I switched the fuse from the other amp, and the amp seemed fine. Not sure why it blew the fuse. Now I need to go buy more fuses.

But it makes me even more inclined to have them checked.

B.

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.

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.

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.

Everything appears to be working correctly now. My guy had to replace caps which had the wrong ones installed, replace tubes which were the wrong tubes, re-solder throughout, re-wire the volume control which had been hacked, and do some rewiring on the power supply/ground. I had the specs, so he redid everything to spec. And now everything works.

He described ARCs in general as great sounding but running on the ragged edge - so innovative and creative that they sometimes pushed the envelope a bit too far, often to the detriment of longevity.