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Topic: Vibrosonic hum balance |
David Guiterrez
From:
Albuquerque, New Mexico, USA
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Posted 31 Mar 2006 10:28 am |
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| I recently purchased a Vibrosonic Reverb, 1975 if I'm reading the transformer correctly. On the back of the amp are a line out and a hum balance. Can anyone tell me how to use these? |
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J Fletcher
From:
London,Ont,Canada
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Posted 31 Mar 2006 12:23 pm
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Listen to the amp without anything connected to the input,controls at zero, and adjust the hum balance with a slot head screwdriver for least amount of hum. It's a 100 ohm pot, as I recall, connected across the 6 volt filament supply, with the wiper going to ground.
If your output tubes aren't balanced or matched up, you have to adjust the output tube balance pot to get rid of that hum.
Depending on your level of expertise, you may choose to have somebody knowledgeable do this for you. Extreme rotation of these controls can cause damage under certain conditions...Jerry |
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Mark Herrick
From:
Bakersfield, CA
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Posted 31 Mar 2006 5:32 pm
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| Quote: |
| Listen to the amp without anything connected to the input,controls at zero... |
I think if you do this, you won't hear anything at all...
Here is a post I made a while back that includes the hum balance adjustment procedure that was in my Vibrasonic manual:
quote:
On older Twins (I have a '74 Silverface) and some other models (my '90's "Custom" Vibrasonic) there is a "hum adjustment" pot in addition to the bias adjustment (or balance) pot. You might check to see if your amp has this feature; seems like it should.
To adjust for minimum hum, you need an open plug (a 1/4" plug with nothing attached to it) in jack position 3 (first input in the second channel) with the second channel volume, mid and bass controls on 10, reverb and vibrato off and all channel one controls off. Then adjust the hum pot for minimum hum.
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J Fletcher
From:
London,Ont,Canada
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Posted 1 Apr 2006 8:19 am
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My experience is that hum introduced by an imbalance on the filament supply, will be audible when all controls are at zero, without anything plugged into the input.
It won't be real loud unless there's a drastic imbalance, but it's quiet audible, and adjusting the pot can usually get rid of it.
But don't take my word for it, the Fender Service Manual I have in front of me says..."turn all controls to the extreme left or off position and turn the adjusting screw clockwise or counterclockwise until the hum is at its lowest level or disappears."
The method you describe sounds very loud!....Jerry
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