Is there supposed to be 120Hz hum, used 2018 Vox AC30S1 OneTwelve?

I am new to tube based guitar amps. I do have hifi tube amps, preamps, and headphone amps.
This Vox, a 2018 AC30S1 is new to me, but came used from online retailer/GC.

Anyways, I have a little bit of hum on the speaker, and according to my scope it is 120Hz.
1692048424058.png

I have 2 questions, as I am used to hifi tube amps being dead mostly quiet.
  1. Is this part of the design of the amp?
  2. Given a no/maybe to question #1, what should I troubleshoot?

Things I have checked/tried:

  1. Tightened all transformer mounts and chassis grounding nuts. Input nut, switch nuts. (I didn't check all the pot nuts. Whoops)
  2. Pull the 2 preamp tubes (2 x 12ax7)
    1. one at a time (no audible change)
    2. both at once (no audible change)
  3. Pull all 4 el84s (hum is GONE)
  4. Pull 1 el84 (hum is LESS)
  5. Pull 1 pair of el84s (no audible change)
  6. Tapped on the circuit board while hooked up to the scope (looking for dodgy connections); this didn't seem to have any effect on the scope output.
  7. Changes in the Gain/Volume/Bass/Treble/Reverb controls do not affect the loudness of the hum.
I assume the full set of tubes are the original tubes. All read "China" no brand name.
What is the diagnosis? Is this normal for the amp design?

Not a lot of AC30s floating around last time I was at the local shop, so I don't have a lot of mental comparison points.
What else should I try before pulling boards/reflowing solder etc?
 
Its a tube musical instrument amp.
So yes after 40 or 60 ish years.
None have figured out to reduce noise.

Assume AC heaters with no center tap
transformer. So common virtual tap
can be added. Or like old Fenders
and others made adjustable for
" hum balance"

Input cord is plugged in
So that would pick up noise
specially a guitar on the other end.

Depending how the input jacks
and outputs are wired.
The amazing chassis ground
is just a huge noise loop.
Obviously we need chassis ground.
But how inputs and outputs are done
usually makes a huge loop.

likely ribbons and flying leads everywhere
which are conveniently arranged to pick up
noise

if tube rec, will have minimal capacitor
with likely no inductor to improve the supply.
No rec tube, will still have dinky caps
 
Some pre disassemble photos of the AC30S1.

The scope pictures include the B+ at the output to the output transformer. And the second scope picture is measured at pin 9 of the first EL84 power tube.

That's some zig zaggy dc being sent to the output transformer.
 

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Pic 4 looks like 120Hz power supply ripple.
To better view the ripple set your scope to AC coupling.

What is the time base setting?
Also use the scope's frequency measuring function.

Are your power tubes fresh and well matched?

Also find and post the appropriate schematic here, so we know what we're talking about.
 
Ok. Before and after shots.

I temporarily added an extra 39uF between the output transformer B+ line and the star ground on the chassis.

The blue is the b+.
The yellow is the output at the speaker.
Seems like the dirty power is related.
 

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@Rikaro I can't find a schematic for this specific model. Vox AC30S1. Thanks for the tip about AC coupling. Never knew what that setting was for.

Tubes are most likely as old as the amp. I bought used, date on transformer is 2018. Time base setting? Is that the number in ms at the top of photo?

Post adding extra cap.
 

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Last edited:
@Rikaro I can't find a schematic for this specific model. Vox AC30S1. Thanks for the tip about AC coupling. Never knew what that setting was for.

Tubes are most likely as old as the amp. I bought used, date on transformer is 2018. Time base setting? Is that the number in ms at the top of photo?

Post adding extra cap.
C'mon, that was easy: https://www.manualslib.com/manual/1568461/Vox-Amplification-Ltd-Ac30s1.html?page=6#manual

AC coupling adds a coupling cap before the channel input, blocking DCV.
This allows you to only view the AC content as well as center it and freely vary the amplitude by changing the channel sensitivity.

Time base setting is a (m)s/div value which is adjusted via the "Horizontal" knob.
It allows to read the signal frequency from the trace.
Consult the scope manual to display it.

Your result with added filter capacitance may indicate that some original cap has partly lost capacitance.
 
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Not sure what you mean with "check the signal output before the transformer".

I generally do not recommend to scope power tube plates (pin7) or the OT primary.
Voltages there can exceed the voltage ratings of scope/probes.
Also sometimes causes oscillation.

It is normal to have a 120Hz ripple of a few volts a B+1 and maybe ~1V at B+2 (you can measure ripple voltage on the power supply with a good DMM).
Don't make filter caps much larger than original as this would change the amp's dynamics.
 
My scope and probe leads are rated 600v. Voltage in this amp is half that, should I be worried?
At B+ 1,(blue) I have scope set to ~26 volt/division. (below, and post # 8)
Across the speaker (yellow), there is roughly 60 mV ripple at 120Hz. (See post 4)
Measuring the B+ 2, (red) shows no ripple, just clean DC.

I didn't check the signal output (orange) from the power amp board. If there is no ripple there, I can narrow in on the 2 caps circled in (purple, c96, c100).

1692637860989.png
 
My scope and probe leads are rated 600v. Voltage in this amp is half that, should I be worried?
Looked up your scope's manual. Says channel input must be below 300V DC or AC RMS. Peak voltage up to 450V max.
http://www.hantek.com/Product/5000P/DSO5000P_Manual.pdf
A 1x or 10x probe does not attenuate DCV in AC coupling.

At power tube plates, peak voltage will be around 600V at full output and even more if the amp is oscillating.

Sorry, I don't understand how to interpret your scope's readouts (much different from my Siglent scope).
Seems you're still using DC coupling.

If B+2 is clean, the ripple will only affect the power stage.
As said earlier, first make sure power tubes are matched.
The EL84s in an AC30 run very hot and don't last long (maybe 2 years depending on operating hours).
 
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These designs are ancient - there is always some hum. Not sure if original Vox tubes are noname China ... don't think so, but I didn't open my AC30 top, so not sure.

Btw you can "jump" the inputs of these amps and use both channels like the old Marshalls. Adds a few cool sounds for these amps.
 
Here is an update for posterity, and future AC30S1 owners.

tldr; Given the info provided by this forum and my experiments, I believe the "hum" I am experiencing is part of the design of this amp.

I never got around to testing the tube set that came in the used (new to me) Vox amp.
However, I did change the tubes out for a new matched set. I went with a set of JJ's from VivaTubes.
I also recorded 3 sound sample of the old set, the new set, and a Spark amp's modeled version of an AC30.
I maintained the controls on the amp and my mic pre for all three recording. The new tubes definitely have more gain. In hind sight, I probably should of decibel matched a known signal for the audio to mean anything. But alas. I think my conclusion would be the same.

Microphone Setup:
2024-03-02 13.01.31.jpg


Knobs:
2024-03-02 13.01.39.jpg


Old Tubes:
2024-03-02 13.25.07.jpg


New Tubes:
2024-03-02 13.26.00.jpg


If you are interested in the signal chain for recording the attached audio samples:
Donner DC20 Condenser Mic => Focusrite Scarlet Solo => Ableton Live
 

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