2. amps-preamps
  3. 1406857519

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?

Thanks,

Brian.
bkrpdx
zd542
5,339 posts
 
What's the rest of your system?
rcprince
3,062 posts
 
If you reverse the channels, does the problem follow the channel or the amp?
bkrpdx OP
24 posts
 
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!
czarivey
5,752 posts
 
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.
bifwynne
2,206 posts
 
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.'

BIF
More to discover