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Old 18th February 2008, 07:30 AM   #1
pftrvlr is offline pftrvlr  United States
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Join Date: Feb 2007
Location: SoCal
Default Strange Hum

I am building a simple one 6688 tube per channel headphone amp. It was working fine on test board. When I put it into a wood frame case, there is vey loud hum from one side. It is not bad tube or wiring mistake. It plays music fine on both side and tube roll does no help. But when I put my finger on the tube, the hum stops. I guess it is oscillating on that side. I need to get my scope out tomorrow to check it. Or maybe it is a loose tube socket? Before I change the socket, I want to see if anyone has had similar experience.

Thanks.
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Old 18th February 2008, 07:35 AM   #2
jnb is offline jnb  Australia
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Try putting your finger down onto the chassis next to it to see whether the vibration isn't coming through from the power transformer.
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Old 18th February 2008, 09:36 AM   #3
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Default induction maybe?

Hi
I just finished a phono preamp based on 2 6688 stages in triode mode and it hummed more than a bee hive in spring time.
based on my experience that might be some form of inductance in the amp. If you touch the tube and the metal chassis does it go away?
Is it possible you arent using a star ground pattern?
let us know how you go
Nick
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Old 20th February 2008, 04:18 AM   #4
pftrvlr is offline pftrvlr  United States
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It turned out be due to bad tubes. Now it is dead quiet.

With the bad tubes, the amp will start or stop hum when I move my fingers near the tube without even touch them.
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Old 20th February 2008, 12:02 PM   #5
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Join Date: Mar 2004
Location: Albury NSW Australia
Default excellent

Oh to have an AVO tube tester...
Glad you sorted it out
Happy listening
Nick
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