• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Bad Grounding,or Cheap Pot??

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I have built a tube preamp,and I have an issue with low level hum,but this only occurs with the gain set half way,by the way I'm using a cheap Alpha Pot,once I turn the gain all the way up the hum completely goes away and it's quiet,and I'm using star grounding!!!! any suggestions?
 
denny9167 said:
I have built a tube preamp,and I have an issue with low level hum,but this only occurs with the gain set half way,by the way I'm using a cheap Alpha Pot,once I turn the gain all the way up the hum completely goes away and it's quiet,and I'm using star grounding!!!! any suggestions?


Have you grounded the pot housing (body) ?

Maybe the pot can have to high a value... ;)
 
Denny,
My guess is that you are using the stereo pot that Radio Shack sells. I guess this because I had a similar problem with equipment that I constructed using it. I remedied the problem by connecting a small (10pf or 20pf) capacitor from the feedback tap to ground. You could also connect the same small cap in paralell with the "top" and the "bottom" of the pot.
 
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