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ZKIT1 Hum (Read 725 times)
Mike B
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ZKIT1 Hum
06/21/13 at 16:53:56
 
All of the previous posts on this topic have vanished, so I will try again.
Just completed a ZKit1 and I have 5.5mv of hum at the speaker terminals.

I tried several input tubes, but no change.  I used tube rectification so I swap out the rectifier tube for new one, no change.

I removed the input tube, as per Steve D's blog, this eliminated the hum.  Voltage at the speaker terminals is 1.7mv. So this indicates the problem is in the input circuitry.

 I am going to isolate the chassis ground from the AC supply ground just to eliminate another source of the problem.

Any recommendations will be appreciated.  I'll try to post a picture when the forum allows me.

Mike B
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Mike B
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Re: ZKIT1 Hum
Reply #1 - 06/21/13 at 16:55:46
 
Here is a picture of the components.
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100_0442_600x800_480x640.jpg
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mboxler
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Re: ZKIT1 Hum
Reply #2 - 06/22/13 at 14:18:21
 
Hi Mike

I see you didn't use the 100 ohm CT resistors for the heaters.  Does your PT have a CT for the heaters?  Can the hum be created from the input tube heaters?  

Mike
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Mike B
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Re: ZKIT1 Hum
Reply #3 - 06/22/13 at 15:25:50
 
Mike, the heater supply is center tapped.  The heater voltage is equal on both legs.  

The only voltage differences I found is the plate voltage on the 6n1p, pin 1 and 6, there is a 10 vdc difference, pin 1 reads 126 vdc pin 6 reads 136 vdc.  the voltage feeding the 47k resistor is the same, so one of the 47k resistors may be suspect.  This is just a swag (statistical wild ass guess) but it is and easy change so what the heck.

Mike B
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« Last Edit: 06/22/13 at 15:27:26 by Mike B »  
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Mike B
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Re: ZKIT1 Hum
Reply #4 - 06/24/13 at 03:40:20
 
Problem solved.  After much checking and verifying, I decided the hum was due to ripple on the mains.  I added a RC filter just after the rectifier tube.  After installing, the hum was inaudible and the voltage on the speaker outs went from 5.5mv to 1.7mv.  All is good.

Thanks for the comments.

Mike B
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Lon
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Re: ZKIT1 Hum
Reply #5 - 06/24/13 at 14:03:15
 
Good news!
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