Marshall 2061x build hum

[JoshBernstein]

Hi all,

I recently built a Marshall 2061x using he ceriatone chassis and layout.

The amp sounds really awesome, but its got one thing that bugs me. Upon power up, there is a deep hum (sounds like 60hz to me, but I dont have a scope. As the tubes warm up, the hum gets louder, reaches a peak volume, and then subsides to a lower volume hum where it stays constant.

It occurs even with no preamp tube or phase inverter, even with the post phase inverter master volume turned all the way down to ground the input of the power amp tubes. I tried putting in a new cap can but no dice. Here are some pics, and i will post a link to a video in a second. The video is taken with only the power tubes in and the grids of the power tubes grounded. I generally don't use the standby, but if i let them amp warm up on standby and then flip the standby switch, there is no swell, the hum just starts at its final idle level. Power tubes are a known good matched pair, I have tried a couple other sets as well. Anybody have any ideas?

https://youtu.be/Nwh5gfoO3SA

[sonicmojo]

I'd try testing the other caps too by temporarily placing another 47uF parallel to each one to see if it helps the hum. Also, I am seeing the black wire off the transformer running directly between the on off switch wires. And the other wires are all kinda jumbled up over there. It is probably not the problem but I'd move those leads around if possible and maybe get them a bit away from the switch.

Also that chassis might be coated with something. Maybe check/sand the chassis a bit at your ground lugs.

[JoshBernstein]

I'd try testing the other caps too by temporarily placing another 47uF parallel to each one to see if it helps the hum. Also, I am seeing the black wire off the transformer running directly between the on off switch wires. And the other wires are all kinda jumbled up over there. It is probably not the problem but I'd move those leads around if possible and maybe get them a bit away from the switch.

Also that chassis might be coated with something. Maybe check/sand the chassis a bit at your ground lugs.

Will do, I have a couple caps I could clip in. Could those filter be the problem even with the preamp tubes removed? Ill move the wires around and see if anything changes.

[JoshBernstein]

I tried clipping in some new caps with no dice, and i moved the transformer wires as far as I could but nothing changed. How could i test the grounds? I measured resistances for each ground point to the ground lug of the iec socket, and all are very low, a fraction of an ohm at most