Vibro Champ Issue

9John9

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I built a Vibro Champ and all is well except there is a scratchiness on the Intensity control as it is initially increased, from say 1 to 4. The rest of its range is quiet. I thought is might be the pot and replaced it, but with the same results. There is DC feeding the pot (see schematic). Could that be the cause, and is there a way to avoid that on the pot? Thanks for any help.

1653329277312.png
 

Phrygian77

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It's just the way it is. I don't see how you could possibly redesign the circuit to eliminate that. The whole thing is kind of a trick to begin with. I guess you could use a cap large enough to block the the DC to the V1B cathode without blocking the signal down around 2-3Hz, but then that would also prevent the intensity control from biasing the stage colder as it's turned up. That also would not eliminate the DC from the wiper to ground, so I don't even know if that would help.
 

corliss1

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Another vote for "how it is" - I think it's a certain model Bassman or Marshall presence control that has this "feature" too, and it's just how that particular circuit is designed.
 




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