Audio Research M100 Thumping

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.'

BIF

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.

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.