Faint 60-Cycle Hum

[jtori]

More prevalent in the left channel and increases with volume.  More specifically, hum in the right channel is lower in volume and remains unchanged with an increase in volume.  Hum in the left channel, however, becomes louder with an increase in volume.  The hum is not noticeable with music playing, but can be heard when there is no signal.  The amp is warmed for roughly 20 mins. before playing.  For info, my C2A is stock with the TwoQuiet upgrade.  Current tube complement is as follows:

-- NOS Mullard CV4003
-- A pair of NOS GE 5 Stars
-- NOS WE421A

Any guidance on what to check?

Thanks,

Joe.

[Doc B.]

Increasing with volume implies that the noise is coming from or ahead of the volume control. To narrow that down try shorting the input and see if the noise changes.

[jtori]

Increasing with volume implies that the noise is coming from or ahead of the volume control. To narrow that down try shorting the input and see if the noise changes.

Thank you, Doc.  Shorting does indeed eliminate the hum.  Before doing this, I did a visual inspection of the underside.  I checked the CAT6 input leads running from the inputs to the selector switch and made sure there was good separation and ensured everything was well soldered.  I should note the hum is faint but audible on every step of the coarse attenuator except 0dB.  At 0dB is becomes noticeably louder.  Then, each step of the fine attenuator increases more.

[Doc B.]

If shorting the input kills the hum it is fairly likely that it's coming in ahead of the C2A. It may be that your cables are picking up some 60Hz from a nearby piece of gear. You might try different cables or at the very least moving the ones you are using around a bit to see if anything changes. The cables we used to do were made from Teflon foamed coax because I found it to be very effective at rejecting 60Hz interference. The STP we tried was not quite as good from that standpoint even though it's supposed to be better on paper.

[jtori]

I thought this might be the case, too.  So, I rearranged the cables, keeping them away from anything that might induce hum.  Still there.  Tried different cables.  Still there.  Disconnecting them completely.  Still there.  Shorting the inputs.  Dead quiet.  Most prevalent in the jump from -9dB to 0dB on the coarse attenuator.  Then, each step on the fine attenuator accentuates the hum incrementally.  Wondering if I've got a grounding issue inside the amp.

[Paul Birkeland]

You could always post photos of your build. 

[jtori]

Thanks, Paul.  You're right. 

[Paul Birkeland]

Did you have this issue with the stock level pot and balance pot?

[jtori]

I don't recall hearing it with the stock pot and balance control. 

[Paul Birkeland]

I would reflow all of the joints on the attenuator upgrade where a black wire lands.

If you have the fine control all the way up, is that the noisiest spot?

[Doc B.]

Are you sure it's 60Hz and not 120Hz? Does the amp sit near another power transformer, like on a shelf above or below it?

[Deke609]

I would reflow all of the joints on the attenuator upgrade where a black wire lands.

For Q2, Q4 and what looks like Q12 (at the 2 terminal strip on the far side of the coarse), maybe pull the black wire out a bit and/or trim off a bit more of the teflon jacket. From the photos, it looks like the wire insulation extends into the joint.

cheers, Derek

[jtori]

I feel confident it's a 60-cycle hum.  I'm fairly meticulous about keeping components and cables separated.  To be sure no transformers I unplugged all the wall warts powering peripheral devices.  While not near the amp, they share the same circuit.  On the outside chance it could be tube related, I swapped out the input and output tubes with others I have on hand.  For reference, I've attached a photo of my setup.

Could the computer be causing this?  My Mainline is dead quiet, as are my other amps. 
Or, might this be something ground related? 
Or, since the increase is most evident in the step from -9dB to 0dB on the coarse attenuator and is louder in the left channel than the right, could it be related to a resistor?

[jtori]

For Q2, Q4 and what looks like Q12 (at the 2 terminal strip on the far side of the coarse), maybe pull the black wire out a bit and/or trim off a bit more of the teflon jacket. From the photos, it looks like the wire insulation extends into the joint.

cheers, Derek

I will do that.  Thanks, Deke.

Joe.

[jtori]

I would reflow all of the joints on the attenuator upgrade where a black wire lands.

If you have the fine control all the way up, is that the noisiest spot?

Yes.  When the coarse and fine are all the way up, it's noisiest.  I will do as you've suggested.

Best,

Joe.