M100 Amps thumping

B

bkrpdx

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Jul 31, 2014
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I just bought what appear to be a nice pair of M100 mono block amps. I hooked them up, and they ran for about 20 minutes. At that time, one channel cut out most of the volume (the music degraded) and began a rhythmic three-beat thumping similar to a heartbeat. This sound persisted even after turning down the volume. No change on the preamp (conrad-johnson pv11) made any change to the noise. The music degradation bled over to the other channel, but once I pulled the speaker wires on the offending channel, the other channel returned to normal. I shut off the system, waited about half an hour, turned it all back on, re-biased the output tubes, and started listening again. It sounded great again, but, again, only lasted about 15 minutes, and then the same behavior occurred again.

Is this a simple issue of replacing output tubes? I have a stack of spare 6550s -- should I just replace all five and see if that fixes it? And if it does not, then what?

Thanks.

B.
 
I would replace the output tubes one at a time with the same replacement tube and see if that is the problem. If not move to the driver tubes.
 
Jack,

Thank you. Upon opening, I discovered 1 6550 and 4 KT88s. I replaced all 5 with 6550s, and I ran it for about an hour with no thumping. I will try again tomorrow for longer and see if it is cured.

B.
 
Sounds like nice amps but which brand are the M100's? I have Ray Lumley M100 tube monoblock amps that are also KT88 (currently using KT120's).
 
Albert said:
which brand are the M100's?
Click to expand...

Well, I posted in the Audio Research Corporation forum :). So, yes, they are ARC M100s.

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.

Brian.
 
Oh, I missed that.;) ARC makes very nice gear. I have a PH3 phono amp that is very well made and works well.

My amp blew a fuse too when I first got them. After replacing I didn't blow another. Perhaps it was a faulty fuse?
 
Yes, to some degree I wonder whether the tube issue and the fuse issue is not just related to rough handling by UPS. My last set of amps blew three tubes and two fuses before stabilizing and did not blow another for years.
 
The M100 amps are classics and still in demand and the price keeps going up. You may want to consider sending them back to ARC and having them recapped and given a general going through if you can afford it.
 
Albert said:
Sounds like nice amps but which brand are the M100's? I have Ray Lumley M100 tube monoblock amps that are also KT88 (currently using KT120's).
Click to expand...

I got a pair of Lumley 100 mono blocks, I'm trying to figure out what kind of fuses they're supposed to run. The owners manual says three amp, but I picked up mine and they only have 500 mA fuses. Any ideas? Thanks very much
 
You have two fuses on the back of the amp. One marked "mains" this should be 3A slo blow, the other is marked "HT" This is the 500ma.
 
I only recall seeing one, but I'll take a look later when I get home. I did end up replacing the 500 mg fuse with a fast low and everything seems to be working. I missed seeing the other fuse holder, but I'll look again


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