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 .

[django420]

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

Thanks for this info! I'm having the exact same issue with my AC30HW60 and this solution helped out a lot. There's still a a bit of hum though, so I did some more troubleshooting... After removing the support brackets for the mains transformer the amp got as quiet as a tube amp possibly can be. I'm not sure if this would be an electrical safety issue though? Is the power transformer supposed to have a grounded chassi like that? Or is the brackets just for weight support?

[g1]

After removing the support brackets for the mains transformer the amp got as quiet as a tube amp possibly can be. I'm not sure if this would be an electrical safety issue though? Is the power transformer supposed to have a grounded chassi like that? Or is the brackets just for weight support?

Not sure how it would be mounted without being grounded, so are these some extra brackets? Or are you talking about insulating it from chassis?
Pictures would definitely help.

[django420]

Not sure how it would be mounted without being grounded, so are these some extra brackets? Or are you talking about insulating it from chassis?
Pictures would definitely help.

Here's photos with supporting brackets mounted and unmounted to the chassi. Obviously the support brackets connect to ground/chassi. When detached, hum/ground loop is completely gone. But is it safe?

[Chuck H]

Well the proof is in the result. If the ground loop is gone with the brackets lifted from the front chassis then there must be something isolating the power transformer and those brackets were grounding it. It does look like there are plasic washers on the corner screws. If the brackets are needed for support then you could retrofit some kind of isolation and reinstall them. But is it safe to not ground the transformer bell and lams? Probably not. In the event of a short could there be high voltage accessible by a hand on the transformer? Not sure. I do know there are amps that isolate the output transformer mounts which would seem to be of equal concern.

The real question is why grounding the PT chassis parts is causing ground loops? Is anything grounded to the PT mounting screws inside the chassis?

[Helmholtz]

To be clear, a mains transformer MUST be grounded for safety reasons. Check using an Ohmmeter.
Are you sure the hum is coming from the speakers?
There's a possibility that the brackets just transfer PT vibrations to the front chassis, which then radiates the noise.
That would mean that no ground loop is involved and you might restore transformer ground using a wire.

[Chuck H]

There's a possibility that the brackets just transfer PT vibrations to the front chassis, which then radiates the noise.
That would mean that no ground loop is involved and you might restore transformer ground using a wire.

I had thought the same thing but didn't itterate it to keep to one subject in the post. Likewise I think if it's not mechanical then the ground scheme could be examined to determine what is grounded where and affect some minor change to the scheme that should eliminate a ground loop while keeping the PT grounded.

[g1]

I think you will definitely need those brackets for supporting the weight. The picture from Vox Showroom attached shows the whole thing, both the PT and the OT need the brackets.
The grounding should be through the transformer mounts to the black chassis, which should mechanically ground to the main chassis. I wonder if maybe the 2 chassis are not connecting together well. I think if you added a ground strap between the 2 chassis, that should take care of the safety ground. Then you could insulate the extra support brackets. This is assuming the black paint isn't insulating the normal part of the transformer mount.

[django420]

The real question is why grounding the PT chassis parts is causing ground loops? Is anything grounded to the PT mounting screws inside the chassis?

I took a look inside the chassi and the paint is missing around one of the mounting points. See attached photo.

To be clear, a mains transformer MUST be grounded for safety reasons. Check using an Ohmmeter.
Are you sure the hum is coming from the speakers?
There's a possibility that the brackets just transfer PT vibrations to the front chassis, which then radiates the noise.
That would mean that no ground loop is involved and you might restore transformer ground using a wire.

Yes there's hum coming from the speakers. The fundamental is 50Hz. But there's also a "vibration"/buzz/hum from the PT which is audible directly when putting my ear close to the PT. This is also true for the OT. It is likely the vibration radiates to the chassi via the support brackets and get's amplified somewhere down the road..

I think you will definitely need those brackets for supporting the weight. The picture from Vox Showroom attached shows the whole thing, both the PT and the OT need the brackets.
The grounding should be through the transformer mounts to the black chassis, which should mechanically ground to the main chassis. I wonder if maybe the 2 chassis are not connecting together well. I think if you added a ground strap between the 2 chassis, that should take care of the safety ground. Then you could insulate the extra support brackets. This is assuming the black paint isn't insulating the normal part of the transformer mount.

Yes the support brackets are needed for the weight. Check new photo of the back of the "black" chassi, where the paint is removed at one of the mounting points.

[Chuck H]

Ok on the missing paint for the mounting bolt area. This would be SOP for painted chassis. BUT... The mounting bolts are slipped through insulation washers. Just what the hell is going on here? Does Vox want this transformer grounded or not? To both chassis or not? They don't seem to know.

I think it's possible the insulation washers are there to mitigate vibration. But Vox can't count on the transformer body being grounded through lamination contact even if they had a no paint area under that surface (which they don't seem to) because the laminates themselves are coated. At face value a lot of this doesn't make sense. It seems like a couple of different problems were being solved for and things got confused in the process.

Notice that the unpainted bolt connection is not one of the bolts with the added brackets. I assume the two chassis are screwed together right through the walls but I don't know. That would make a connection but isn't really reliable. You need to test to see that both chassis are connected to safety ground (earth) and that the transformer body shares continuity with that connection. After all this is certain...

I think it's possible that the two chassis ARE connected somehow at the input end of the chassis but not elsewhere unless those extra support brackets are connected. Hence the ground loop. I've never experienced much ground looping via chassis connections before. Probably because they are a plane area and of relatively low resistance compared to lead wire. So a ground loop through a transformer mount is a new one on me.

To be honest I don't trust the way vox has that PT grounded. And perhaps even the two chassis connected. I would remove the insulation washer from (at least) the bolt with the unpainted contact and make sure there's a secure connection between the two chassis at the input end of the amp. Then, with the brackets still disconnected... As long as the two chassis and the PT body show continuity with safety ground you're good to go. You can then isolate the brackets as g1 suggested and hopefully solve your problem.