After 9 months of trying to resolve the hum in this awesome Vox V125 amp, I discovered it was as simple (and free) as this... 'one weird trick'. 😂

Most amp hum diagnosis checklists tell you to check and usually replace the tubes first. The argument is that tubes commonly fail and that they are cheap to replace. Sure, they are a major point of failure in a tube circuit, but nothing is cheaper than tightening your power transformer bolts. 😄 It's at least worth a shot.

The reason for the hum in this case is because the E core transformer (EI) is made up of laminations of thin metal plates. Since they conduct magnetism, if they are slightly loose, the magnetic flux of the transformer (oscillating at 50 or 60Hz) can cause them to vibrate. Annoyingly, this will induce electrical noise into the secondary windings and cause noise that not even the freshest, highest capacity power supply capacitors will be able to filter out.
In my case, I was able to feel the chassis physically vibrating when the amp was powered up.


I hope this will help someone else who's been hunting that hidden source of hum in a guitar amp.

All the best!
Dax.

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