Vox AC30hw60 Hand wired Combo Ground Loop issue

[Helmholtz]

I'm not convinced that the brackets are needed for stability.
Neither of my 2 old AC30s has them.
The lower horizontal chassis with the transformers is from sturdy steel supported by the wooden board, while the vertical part is thin aluminum and flexes easily.

It seems the transformers are mounted with their magnetic fields in parallel, so some direct hum coupling from PT into OT is possible.
This would result in some speaker hum even with power tubes pulled.

I already advised to check proper grounding of PT parts using an Ohmmeter.
Also suggested a ground loop test using a (short, heavy gauge) grounding wire from the PT to the aluminum chassis part.

[Chuck H]

I'm not convinced that the brackets are needed for stability.
Neither of my 2 old AC30s has them.

Great observations. The only caveat I have would be if the new chassis are of a thinner gauge. One guy I worked with had an amp line with slightly bigger than average transformers and slightly thinner than average chassis. There were several reports of transformer mounting bolts tearing through the chassis after some hard road use. And this was no small player in the market.

As long as the amp isn't being treated carelessly rough I don't imagine removing the brackets would be a problem.

I already advised to check proper grounding of PT parts using an Ohmmeter.

Yes you did. I was just backing your suggestion since there was no report. And I'll repeat that the way the PT is grounded now is weird. With the bare chassis area under a bolt insulated by a nylon skirt washer. I don't understand how that design went into manufacture. Even if the PT does show continuity to ground it could be incidental and I think this should be changed to something more sure.

EDIT: I think the best test points for checking transformer to earth continuity would be between the actual bell end and the AC safety ground connection to the chassis.

[django420]

Update:
- I removed the support brackets and tested PT housing/bell for grounding = still connected to ground without the brackets.
- Isolation between the brackets and chassi with poly/nylon washers = still a bit hum, but a bit less than before.
- Removed the brackets completely = lowest hum of all test configurations.
- I removed the nylon washer as pointed out by Chuck H = made no difference really, but now ground connection would be more "safe" in theory I guess, since isolation is gone.
- The two chassis (painted and unpainted) has a solid connection to each other when testing for connection between the two, thanks to the construction.
- The audible hum/buzz from the PT itself = got less when tightening it down really really hard, but far from gone.

Without the support brackets the hum is at a level where I can record with the amp. BUT, there's still a low hum, and I noticed it get's amplified when turning up the Bass control. This is true for both the Normal and Brilliant channel, even though Bass and Treble should only affect the Brilliance channel... On the other side there's bleed between the channels anyway... Not sure if this is just what it is for a AC30, or another issue..

Overall really concerning issues since the amp had an MSRP at $5000. I got it for $2500 new, so guess that makes this more acceptable, but still... Come on VOX...

Anyhow the amp sounds absolutely amazing, and I wish the hum issue were none, which would make it my go to amp for recording.

[django420]

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

I also did this trick to start with, which helped a lot. But seems there's still an amplification of the hum when turning up Bass-control.

[django420]

Adding another note:
Hum is present in speakers as soon as the amp turns on. When inrush current limiter period is over the hum gets a bit louder in speaker.

Clues?

[Helmholtz]

Adding another note:
Hum is present in speakers as soon as the amp turns on. When inrush current limiter period is over the hum gets a bit louder in speaker.
Clues?

What inrush limiter?
Did you try the hum test with power tubes pulled?

[django420]

What inrush limiter?
Did you try the hum test with power tubes pulled?

Since many modern amps doesn't have a standby switch anymore (due to EU regulations I think), manufactures put inrush current limiters on the mains input to protect the tubes from getting current directly when powering on the amp. As I understand it.

Haven't pulled the power tubes, but tested with other known good set of power tubes. Will try to pull them tomorrow when I'm back in the studio.

[Helmholtz]

Since many modern amps doesn't have a standby switch anymore (due to EU regulations I think), manufactures put inrush current limiters on the mains input to protect the tubes from getting current directly when powering on the amp. As I understand it.

If the amp uses a GZ34, this will act as an inrush limiter because of its delayed heat-up time.
Do you have a schematic?
Does the amp have a heater balance pot (humdinger)?

[Chuck H]

Do chase Helmholtz path of determining balance on the output halves. But just to add a note...

There have been a few "handwired" reissue amps from major manufacturers for vintage models that have had lead dress and/or ground scheme that was less than ideal. The reality is that the people in charge now don't have the same understanding of these matters as those who made the originals. In the golden age when matters like this came up they were addressed on following batches until it was fixed. Hand wired amps aren't like PCB amps in a few regards and issues with lead dress and ground scheme probably aren't as quick to be addressed or even recognized as they were in the golden age for these "modern" "hand wired" reissues.

JM2C

EDIT: Also see post 16 where Helmholtz called out the transformer orientation. You could pull the tubes and see if this small amount of hum remains. If it does then there's nothing to be done circuit wise to correct it.