Bad Grounding,or Cheap Pot??

[denny9167]

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?

 

[leadbelly]

Hey, whaddya know, no psychics here today. Why don't you humour us and give us clues like your schematic and layout info?

 

[Tube_Dude]

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...

 

[6f6]

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.

 

[woodturner-fran]

FWIW, I had this on my aikido pre until I sorted out the grounding (which I had done incorrectly). I had this effect on both an alps pot and then a stepped attenuator.

Fran

 

[6f6]

Did you get it fixed?