Problem - 100hz Hum in Vox AC30 CC2X

[whoops]

Hi everyone,I have at the shop a Vox AC30 CC2X (Reissue), the amp works fine and sounds good. All tubes measure fine in my tube tester.

The problem is the amp has a 100Hz hum coming from the speakers (it's now transformer physical buzzing)The 100Hz Hum is independent of the Volume controls, meaning it's always the same being all the volumes down, or all the volumes at maximum, it doesn't change.

Schematic is here in case anyone wants to have a look:
https://usermanual.wiki/Collections/magnet/files/9365%20Audio%20Schematics%20and%20Service%20Manuals/Vox%20AC30CC2,%20AC30CC2X%20Service%20Manual.pdf.html


I'm in Europe so 100Hz is double my mains frequency.The Filter caps look pretty good, and the amp is not that old, I'm not saying it can't be bad filter caps but I think that's highly improbably.I didn't change any grounding on the amp (all original grounding)The amp uses a GZ34 rectifier tube

What do you guys think the problem might be or what could it be related to?
Thank you so much

[whoops]

Any help?
Thanks

[sluckey]

Filter caps have to be at the top of the suspect list. Looks mean nothing.

[Latole]

Filter caps, filter caps and filter caps is often the issue.
Or broken printed circuit in filter cap circuit
One or more output tube is bad, tubes set can't  work properly in push pull.

Tubes tester ( wich model ? ) is unreliable to test noisy tube.
Unreliable to test power tubes unless they are dead.

[whoops]

Thank you so much Sluckey and Latole, it's very helpful of you.
So I quickly tested the Filter caps by wiring some New and known good caps in Parallel with the existing ones in the circuit (same capacitance and voltage).The noise stays exactly the same.
I don't have stock of good EL84 tubes at the moment to test in the amp, but I asked a friend to spare me some EL84 so I could test by swapping output tubes one by one. I will have that soon to test.
Yes for sure no Tube tester tests tubes for noise. I tested them in the Orange Tube Tester which gives a quick indication if the tube in good, worn or fail, and also gives a number to be used in matching. Off course the Tube passing the test on the Tube tester doesn't mean they cant have other problems like noise.That will be my next step.
Could it be also the Rectifier Tube?
Thanks

[Latole]

Unbalanced( matched output) tube may make hum.
Balanced /matched output tube work as hum cancelling.

Putting one by one unknow power tube is wast of time.

Best test is to remove one by one preamp tube and listen the amp each time.
when the hum disappears, the issue is from the last tube you remove or the circuit before.

if this doesn't work, I can't help you anymore. I would have to do the tests myself because it's all in the details that can escape a non- amp technical person

[whoops]

Hi,
I got some new output tubes to try out, and replaced the output tubes one by one and found the problem, it was one of the output EL84 tubes that was making the hum.
Replaced that one with a new one that I selected to be matched to the other 3 good ones (tubes were already matched before) and all is good now.


Problem solved


Thanks

[Latole]

Good news, congrat !