Vox AC30 6TB hum problem; bad power transformer or something else?

[vintagekiki]

You said that means the power transformer must be good, why is that?

Because the power transformer does not heat up, the hum comes from the speakers, most likely due to some defect in the preamp section (V7-V11)
In what condition are the +HV electrolytes C30, C35, C35, C45, C46?
In what condition are the grounding of the main cable?

[The Jonald]

Because the power transformer does not heat up, the hum comes from the speakers, most likely due to some defect in the preamp section (V7-V11)

What do you mean? The power transformer must heat up, and specifically the high voltage must be on, for the speaker to draw current and make sound.

The filter caps look fine and the grounding is all good, as far as I can tell. I have checked everything I can think to as far as grounding.

[g1]

Pulling any or all preamp tubes has no effect, except V6. Pulling V6 removes the hum. You said that means the power transformer must be good, why is that?

If the hum is caused by the power transformer, it should still hum with only the power tubes installed (or with only V6 removed).

[vintagekiki]

I'm having an issue with a humming Vox AC30 6TB ... I've performed the modifications described in the following video ...

At AC3093, instead of one problem (humming), now appeared another problem (modifications).

In order to perform any modification, the first amplifier should be repaired and with the schematic brought amplifier to the original state (mint condition)
In order for the amplifier professionally quickly and efficiently repaired, all the tubes in it must be new (no as new), find the cause and rectify the fault.
Any modification on the faulty amplifier represents an unsuccessful elimination of the consequence of the fault, and the cause still exists.

Modification factory wiring and (or) ground wiring always bring to a ground loop problem (main frequency hum) that is difficult to find. The development engineers in the factory solved this problem on the prototype chassis by carefully searching the place where they would perform the grounding.

I'll be free to suggest the following
- First, return the AC3093 to original condition (mint condition), ie remove any ad hoc modifications.
- For repair remove all existing tubes and install only new tubes (not as new).
- Never eliminate the consequence of a fault, approach finding its cause.

What do you mean? The power transformer ... ...

It is illusory to enter into a debate about the power transformer, when you already have a preconceived opinion about its condition.

Finally advice
In advance inform yourself about the AC3093, and if you are able to borrow from your colleague musicians AC3093 that works and that is correct, and compare them with each other so as not to unnecessarily heat hot water.
Don't forget the Vox brand has existed for many, many decades and there is an army of educated development engineers in it ... ...

1)
https://www.voxac30.org.uk/

https://www.backfromthesixties.co.uk/vox-ac30

http://www.chambonino.com/work/vox/vox1.html

http://turretboard.knucklehead.dk/tag/ac306/

http://www.tedweber.com/media/kits/6v30_layout.jpg

http://www.tedweber.com/media/kits/6v30_schem.jpg

https://music-electronics-forum.com/forum/schematic-requests/41587-voicing-vox-ac-30 ²⁾

[The Jonald]

If the hum is caused by the power transformer, it should still hum with only the power tubes installed (or with only V6 removed).

Yes, the why of that is what I'm not understanding. Isn't it also possible for the noise from the PT to be coupled into only part of the circuit?

[Helmholtz]

I'd say, if the OP is convinced that the PT is the culprit, let him exchange it. After all, it's his money.
There is a slight chance, that a "better" (meaning lower stray field, better regulation) PT may reduce 60Hz (certainly not 120Hz) hum.

Curious about result.

[The Jonald]

At AC3093, instead of one problem (humming), now appeared another problem (modifications).

It is illusory to enter into a debate about the power transformer, when you already have a preconceived opinion about its condition.

I checked the amp after every modification. Before the mods, it was humming away even on a current limiter. After the mods, its condition is improved, but its still unusable on wall power. It's better, not worse, just not fully healthy. I don't think whatever problem it has remaining was caused by what I did to it. These mods were asked for by a customer, they fix known issues with this particular model, there is information about them which I have linked.

I'm trying to figure out the power transformer angle because it's a known issue with this specific model of amp--I have linked the info--and because I can hear the PT physically humming at what sounds to be the same frequency as the hum in the speaker. I think a bad rectifier could also be the culprit?

I'd really like to understand why the hum is reduced when I dime the volume on the normal or brilliant channel.

[Helmholtz]

I'd really like to understand why the hum is reduced when I dime the volume on the normal or brilliant channel.

Sounds like a (phase) compensation effect. Those AC30 are known to have more than one hum source. For analysis you'd need a dual channel scope.

[vintagekiki]

I'd really like to understand why the hum is reduced when I dime the volume on the normal or brilliant channel.

Setting VR4 and VR5 to min, you conditionally connect the signal from preamp (V8) to gnd.
The same happens when reduced the sound on the TV.

[vintagekiki]

https://www.youtube.com/watch?v=Kfoj4ft_yuY

https://psionicaudio.com/services/services_amp_mod.htm
Vox AC30 Reissue "JMI" Modifications

https://www.youtube.com/watch?v=L9hXd-ajJ9c
Part One: Factory Ground Loops

https://www.youtube.com/watch?v=j50s1KIFEWs
Part 1 : Unpacking, Inspection, & That Awful Buzz

etc ... ...

[Mick Bailey]

I'd really like to understand why the hum is reduced when I dime the volume on the normal or brilliant channel.

I'm in agreement with Helmoltz; you have more than one source of noise with a difference in phase between them. This often shows up where you get a control (especially volume) where there's a point in rotation where the noise reduces. Sometimes this can be mid point and moving the control either way increases the noise, or it can be towards one end of rotation. The control is acting to null out the noise - that is, one noise source is largely cancelled by the other that's out of phase at that particular point.

[Dave H]

It still hums with all the pre-amp tubes out except V6 and all controls at minimum. Does grounding the junction of R5,R6,C3 reduce the hum?

He eliminates several ground loops in the video but then introduces one of his own by connecting to chassis at both ends of the amp.

[The Dude]

Sounds like a (phase) compensation effect. Those AC30 are known to have more than one hum source. For analysis you'd need a dual channel scope.

THAT!

[garytoosweet]

I've just completed the same mods on the same amp and had the same issue. There was a thread on this forum about it that I started a couple weeks ago you can reference.

Humdinger was the solution.

I also suspected the PT (after ruling out everything else that I could think of) but did not try to swap it, I was able to get the amp to total silence with the humdinger pot.

[vintagekiki]

https://dalmura.com.au/static/Hum%20article.pdf
Valve Amplifier Hum