Troubleshooting tube preamp hum

[Dylan W]

I'm getting 60 cycle hum + harmonics from a MILA tube preamp I just finished. When I short all 3 input pins, the harmonics disappear, though a lesser hum at 120 Hz remains.

• Input wiring is with shielded cable, unless I miswired a shield
• The preamp and PSU are in a small enclosure (50 cal ammo case), with toroidal power tx
• Heaters are unregulated DC (CRC w/ 10000uF caps)
  

 

[Dylan W]

Attached below is a measurement with the input open, with 48 db gain added in DAW.

 

[Dylan W]

And with input shorted

 

[Dylan W]

Engaging the pad drops the harmonics ~20db. So the source of noise must be coming from before the pad.

Edit: The noise goes away entirely when plugging a 1/4" cable into the DI. (DI is a shorting jack, which lifts the input network and goes straight to V1 grid.) So obviously a problem with the wiring between input XLR and DI. Will attack tomorrow.

 

[abbey road d enfer]

• I'm getting 60 cycle hum + harmonics from a MILA tube preamp I just finished. When I short all 3 input pins, the harmonics disappear, though a lesser hum at 120 Hz remains.
  
  
  The preamp and PSU are in a small enclosure (50 cal ammo case), with toroidal power tx

  Have you tried rotating/moving the mains xfmr?
  
  [/quote]Heaters are unregulated DC (CRC w/ 10000uF caps) [/quote] This is a serious mod to the standard circuit. Have you noticed that in the original the heater voltage is elevated? Have you also done it with your DC heater voltage?
  

[/quote]

 

[Dylan W]

Have you tried rotating/moving the mains xfmr?

I rotated the mains transformer and was able to get about 3db improvement.

The main culprit was the input transformer, it was picking up gunk from the power toroid. I had been using a Sescom MI-15, which has a mumetal can but an EI core. I switched to a UTC humbucking type I had banging around and played around with the rotation, and I was able to pretty much kill the hum. I'm getting around 115 db EIN with 200R termination, which I'm happy with given the proximity of the power supply. And actually most of what's left is white noise, so I think it will improve another ~6db when I get quieter tubes.

Heaters are unregulated DC (CRC w/ 10000uF caps)

  This is a serious mod to the standard circuit. Have you noticed that in the original the heater voltage is elevated? Have you also done it with your DC heater voltage?

I actually didn't notice the elevated heaters - good catch. Do you think moving to elevated AC heaters would quiet it down any?

 

[abbey road d enfer]

I actually didn't notice the elevated heaters - good catch. Do you think moving to elevated AC heaters would quiet it down any?

I must say I don't know for sure, because I don't know why it's been done originally.
very often it's done to accomodate a cathode-follower, but there is none in this circuit, so I would surmise it's done for noise improvement. Only experimenting will tell.