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

[Helmholtz]

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

Another excellent article by MEF member trobbins .

[vintagekiki]

I'm afraid that some terms have been misunderstood ...

At amplifiers can appear two types humming.
- Humming as a consequence of heating cathode filament at tubes with AC voltage.
- Humming as a consequence of the ground loop or earth loop.

https://www.soundonsound.com/sound-advice/q-what-ground-earth-loop
https://www.epanorama.net/documents/groundloop/basics.html
https://www.epanorama.net/documents/groundloop/
https://www.analogictips.com/faq-ground-rules-earth-chassis-signal-ground-faq/
https://sound-au.com/earthing.htm

1)
https://music-electronics-forum.com/forum/amplification/guitar-amps/45595-best-way-to-reduce-hum

[The Jonald]

Minor update: I moved the heater center tap to its own ground and the amp got a little quieter. It's still humming, but better.

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.

Grounding that junction changes the character of the hum. It seems to drop in frequency, or lose treble and gain bass. It becomes, I think, somewhat less offensive, but I'm not sure about the absolute volume. What do you think it means?

When you say he introduces ground loops by grounding to the chassis in multiple places, are you suggesting to use star grounding, with only one chassis ground? It seems like a lot of work and very tricky to get right for little benefit. I have ignored star grounding on all my amps, from 5 to 100 watts, and simply grounded things in logical nearby places, following a few rules such as grounding the HV center tap and the first filter cap in the same place and separate from anything else, and keeping the preamp and power amp grounds separate. I've never had issues with noise from doing things this way.

[The Jonald]

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.

My scope is isolated and dual-channel. I think mostly it works, just has a loose connection somewhere. If I can get it sorted, what would you suggest looking at?

I'm getting very tempted to build a Carlson Super Probe to help with things like this.

[vintagekiki]

Best Way to Reduce Hum?

Bias a heater supply with humdinger? - diyAudio

Where to place the humdinger pot?

[Helmholtz]

If I can get it sorted, what would you suggest looking at?

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

Please post pics with scope settings.

[Dave H]

Grounding that junction changes the character of the hum. It seems to drop in frequency, or lose treble and gain bass. It becomes, I think, somewhat less offensive, but I'm not sure about the absolute volume. What do you think it means?

It sounds like you've connected it to a different noisy "ground" Did you connect it to chassis? Try connecting it to the bottom of the tail resistor on the pcb, that's R1 47k on sheet 2.

[vintagekiki]

Before you start modifying your AC30, get to know it well, because development engineers have done it much better before you.
Annoying buzzing, humming, whistling, whistling, etc. which occurs in worn-out amplifiers and even in AC30, is mainly caused by the aging of the tubes and +HV elco.

Rule No 1.
Do not modify anything related to shielding and grounding, so as not to pluck your hair due to the unwanted buzz of the grounding loop.
The factory did it better than you.

Rule No 2.
Do not modify anything related to grounding at input jacks, potentiometers and vib/trem switches (that long wire)
There are 3 grounding points on the preamp chassis. OT ground, AC power cable ground and ground at input preamp tube V1.

Rule No. 3.
When you replace tubes in AC30, replace all tubes with new tubes (not like new ones) because that way you will get an amplifier that will serve you faithfully for several years.
The EL84s is assumed to be matched (for operating voltage) and to be "heavy" (intend for higher voltage, not for receivers)

Reduce hum in preamp tubes
In terms of eliminating hum due to preamp tubes heating (6.3VAC), the AC30 evolved between 1959 and 2004.
There are basically 4 ways to heater preamp tubes

1. AC301960-67
AC heating, twisted wires, screw ground at input preamp tubes for normal / bright ch


AC301960-67 gnd
https://thevoxac30guide.com//chassis_final.jpg ¹⁾

2. AC301978, 86, 89
Heating AC, twisted wire, with hum cancel preset pot 500 ohm.


AC301978, 86, 89 RM hum
http://www.voxshowroom.com/ct/amp/uk/1982_RM_AC30/chassis_preamp2_12.jpg ¹⁾


AC301978, 86, 89 DA hum
http://www.voxshowroom.com/uk/amp/ac30_dallas_pcb_hood.html ¹⁾

3. AC301990, 92

Heating preamp tubes with DC voltage, heating EL84 AC


AC301990, 92 DC voltage
https://i.imgur.com/BD32juI.jpg ¹⁾

AC301990, 92 DC voltage
http://www.voxshowroom.com/ct/amp/uk/rm_sac-30/Issue1_%20PCB_green_8.jpg ¹⁾

4. AC301993

Heating AC, mains transformer on the heating coil has a middle terminal (2x3.15V) which is grounded.


You can make modifications only on amplifier in mint condition (100% correct).

[Dave H]

When you say he introduces ground loops by grounding to the chassis in multiple places, are you suggesting to use star grounding, with only one chassis ground?

He has sliced up the pcb ground bus in that video so may not have any ground loops with his two chassis connection points but I would have preferred to keep the bus intact on the pcb and then connect it to chassis at one and only one point.

One thing I don't like the look of is the right hand chassis ground. The PT B+ winding CT is connected to chassis there then a wire is taken from that chassis point to the pcb. The PT CT wire should really go directly to the reservoir cap -ve terminal. It shouldn't be stopping off at the chassis on its way there as doing so risks injecting rectifier current pulses into the amp's grounding system especially with the amp ground also having a chassis connection at the input end of the pcb. I would keep the green wire from the RH chassis connection to the pcb but disconnect the PT CT wire from chassis and connect it directly to the reservoir cap -ve terminal.

[Helmholtz]

Scope pics would tell us if it's ripple or heater hum.

[echoplexi1974]

I too am having issues with 120hz hum on my 1993 Vox AC30 tb /6. I used the Spectroid app (frequency analyzer) on my phone to determine the hum frequency. I not sure how accurate that method is…

I also watched the Psionic video (5) AC30 Comparisons - TB/TBX Hum Fix - YouTube I could not find the trace he was talking about (from pcb ground to the body/mounting of the brilliant channel pot). The only way the body of the pot is grounded is when it is physically touching the chassis when the mounting nut is installed. So his video confused me and I did not follow through with any of it.

[echoplexi1974]

and connect it directly to the reservoir cap -ve terminal.

Is this an actual terminal on the pcb board?

[vintagekiki]

Nothing personal.

You can't compare 2 conceptually similar amplifiers because their layouts are different, so and wiring is different. When designing an amplifier, schematic is technical, wiring is technological part of amplifier conditioned by choice and arrangement of the components.

Any unprofessional intervention (mod proudly echoes) upsets balance of projected wiring and grounding.
In each factory, team of engineers project amplifier, and they certainly have a better view on the end product, from semi-informed musicians who are inspired and educated through various 10-minute video clips.
After all, if everything was as described in the clips, you wouldn't be looking for advice here. A clip would suffice as it would give an answer to all your questions.

Tip
If you are a musician, do what you are successful at and what you know (play) because you don't know electronics.
Sorry for the honesty.

[The Jonald]

It sounds like you've connected it to a different noisy "ground" Did you connect it to chassis? Try connecting it to the bottom of the tail resistor on the pcb, that's R1 47k on sheet 2.

Okay, I feel stupid, but the cable I was using to jumper that spot to chassis earlier was picking up interference and I didn't realize it until trying to jumper to R1 today. So now, with that eliminated, connecting R5/R6/C3 to either chassis or R1 makes the amp virtually silent. There's only a very faint low-level hum coming in from the speakers that's drowned out by the physical hum of the transformers.

[vintagekiki]

That means you fixed your AC30.
Congratulations