# gah! VOX AC30 help needed!!!



## Wired (Jul 21, 2009)

I have an early 70's AC30. I think we got it pegged as a 1973.

I have a continuing problem with it and my usual tech (who rebuilt the preamp section already) has no idea how to fix it...

The amp, once warm, will make a whump/whump/whump noise, it's not motor boats! It's more like a clicking noise. Sometimes if it's not clicking...its just oscillating and speeds up and slows randomly...

The noise happens even if the volumes are turned right off and nothing plugged in.

The other interesting thing is... no matter what background noises it's making... the amp still sounds stellar and works fine in every way other than the noises... It will sometimes even not make any background noises at all!


If anyone knows of a good AFFORDABLE VOX tech that could help me out that would be fantastic. This amp is my life blood... and I really need her healthy again.


As is, I'm incredibly close to just trading in my Gibson Les Paul to get an AC30CC just so I can have a reliable AC30...even though the tone on my vintage one is so much warmer and lively.


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## jimihendrix (Jun 27, 2009)

hey there...this thread mentions a tremolo circuit...

http://vintageamps.com/plexiboard/viewtopic.php?p=458512&sid=5e7bea24d7a5fdd3a166f3953110e3b2


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## Wired (Jul 21, 2009)

jimihendrix said:


> hey there...this thread mentions a tremolo circuit...
> 
> http://vintageamps.com/plexiboard/viewtopic.php?p=458512&sid=5e7bea24d7a5fdd3a166f3953110e3b2


yea, but thats on the CC2... this is a vintage Vox. and the trem channel is not in use, and i've even tried bypassing it using the footswitch.

The trem does work properly if I use that channel tho.


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## Ti-Ron (Mar 21, 2007)

Try with WilBill, many people here have some amp repaired by him!


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## greco (Jul 15, 2007)

I can't offer any help...certainly wish that I could.

This problem is very interesting (although, understandably, extremely frustrating for you).

I'll be following the thread...hope you can find a solution soon.

Cheers

Dave


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## Greenbacker (Mar 29, 2007)

I used to have a Korg era Vox AC30TB. There was serious bleed from the trem channel into the regular/TB channels. I had it modded by Tim who used to work at Songbird in Toronto. He is incredible and takes great pride in his work... I'm not sure who he is working with now but he is amazing. Maybe somebody else can chime in on this... He made a lot of amps sound better for a lot of years.

Anyway, he modded my amp so the trem channel was totally disengaged when I wasn't plugged into it (and I never was) or until I had a footswitch plugged in. Your amp would have a pretty similar circuit to my old one so I think the problem/fix would be pretty similar.

Let me know if I can be of any more help. Please don't buy a Custom Classic. No offence to owners out there, but they sound like crap in my opinion.


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## Greenbacker (Mar 29, 2007)

Tim Dudley! That's his name. 

Apparently he's working out of here now. Couldn't hurt to drop him a line. I bet he's heard of this before.


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## nonreverb (Sep 19, 2006)

Here's something to try...pull the preamp tubes one at a time and see if it makes a difference. If you have any other preamp tubes around replace each one with a different one. If the sound changes, it might indicate the area where the fault is.


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## Wired (Jul 21, 2009)

I did a full retube tonight, no change.

I'll try pulling them out. 

This will not damage the amp?


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## Blue Apple (Feb 8, 2006)

When plugging into the trem channel, did you notice if the noise is in sync with the tremolo?


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## Wild Bill (May 3, 2006)

Wired said:


> I did a full retube tonight, no change.
> 
> I'll try pulling them out.
> 
> This will not damage the amp?


Only pull the preamp tubes (the little ones!). If you pull the output tubes you'll likely not hear anything.:smile:

Some folks are suspecting tremolo bleedthrough. Does the noise sound like this could be it? The AC30 has a trem on/off switch that should kill the trem output but it's possible that the trem oscillator signal could be bleeding by capacitive coupling between two wires or parts running closely parallel for a ways. You did say that your tech had rebuilt the preamp. Was the problem always there or did it occur after the preamp work? If so, I'd take a plastic wand of some kind or a wooden dowel and gently push wires and parts around, listening to hear if that causes any change to the noise.

If it's not trem bleedthrough then things could be more ugly! You could have a cap that's leaking, making a 'snap' every time it leaks. You could have an arc in your output transformer. 

It's really hard to guess 'cuz frankly, you haven't given us much more than a vague description of the offending noise. If you don't think it has anything to do with the trem then give us some more detailed info, so we can make even wilder guesses!:smile:

:food-smiley-004:


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## nonreverb (Sep 19, 2006)

Wired said:


> I did a full retube tonight, no change.
> 
> I'll try pulling them out.
> 
> This will not damage the amp?


No, the amp will be fine. What your doing is breaking the signal at different points in the signal path. I have to agree with Bill as it's hard to determine with the available information. Pulling the preamp tubes will only give you the general area where the fault is.


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## Wired (Jul 21, 2009)

Blue Apple said:


> When plugging into the trem channel, did you notice if the noise is in sync with the tremolo?


it is not in sync


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## Wired (Jul 21, 2009)

pull the pair of preamp tubes farthest from the inputs on the amp, which I can only assume are the Vib/Trem channel tubes... amps been running for 1 hour, there is no noise.


