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Old 21st October 2010, 08:14 PM   #21
240z4u is offline 240z4u  United States
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Nice. Done deal.
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Old 21st October 2010, 09:17 PM   #22
DF96 is offline DF96  England
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If hum is varying with volume setting then it is probably being picked up on the input or the grid wiring. Hum from heater leakage or HT/B+ would stay the same. Are you using screened cable?

Incidentally, you ought to put a 1M from the wiper to ground. This will protect against the wiper coming off the track.
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Old 21st October 2010, 09:42 PM   #23
scott17 is offline scott17  United States
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Just a thought, if heater leakage was in close proximity to the input tubes, would not the hum vary with the input level?

Also I agree, make sure you are using screened (shielded) cable for all low-level input signals.

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If hum is varying with volume setting then it is probably being picked up on the input or the grid wiring. Hum from heater leakage or HT/B+ would stay the same. Are you using screened cable?

Incidentally, you ought to put a 1M from the wiper to ground. This will protect against the wiper coming off the track.
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Old 21st October 2010, 10:03 PM   #24
DF96 is offline DF96  England
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Leakage is internal, from heater to cathode. This would not vary with volume. Adding a cathode decoupling cap would check this: less hum with cap means heater leakage or HT/B+ ripple, more hum with cap means something else.

Capacitive coupling to the input becomes part of the signal, so varies with volume. Capacitive coupling to the grid circuit basically injects a hum current, so the voltage developed depends on the volume setting - probably highest at mid-volume.

What happens to the hum when the input (i.e. top of volume pot) is shorted to ground? If it goes away, whatever the volume setting, then it is input pickup. If hum is still present but disappears at min and max volume then it is being picked up on the grid wiring.
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Old 21st October 2010, 10:13 PM   #25
240z4u is offline 240z4u  United States
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Good food for thought here guys. I am going to begin working on it here in a second.

All the signal wiring is shielded.

Evan
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Old 21st October 2010, 10:14 PM   #26
scott17 is offline scott17  United States
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Excellent description, makes perfect sense, thanks.

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Originally Posted by DF96 View Post
Leakage is internal, from heater to cathode. This would not vary with volume. Adding a cathode decoupling cap would check this: less hum with cap means heater leakage or HT/B+ ripple, more hum with cap means something else.

Capacitive coupling to the input becomes part of the signal, so varies with volume. Capacitive coupling to the grid circuit basically injects a hum current, so the voltage developed depends on the volume setting - probably highest at mid-volume.

What happens to the hum when the input (i.e. top of volume pot) is shorted to ground? If it goes away, whatever the volume setting, then it is input pickup. If hum is still present but disappears at min and max volume then it is being picked up on the grid wiring.
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Old 21st October 2010, 10:44 PM   #27
240z4u is offline 240z4u  United States
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Okay, when I short the input to ground there is no change. If I touch a probe to the input signal I get another "different" sounding hum. I think the hum may actually be 120hz. I wish I had a signal generator so I could know for sure what I was hearing.

Changing the volume with it shorted gave a linear rise in the loudness of the "buzz" which is a more accurate term I think.

Anybody know of an online signal generator where I could hear 120hz in the flesh?
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Old 21st October 2010, 10:47 PM   #28
240z4u is offline 240z4u  United States
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I am majorly re-evaluating (going to redo) my grounding setup. It is fairly ugly. Should I have two seperate ground planes? One for "safety ground" and one for audio signal grounds?

I am going to see if I have AC on my B+.
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Old 22nd October 2010, 12:14 AM   #29
Doz is offline Doz  United Kingdom
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Sound Generator free download. The audio frequency sinusoidal signal generator is intended for adjusting and measuring parameters of audio equipment. Works with 16, 24 and 32 bit so...

Signal generator ... set to 120Hz and enjoy
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Old 22nd October 2010, 12:22 AM   #30
240z4u is offline 240z4u  United States
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dumb laptop won't run it. Ill do it at work tomorrow.
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