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Old 24th February 2008, 11:21 PM   #1
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Default The sound of a failing tube rectifier?

I have an Audio Note preamp kit, M7 variant, that operated quietly at my old house. After a move it has developed an impressive hum. The overload indicators on my ss amp light up with no signal.

I have checked wiring and can find no grounding issues. I have put shorting plugs on all unused inputs and that seems to help (but not cure) the hum. While the audible hum drops with the shorting plugs, the power amp overload lights remain on making me think something sub-sonic is going on..

While the obvious culprit is a ground loop, I haven't been able to chase it down. This is my first piece of gear with tube rectification so I wondered... could what I'm experiencing be explained by a failing rectifier? What would that sound like?

Any thoughts would be greatly appreciated.

Cheers...TG
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Old 25th February 2008, 12:14 AM   #2
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Hi Tgoram3,

Your question is difficult to answer - a failing rectifier is best checked by measuring the H.T./scope-on-the-rails.

But the problem appears to have arised as a result of moving, thus shorting inputs and other basic things may not be going in the right direction. I would start (as you mentioned) by checking gound issues: Check your wall outlets (with the necessary care!) for broken ground leads, plugs, sockets, etc - things that might have been damaged during the move. It may be that by sheer misfortune the rectifier is starting to fail just when you moved, but I consider that unlikely.

Good luck!
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Old 25th February 2008, 03:13 AM   #3
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Quote:
I have an Audio Note preamp kit, M7 variant, that operated quietly at my old house. After a move it has developed an impressive hum. The overload indicators on my ss amp light up with no signal.
Does the SS amplifier act correctly when it is unhooked from the preamp?

Have you taken a voltage measurement at the output of the preamp's RCA jacks?
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Old 25th February 2008, 04:18 AM   #4
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Burned Fingers,

1) Good question. When the amp in my test rig (a sturdy old Apt Power One) is driven directly it is well behaved and the overload LEDs do not come on.

2) Without a great deal of confidence in my multimeter (millivolts just plain aren't part of my day-to-day) it looks like I have about 4-8 mV (dc with no signal) at my outputs.
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Old 25th February 2008, 04:41 AM   #5
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You want to be measuring for AC on the output.
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Old 25th February 2008, 04:55 AM   #6
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AC measures about 45 mV.
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Old 25th February 2008, 06:31 AM   #7
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I suspect oscillation being caused by something. It's not easy to get an SS amp to show overload unless somehow you are causing DC offset (only detected in some designs and even then, coupling caps in most designs will be eliminating the DC offset from ever happening.) or you are inputting enough high frequency stuff to cause the output stage to have shoot-through current (both devices in a push-pull pair turning on at the same time becaues things are going too fast.

It is best to use an oscilloscope to determine what's going on here.
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Old 25th February 2008, 11:35 AM   #8
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Quote:
You want to be measuring for AC on the output.
Actually I look for both AC and DC on the output. The AC out of 45mv looks somewhat high to me. I would have suspected it to be less than 3-4 mv. The DC out you measured tells you that your coupling caps look to be good.

If you feel ok about opening up the preamp I would look on the power supply rail. If I were to make a guess I would think a power supply cap may be suspect.
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Old 25th February 2008, 11:40 AM   #9
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If the frequency is high enough, a DMM may read the voltage quite erroneously. There could be a lot more AC voltage in reality.
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Old 25th February 2008, 11:43 AM   #10
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It is best to use an oscilloscope to determine what's going on here.
I agree
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