slight hum

[jt]

I have recently built a stock crack. It works great but there is a slight hum when volume is between 12 o'clock and 3 o'clock. It has been there since the initial build.

I have performed the following but the hum is still there in the same form:
  - switched input and output tubes - currently using TS5998, TS6SN7GT
  - upgraded potentiometer to stepped attenuator
  - upgraded power cable to shielded cable
  - switched of all electronic devices close by

Also, when I touch the faceplate hum diminishes but is still evident.

[Paul Birkeland]

What is the resistance between the plate and pins 7/8 on the 6SN7?

Is the ground lug back by the IEC entry nice and tight?

It sounds like there is a grounding issue between the signal or power ground and the chassis, or perhaps you have the heater winding floating.

Is this noise there with no source plugged in?

[jt]

Resistance at pin 7 is 0.5, pin 8 is 0.
Tightened ground lug and RCAs.
Visually checked heater wiring - looks ok.
Noise is there with no source plugged in.

[Paul Birkeland]

Try connecting a piece of wire between your Crack and the metal chassis of the source you are using.

[jt]

I tried with the wire connection and it made no difference.
So I replaced the SN7 with the original 12AU7 and the hum is gone!
I guess I must have lost track and introduced the hum with the SN7.
I also tried with a 6F8G - same hum.
I'm thinking it's a crappy 6SN7 to 12AU7 adapter.
Will try again when I get another adapter.

Thanks for your help!

[Laudanum]

Could be the adapter.  I think there was another Crack owner who experienced some hum with the 6SN7 and adapter that was narrowed down to the adapter itself.
Mine was dead quiet with 6SN7's on an adapter so an adapter can work without adding any audible noise.   You could always open up the adapter and see if anything looks sloppy.  I dont know what's in there but really cant hurt if you arent going to use it anyway. 

[Paul Birkeland]

Yeah, the adapter could potentially cause some problems, or you could have a noisy 6SN7 (sounds like you tried a few different ones).

-PB