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

[Helmholtz]

I think it only means that the hum is not coming from the power amp.

[The Jonald]

I think it only means that the hum is not coming from the power amp.

Precisely. I originally posted mostly to see the response about the power transformer theory. It seems the consensus here is that the power amp would be humming too, despite the push-pull cancellation, and it's probably not the PT. So I'm looking at a problem in the preamp somewhere. Pulling all tubes except the PI does nothing, so I would expect perhaps a problem in the vicinity of the PI, but I can't find anything there so far. The other thing I've been looking for is a ground problem, either a loop or a missing connection somewhere, but I've been looking at this thing quite a bit and I can't find it. The video I've been following seemed straightforward, especially after inspecting the board. Perhaps I've missed something, or perhaps there's another problem I'm not seeing.

I have a few things to try. I haven't elevated the heaters yet, so I'm going to do that, and as per the suggestions here and in other threads in the archives, I'm going to make it a humdinger to see if that helps. I know that this would be an excellent time to have a scope so I'm going to try and get that working too, and also give the filter capacitors and power supply another look. If I find anything useful I'll report back.

[vintagekiki]

No need to report. Now you know, what you don't know.
Do this what you know. If you are musician, play (drums, guitar ... ...) or accept real books. A little knowledge will not hurt

[echoplexi1974]

No need to report. Now you know, what you don't know.
Do this what you know. If you are musician, play (drums, guitar ... ...) or accept real books. A little knowledge will not hurt

Yes please do report! I'm new to the forum and I must admit vintagekiki, you have added nothing but confusion to this thread! It's really annoying. Please stop.

I am having a similar issue with my Vox AC30 and I look forward to the fantastic replies from the members of this forum who are trying to help with good troubleshooting advice.

Please proceed!

[vintagekiki]

The only way the body of the pot is grounded is when it is physically touching the chassis when the mounting nut is installed.

Mounting nut can and it doesn't have to. Body of the pot is grounded over its specific construction.

https://www.youtube.com/watch?v=jUE9qeoMSeM

[The Jonald]

I would scope both PI grids for hum signals (AC coupling).

Please post pics with scope settings.

I'm an oscilloscope novice and haven't used it to check for hum like this. So my apologies, but how do I go about getting it to trigger? Do I need to set up line triggering and is that why you said a dual channel scope was needed?

[The Jonald]

I took a pic of the scope hooked up to the speaker terminal. 5 milliseconds per div, 5 millivolts per div with a 10x probe. It looks like the big spike is close to the 16.7 milliseconds peak-to-peak for 60 cycle hum.

[Dave H]

That looks like power supply ripple to me but it should be at 120Hz unless one side of your rectifier tube isn't working. Scope the reservoir capacitor using a x10 probe AC coupled. Fuse F2 is an easy place to connect to. Those big (rectifier?) spikes will probably make it sound more like a hard buzz than a smooth hum.

[Helmholtz]

Looks like typical heater buzz, caused by heater-cathode capacitance (or capactive coupling between heater and grid wiring). See Merlin's book pp 75/76, where he explains the reason for the spikes.
The capacitive coupling differentiates (in mathematical sense) the distorted heater voltage and sharp steps in heater voltage show as spikes in the noise.

We've seen this waveform here before and the most effective countermeasure was a humdinger.

[Helmholtz]

Do I need to set up line triggering and is that why you said a dual channel scope was needed?

Yes, line triggering.

I recommended a dual channel scope because it allows to see both PI grids' signals simultaneously.

The PI is where the channels are mixed. A speciality of the AC30 is that the vibrato channel feeds the inverting input.
A LTPI is a differential amplifier and when the (hum) signals at both inputs/grids are equal and in-phase, the outputs at both plates should be zero.

[vintagekiki]

... the most effective countermeasure was a humdinger.

No need, because mains transformer on the heating coil has a middle terminal (2x3.15V) which is grounded.

Look post #38 (4. AC301993)

[Helmholtz]

No need, because mains transformer on the heating coil has a middle terminal (2x3.15V) which is grounded.

Look post #38

Wrong. A humdinger is always superior to center grounding.

[Dave H]

Jonald, how was the amplifier set up for that scope shot? Were the pre-amp tubes in or was it only the PI tube and where were the controls set?

[vintagekiki]

What wrong. Be specific in your response

[Helmholtz]

What wrong. Be specific in your response

Both, heater CT grounding as well as equal value balancing resistors result in heater voltage being balanced/symmetrical wrt ground (3.15V - 0 - 3.15V).
A humdinger allows for symmetrical as well as asymmetrical ground reference, which often gives better hum compensation results.
So a humdinger is simply more versatile and often much more effective than simple balancing.
Of course the heater CT must be disconnected from ground when using a humdinger.

For further info see Merlin's book and the article by trobbins I linked somewhere above.