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Mark III odd hum after warming up

207.38.43.30
Posted on September 14, 2009 at 13:40:49
Japhy
Audiophile

Posts: 106
Joined: November 2, 2007
 Howdy folks,

Have a hum question for you. I have a set of Mark III's that I've been using for a while now. They have been driving a set of ACI Sapphire XL's up until this weekend as I picked up a set of Klipsch Fortes. I don't hear any appreciate hum upon first firing the amps up and on one, it stays very quite even after a few hours of listening. The other hum starts to build up a hum which sounds like a little 60Hz line hum on top of higher frequency hum that may ebb and flow in and out. At its loudest I can slightly hear it while at the listening position. I haven't had a chance to swap output/rectifier tubes, but did swap the 6CG7 driver tube of the Triode Electronics driver board and this did reduce the noise a little bit, but not all that much. Something else I think is going on. As I said, the other amp is totally quiet and I build both at the same time and should be the same throughout. The weird thing is that it takes a while for this to show up.

Any thoughts?

Cheers!

RE: Mark III odd hum after warming up, posted on September 15, 2009 at 08:34:52
Joe Curcio
Manufacturer

Posts: 1094
Location: Northern CA
Joined: October 10, 2001
Contributor
  Since: July 1, 2002
 The first thing you need to do is be sure that the hum is not originating elsewhere. I'd suggest placing RCA shorting plugs into the inputs of both amps and just letting them sit for a few hours to see if the problem develops.

If you find that it does, pull the driver tubes (small tubes) and power up the amp again and let it sit for a few hours (again until the hum develops). If it does, this indicates that the source is in the output stage (output tubes, sockets, rectifier tube or bias supply). If not it is in the driver stage and there is a problem with the driver board.

RE: Mark III odd hum after warming up, posted on September 16, 2009 at 23:07:15
sford@altair.com
 Thanks again Joe for the response. I'm going to set aside some time this weekend hopefully to diagnose what is up.

Cheers,
Steve

RE: Mark III odd hum after warming up, posted on September 21, 2009 at 11:05:54
Japhy
Audiophile

Posts: 106
Joined: November 2, 2007
 Checked this weekend and it was the V1 (12AU7) tube. Guess I've had it in there for 2.5 years with a fair bit of use so time to pick up a new pair!

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