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 Post subject: Vox AC30/6 TB hum/buzz
PostPosted: Fri May 05, 2017 4:12 am 
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Hello,

I think I have a fairly common issue, but I still need help. I posted in a Vintage Amp forum already, but have received no responses... figured someone here might have some insight.

I've got a buzz / hum on my 2003 Vox AC30/6 TB. I only use the Normal channel, and it buzzes with the volume at zero and nothing plugged in. It goes away when I turn it half way up, and it comes back louder when I turn it all the way up.

Here's what I've done:

I replaced all the EL84 power tubes with a matched set of JJ's - No change.
I swapped the 12AX7's around - No change.
I pulled all but the two 12AX7's needed for the normal channel - No change.
I changed out the power cord - No change.
Then I attempted to hunt for the "lead dress" issue - No result.

Video - https://youtu.be/6RyKUr_KIwI

Are there any other easy to resolve issues I can focus on? If there are any threads or repair videos someone can point me to, that would be great. I have searched, but most of the results I found say to do what I already did.

Thanks.

And just for fun, since I recently dusted it all off... here is a pic my old rig.

Attachment:
Pedal Board 2017 IMG_8796.jpg
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 Post subject: Re: Vox AC30/6 TB hum/buzz
PostPosted: Fri May 05, 2017 4:57 am 

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To get to the bottom of this, if I were you I would send Nigel Knight at Kat an email explaining the problem, I think with Amps and other electronic equipment, you really need to seek professional help, there is every chance you could kill the amp (and yourself) if your not sure what you are doing.


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 Post subject: Re: Vox AC30/6 TB hum/buzz
PostPosted: Fri May 05, 2017 4:58 am 

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Nice rig by the way


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 Post subject: Re: Vox AC30/6 TB hum/buzz
PostPosted: Fri May 05, 2017 5:21 am 
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Luke Holwerda wrote:
Hello,

I think I have a fairly common issue, but I still need help. I posted in a Vintage Amp forum already, but have received no responses... figured someone here might have some insight.

I've got a buzz / hum on my 2003 Vox AC30/6 TB. I only use the Normal channel, and it buzzes with the volume at zero and nothing plugged in. It goes away when I turn it half way up, and it comes back louder when I turn it all the way up.

Hi Luke.

I don't have that much experience with Vox AC30s. As you probably know I have a vintage copper top. I got a bit obssessed with them at one point and bought all the literature, viz:

- The Service Engineer's Guide: http://www.thevoxac30guide.com/
- The Vox Story: A Complete History of the Legend (Guitar History), 1991 David Petersen
- Vox Amplifiers: The JMI Years by Jim Elyea (Deluxe Edition with schematics)

So if you ever need some info from these, give me a shout, but it has been a few years since my obsession quelled.

Mine hums/buzzes a bit but nothing more than I'd expect with a vintage high power amplifier. The only issues I've ever had with this one or its grey top predecessor was popping and crackling. I believe that's generally an indicator of degrading pre-amp valves.

All AC30s hum, you've probably Googled it just as I have and found the same answers. If you have swapped out all those parts, then there can't be much left. If you feel that it is excessive, you should take it to a recognised amplifier technician for diagnosis.

I think the best and clearest explanation I saw on the internet is given here:

http://tone-lizard.com/vox-myths/

"Part of the reason is the output transformer is not a true Class A transformer (i.e. one which is perfectly balanced and rated for a 100% duty-cycle). Any imbalance in the transformer primary [winding causes] unbalanced DC through the transformer core... Seconnd order harmonic distortion (and therefore more THD), and the common mode (hum, noise) rejection benefits are reduced. Add to this the 16 microF filter capacitors found in AC30 amplifiers, and you end up with hum in AC30 amplifiers."

So it seems to be primarily related to the quality of the transformers. I hope this helps a bit.

Lastly, your old mate Greg Fryer has probably forgotten more than anyone except Vox Engineers have ever known about Vox AC30s. Perhaps you could drop him a line and post up his response? ;)

Doug


Last edited by Doug Short on Fri May 05, 2017 7:07 am, edited 1 time in total.


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 Post subject: Re: Vox AC30/6 TB hum/buzz
PostPosted: Fri May 05, 2017 5:55 am 
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Thanks Peter. And good call.

Doug.. I was JUST today considering which body parts I could do without to afford an old copper and fawn AC30. Color me jealous.

All good information as ever. I was resolved to think it was either just a trait of the amp sitting, or a bad transformer, because, as you said, there isn't much left to change. As to your last point, I'll give it a go. Thanks!


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