I called my tech, he seems to think it's trem bleed as well... and no, the problem only started a year ago. he rebuilt the preamp 2 years ago.


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## Wired (Jul 21, 2009)

I took the following with my black berry 5 minutes after I posted that last message


http://www.youtube.com/watch?v=9bLWP_kYMRY


its come back 

I guess I spoke too soon.

However, it's gone again, and has been running for 20min without it...


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## Wired (Jul 21, 2009)

Blue Apple said:


> When plugging into the trem channel, did you notice if the noise is in sync with the tremolo?


the notes are not in sync.... 


I have put a AC30 Handwired head on order.  I need my Vox toneS! lol


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## Greenbacker (Mar 29, 2007)

Nice choice! I've got the AC15 head from that series. Really nice stuff! Seem to be reasonably priced too.

What are you going to do with your old amp?


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## jimihendrix (Jun 27, 2009)

Wired said:


> I took the following with my black berry 5 minutes after I posted that last message
> 
> 
> http://www.youtube.com/watch?v=9bLWP_kYMRY
> ...


whoa...the "helicopter" noise sounds like what this guy is experiencing too...

http://www.youtube.com/watch?v=E5y9l2yFgBA


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## Wired (Jul 21, 2009)

Greenbacker said:


> Nice choice! I've got the AC15 head from that series. Really nice stuff! Seem to be reasonably priced too.
> 
> What are you going to do with your old amp?


Get it working 


I'll never part with this amp. Unless it becomes completely unrepairable... but even then, I'd probably just find a working chassis somewhere and throw it in the cab or something.

The AC30 handwired head will be my gigging amp, and if this vintage one is not being finiky I'd use it for recording.

However... for two hours now it has not made any noises like that in my youtube video. 

So I'm confused on what to do now... I gotta get rid of my Les Paul to get the new head... and I love that guitar...until I plug it in anyways....


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## Greenbacker (Mar 29, 2007)

Ugh. That's crappy man. Nothing sucks more than getting rid of a nice axe... That's too bad.


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## bolero (Oct 11, 2006)

have you tried moving the amp somewhere else? ie: another building?

sometimes those lightswitches/dimmers & fan controllers etc can make an amp do things like that


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## Scottone (Feb 10, 2006)

bolero said:


> have you tried moving the amp somewhere else? ie: another building?
> 
> sometimes those lightswitches/dimmers & fan controllers etc can make an amp do things like that


And Blackberry's can induce noise into tube amps :smile:


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## Wired (Jul 21, 2009)

Scottone said:


> And Blackberry's can induce noise into tube amps :smile:


Any GSM phone can, and I know the diff between those pulses and what the amps doing. Besides, the amp was doing this before I got my GSM phone. (CDMA phones don't affect amps or stereos with the clicking. In the youtube vid you can hear the GSM click, but its near the end of the video. what it's doing right off the bat in the video is the issue I'm referring too. 


But yes, I have put the amp in different buildings... three in fact. My condo, parents place, and jam space. Not to mention my techs building.




> Only pull the preamp tubes (the little ones!). If you pull the output tubes you'll likely not hear anything.
> 
> Some folks are suspecting tremolo bleedthrough. Does the noise sound like this could be it? The AC30 has a trem on/off switch that should kill the trem output but it's possible that the trem oscillator signal could be bleeding by capacitive coupling between two wires or parts running closely parallel for a ways. You did say that your tech had rebuilt the preamp. Was the problem always there or did it occur after the preamp work? If so, I'd take a plastic wand of some kind or a wooden dowel and gently push wires and parts around, listening to hear if that causes any change to the noise.
> 
> ...



My local tech was thinking the same thing with the vib/trem bleed through, as was a few guys at the local L&M. But my tech thought it was that last time and couldn't figure out how to fix it.

When I last got it back from him, it would be okay for 5 minutes after I threw the standby (I had a standby installed), and then it would pulse for 5 more minutes, then it would be fine after that. That was a year ago.

In the past 6 months the pusling hasn't been going away. Sometimes its quiet enough that if I'm playing you don't notice it. Other times it's right up there in volume with the amp running @ 10% Sometimes a nice thump on the side of the amp will set it straight...

Just flicking the power on and off a few times in rapid succession and then leaving it on will change what it's doing... from doing the pulsing, to do this whiiir whiiiir whiiiiiir whiiirrrr whump whump whump whump kinda thing. (each whiiir is longer than the proceeding. And whiriir is what it sounds like if you sound it out phonetically at a high pitch)... Sometimes it just puts out a low freq hum thats not standard old-tube amp hum... until the tone goes down low enough that it dies out.

But both the two above issues are no where near as common as the general helicopter sound in the video.


I had a set of MesaBoogie EL's and EHX 12AX7's in it till last night when I retubed with a complete set of Groovetubes that were given to me when I bought my Orange AD30 (that I recently sold)


This amp has a solid state rectifier. Could something be amis with that?


