Background hum issue (volume control?)

[digicosmos]

Hello guys,

I've recently got an assembled Bottlehead Crack w/ Speedball upgrade from someone on Head-fi.  The amp sounds good but there is some background hum in the right channel, which is still very present during playback.  Strange thing is, it's only there at some volume levels (about 1/2 to 3/4 of maximum volume).  Unfortunately, it is the volume level that I use most.

What do you think is causing the issue?  Is there any easy fix?

Thank you!

[Paul Birkeland]

I would try applying a bit of contact cleaner in the volume control. 

Is this hum present with no source plugged in?

-PB

[Paul Joppa]

If the contact cleaner does nothing, then I'd say that volume control behavior suggests it's capacitive coupling, or at least high impedance coupling, to a grid. It would most likely be the 12AU7 grid, since that is the most sensitive part of the circuit.

One possibility is a tube whose heater voltage leaks to the grid. Trying a replacement tube is the easiest way to test for this.

Another possibility is something off in the layout, with a heater power wire too close to a grid terminal. Perhaps you could post a picture of the wiring? You may have to unscrew and push aside the PC boards to get a good shot of the tube socket wiring.

[digicosmos]

I would try applying a bit of contact cleaner in the volume control. 

Is this hum present with no source plugged in?

-PB

Thank you for your reply, Paul.  Yes, the hum(or hiss) is still present with no source plugged in

[digicosmos]

If the contact cleaner does nothing, then I'd say that volume control behavior suggests it's capacitive coupling, or at least high impedance coupling, to a grid. It would most likely be the 12AU7 grid, since that is the most sensitive part of the circuit.

One possibility is a tube whose heater voltage leaks to the grid. Trying a replacement tube is the easiest way to test for this.

Another possibility is something off in the layout, with a heater power wire too close to a grid terminal. Perhaps you could post a picture of the wiring? You may have to unscrew and push aside the PC boards to get a good shot of the tube socket wiring.

Thank you, Paul

Let me try the contact cleaner first.  It could also be the problem of the tubes.  They seem rather old to me.  Can you please recommend some better sounding tubes?