Guitar Amplifier Hum Varies with Volume Knob - diyAudio
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Old 6th September 2013, 02:36 PM   #1
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Default Guitar Amplifier Hum Varies with Volume Knob

I am pretty well along in re-purposing a tube phono amplifier for guitar use. It is a push-pull 6v6 design with a 12at7 phase splitter and a 12ax7 preamplifier tube. I gutted the phono stage and I'm currently using one half of the 12ax7 for the input section (using both sections provided unnecessary gain and noise). But for the lack of a series grid resistor, it is configured similarly to a standard Fender input stage.

Everything works as expected, but for considerable power supply hum that varies with the guitar's volume knob. The hum is fine at maximum volume or minimum volume, but it gets obnoxiously loud as you approach the 50% volume setting.

My wiring is pretty much a rat's nest at this stage. Is this the sort of thing that is likely to improve with better layout and shielding? Any thoughts would be appreciated.




-Nick
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Old 6th September 2013, 03:12 PM   #2
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Most likely it's down to poor or incorrect wiring - can you post a picture?.
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Old 6th September 2013, 04:23 PM   #3
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Thanks for responding - I've attached a sketch of the circuit. You'll have to take my word that it is wired per the drawing, as I'm away from my workbench.
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Old 6th September 2013, 10:51 PM   #4
h2mster is offline h2mster  Germany
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try a cap(maybe 100pF) in parallel to the 1K resistor, a 68k+10pFCab before the tube

you can vary the 100pF until you hit the "hum"
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Old 6th September 2013, 10:56 PM   #5
h2mster is offline h2mster  Germany
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ok @60Hz a 10uF should do
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Old 6th September 2013, 11:12 PM   #6
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I suggest a grounding issue (loose ground, ground loop...). Is the input isolated from the chassis?
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Old 6th September 2013, 11:36 PM   #7
h2mster is offline h2mster  Germany
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Click the image to open in full size.

around like this
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Old 6th September 2013, 11:37 PM   #8
h2mster is offline h2mster  Germany
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@funk1980 indeed it would be nice to see the whole schematics to get an picture
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Old 6th September 2013, 11:54 PM   #9
Enzo is offline Enzo  United States
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But you said the hum varies with the GUITAR's volume control? SO if you turn the guitar control to zero, you get zero hum?

If so, that tells me the guitar is the source of the hum. If turning the guitar to zero kills the hum, then the input jack and cord and everything back to the volume control seems to be working. Iif the input jack on the amp had a bad ground, it would hum regardless of guitar control setting.

With the guitar up and hum happening, step back away from the amp as far as possible. ANy effect on hum? Also, turn side to side each way. Does "aiming" the guitar different directions make a difference in the hum? Got flourescent lights? Turn them off, any difference.
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Old 6th September 2013, 11:58 PM   #10
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Quote:
Originally Posted by Enzo View Post
But you said the hum varies with the GUITAR's volume control? SO if you turn the guitar control to zero, you get zero hum?
I totally missed that. I assumed the input's gain control
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