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## jimihendrix (Jun 27, 2009)

have you had the amp exorcised by a roman catholic priest...???...


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## Wild Bill (May 3, 2006)

Wired said:


> When I last got it back from him, it would be okay for 5 minutes after I threw the standby (I had a standby installed), and then it would pulse for 5 more minutes, then it would be fine after that. That was a year ago.
> 
> Just flicking the power on and off a few times in rapid succession and then leaving it on will change what it's doing... from doing the pulsing, to do this whiiir whiiiir whiiiiiir whiiirrrr whump whump whump whump kinda thing. (each whiiir is longer than the proceeding. And whiriir is what it sounds like if you sound it out phonetically at a high pitch)... Sometimes it just puts out a low freq hum thats not standard old-tube amp hum... until the tone goes down low enough that it dies out.


I just noticed that this is an original and not a re-issue. Surely the filter caps have been replaced?

Filter caps don't just smooth out power supply ripple (120 cycle hum). They also do something called "decoupling". This is where each power connection to a preamp tube plate sees its ground return through the filter cap at its power supply node. If the cap is old and not working properly signals can 'couple' to other stages, often causing low frequency oscillations. 

Filter caps were only ever guaranteed for 1-5 years, depending on the manufacturer. Every engineer and tech knew that commonly they would last at least 10-15 years. Your amp has been on borrowed time for a lot longer than that if the filters are original!

Let us know so we can eliminate that possibility.

:food-smiley-004:


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## Wired (Jul 21, 2009)

Wild Bill said:


> I just noticed that this is an original and not a re-issue. Surely the filter caps have been replaced?
> 
> Filter caps don't just smooth out power supply ripple (120 cycle hum). They also do something called "decoupling". This is where each power connection to a preamp tube plate sees its ground return through the filter cap at its power supply node. If the cap is old and not working properly signals can 'couple' to other stages, often causing low frequency oscillations.
> 
> ...


They did noticed that it was decoupling before and fixed that. I remember him mentioning he recappd a god part of it... but I'll give him a call next week (he's on tour) so I can get a confirmed yes or no.


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## Scottone (Feb 10, 2006)

Wired said:


> Any GSM phone can, and I know the diff between those pulses and what the amps doing. Besides, the amp was doing this before I got my GSM phone. (CDMA phones don't affect amps or stereos with the clicking. In the youtube vid you can hear the GSM click, but its near the end of the video. what it's doing right off the bat in the video is the issue I'm referring too.


I was just pulling your chain with the Blackberry comment :smile: 

Since you have encountered this with 2 different sets of tubes, I'm guessing that it's a filter cap problem on one or more of the nodes in the circuit. It's kind of puzzling that the problem is intermittent though.

If it were me, I'd box up the chassis and send it to a more experienced tech. Our own Wild Bill would be a good choice as would Martin Newall or Tim Dudley in Toronto. Since you love the amp, it would be money well spent.


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## Wired (Jul 21, 2009)

Well its a year later and things have semi-improved.

The amp has been giggable and has seen the stage 5-7 times we have played in the last 6 months, each time operating without a hitch. Only once did I not bring the AC30HW head along for the ride just incase. 

The '73 now sits in my condo as my "practice amp" and has behaved itself very very well. I've only gotten the noisy helicopter a handful of times since I was last here. As before a quick flicking back and forth of the power switch does fix the issue if it comes back.

It seems to operate better when there is less humidity outside and when the overall temps are colder. I have noticed the noise coming back more often as summer has finally appeared and we've been getting a lot of rain. Even then, its only done it once in two months, and it went away after a few minutes of letting the tubes come up to temperature fully.

So I'm still confused, and still trying to figure out why its behaving so badly. But right now it seems to be living in fear of me permanently replacing it with the hand wired reissue, so it behaves. I'm sure if I sold that other head it would be back up to its old tricks. Damn thing has a mind of its own I tell you!


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## traynor_garnet (Feb 22, 2006)

Who is your regular tech?

TG


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## Wild Bill (May 3, 2006)

Wired said:


> yea, but thats on the CC2... this is a vintage Vox. and the trem channel is not in use, and i've even tried bypassing it using the footswitch.
> 
> The trem does work properly if I use that channel tho.


First step is to determine if it indeed is the trem bleeding through. That's easy! Listen to the 'thump' and crank the trem speed control to see if the rate changes.

It probably is trem bleedthrough. Lots of amps do this. Older amps like this Vox will have many uninsulated wires running all over the place. Find the ones running to the Trem intensity pot and use shielded wires instead. That should kill it but if there's a bit left try shielding the wire coming from the speed control. Wires can radiate unwanted signal or pick it up from other wires. Wires that cross at right angles pick up mice nuts of unwanted signal. Wires that run for a ways beside other wires are begging to have problems.

This has always worked for me. I first started using it with BF Fenders. If it doesn't, get back to us and we'll think of something else.


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## Wired (Jul 21, 2009)

Naw, it does not seem to be the vibrato/trem bleed. The tempo/rate of the noise does not change with the trem speed control. 

Usual tech is Dove Brown of Lillo's Music, previously Long + McQuade.


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