Vox AC30hw60 Hand wired Combo Ground Loop issue

[Mars Amp Repair]

Hello folks,
Just entering an issue I had with one of these limited edition Vox AC30HW60's that are British made from a few years ago that were probably around $4K when new.
The issue the customer stated was that with both volumes down (or up) if he increases the bass control, there is a hum that he felt shouldn't be there. It also changed with the treble setting. Ya gotta know anyone who pays that much for an amp is going to be a bit picky. I decided to pursue it rather than just tell him that's how it is.
If I pulled V2 the hum was gone.
Turns out that the shielded cable shield that leads to the brilliance vol pot is grounded on the turret board (& Ultimately to the star ground thru the turret board connection) and is also chassis grounded at the Brilliance channel vol pot case which of course goes thru the pot to the chassis thereby creating a ground loop.
You can unsolder the 2 shielded cables at the brilliance pot case & run a wire to the shielded cable ground at the Bass control which is connected thru another shielded cable to the turret board star ground.
It's a tube amp & of course still has some errant noise, but this countermeasure resolved the hum w/bass control turned up.
FYI for anyone who may encounter this issue.
Thanx, glen

[nosaj]

is he one of those guys who plays with volume turned down?
nosaj

[Jazz P Bass]

Nice catch Glenn.
Thanks for posting that!

[Chuck H]

Here's a neat trick I ran across accidentally. I was troubleshooting hum in a reverb circuit (notorious for ground loops) and was using an alligator clip lead from the ground point at the chassis jacks to other potential ground points in the amp. But I hadn't lifted any existing grounds yet. So creating a redundant ground that did reduce the hum almost anywhere I connected it. It wasn't reducing the hum enough, but better is still better so I relocated the ground for that circuit where the redundant ground made it quietest. And... The full hum returned??? So I tried the redundant alligator clip ground again with the same result. It took me a second to reason it out but I realized that I must be grounding a ground loop. Once I knew the hum was caused by a ground loop I was able to find it. So...

If you want to check if a ground loop might be responsible for a hum issue you can always try this redundant ground trick and see if the hum is reduced. If it is there's a good chance it's caused by a loop.

[Mars Amp Repair]

Another great troubleshooting technique which we all need on tough issues like ground loops! Years ago I cobbled together a computer monitor & a magnetic phone preamp to become a fairly sensitive signal tracer. I added a series cap to block DC & some other tonal mods to tame down the RIAA tone curve as it all comes out bassy if you don't. This silly looking device allows me to hear the ground loops in most cases, too.
I like your solution given its simplicity and have actually used that method, too. All great tools to have.
Thanx for posting Chuck H.

[Mars Amp Repair]

is he one of those guys who plays with volume turned down?
nosaj

I actually think this guy just happened to notice the issue and was wondering if it was 'Normal' given the special price he paid for it .