Hoffman Stout TMB Hum Issue

[coltojt]

Greetings all,

I've reached a wall in my troubleshooting attempting to identify the source of hum in this amp build and am needing input from the community. The hum does not change regardless of volume and is ever-present. 

 I've made three changes to the original layout: Fender-style pilot lamp, Pure Tone Output jacks, and the tube sockets were positioned for labels to face outward -silly in hindsight? Now I’m wondering if the latter is the source of hum as the heater wires are floating instead of snug to the chassis. That change did also shift the orientation of wiring to the board.

The amp sounds awesome minus this audible hum which is driving me bonkers… I believe it may be 60Hz? I do have an analog oscilloscope at my disposal, I just don’t know where to start.

Hum Audio:
https://www.dropbox.com/scl/fi/8kj7pd6k9m0iwojvhe6ff/New-Recording-563.m4a?rlkey=71nqj06vodyqt9j5lj0f29moo&st=3zv9mn7i&dl=0

[TIMBO]

hey mate,
Not ideal for the heater wires to be up near all the sensitive signal wires.
If you are able to redo the heater wires and get then into the internal corner under the pots would be a lot better.
You can also connect the heater CT to the cathode of the power tubes

[zircontweezer]

Have you tried taking out tubes one by one in the preamp. Start first tube in preamp working towards phase inverter and if it still hums remove the power tubes and tell us what happens.

[coltojt]

Thanks for the input y’all,

I’m thinking I may trying wiring the heater ct to the cathode node first before redressing to see if that helps.

What’s tricky is if I remove V1 the hum stops but also stops if I remove V2 with V1 in place. So I’m struggling to tell if the hum is being injected at the preamp or PI stage.

[stratomaster]

That means it's coming from the first tube. Putting V1 back in with V2 removed is immaterial.

To narrow it down further you can ground the grids (and/or bypass plates with a large value cap) to zero in on the exact stage that is problematic.

Update: just looked at the schematic. It's even easier than that.  If the Volume knob controls the volume of the hum, then it's in the first stage. If it doesn't, it's in the 2nd stage. To confirm you can jumper across the 470k to ground after the 2nd stage coupling cap. This will kill the signal after the 2nd stage. If that kills the hum but the volume control doesn't, then it's for sure the 2nd stage.

[coltojt]

I grounded the circled node on the layout and that majorly killed the hum. I also tried grounding Pin 7 on V1 but that made no difference. To confirm, volume control has no effect on hum. Any advice where I go from here?

[stratomaster]

If you disconnect the negative feedback does the hum go away?

[coltojt]

I’m embarrassed to say, I think the issue was a faulty preamp tube… I swapped them between sockets and was having the same issue then by luck replaced the faulty one. Haven’t experienced that type of hum from a tube before… must’ve been leaking heater current. I’ll tinker with the amp the next couple of days to be sure

[tubeswell]

I’m embarrassed to say, I think the issue was a faulty preamp tube… I swapped them between sockets and was having the same issue then by luck replaced the faulty one. Haven’t experienced that type of hum from a tube before… must’ve been leaking heater current. I’ll tinker with the amp the next couple of days to be sure

With any tube amp hum, rolling the tubes is a first easy-to-solve step. Always carry spares for this reason. It's not that uncommon.