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?


What's the rest of your system?
If you reverse the channels, does the problem follow the channel or the amp?
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.

find tube tester and test out the good ones from the bundle.
you don't have to purchase it, but try to cash-nego for testing few bulbs.
If your tubes are not fresh, it's alwasys a good idea to freshen them up. If that solves the problem, you are good to go.

I have another suggestion, regardless of whether a retube fixes the issue. Your amp is long in the tooth. Some parts do degrade over time, like caps.

I suggest that you call Kal at ARC and ask him if he thinks it would be a good idea to have the amps checked over. If he says "yes," depending on where you live, you could take it to an authorized ARC service tech, like Gary (Hifigeek1) in LA, ARC if you live in the Midwest, or if you live on the East Coast, there's a fella near me who makes house calls.

I strongly believe you should only use an authorized ARC tech. There was a story running here on A'gon about a well meaning and I'm sure talented tech who tried to fix an older ARC amp. He just started replacing parts. After the job was done, Gary picked up the thread and told him exactly what and where the problem was. No surprise since this is what Gary does.

Just sayin.'

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.

You should have them checked pronto. They have some issues and you don't want them to damage themselves or your speakers.
Bkrpdx ... call Kalvin Dahl at ARC. He can properly advise you.
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.


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.
You may have a bad Hexfred in the power supply.
Is there an easy way to test for a bad Hexfred?
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.
Yup I have an ARC SP-10 that's been modified and the work was simply awful. It's taken me forever to find all the problems wrong with it but I think I'm finally in the home stretch.
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.