Vox a30 needs some direction

Vox and Hiwatt Discussion

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ampmike
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Vox a30 needs some direction

Post by ampmike »

My buddy ask me if I could take a look at this amp.They lost sound and i think there was a flash.Sounded like I could check the tubes out.In doing so one of the power tubes was dead and one of the ec83 preamp was cooked.I have it working with some doner tube but the top boost channel is sounding bad,kinda farts out into bad distortion.Ill take any ideas from you guys.I posted a picture just to show how nice the lead dress is.All the soilder work is fine!!!I m not crazy about the tube sockets being separated but its a clean looking build,thanks
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andyfromdenver
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Re: Vox a30 needs some direction

Post by andyfromdenver »

if you'll take "any" idea, I suggest taking it to someone who fixes amps. You tried a tube swap and no dice.

An aside, your head cabs are very nice.
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xtian
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Re: Vox a30 needs some direction

Post by xtian »

andyfromdenver wrote:if you'll take "any" idea, I suggest taking it to someone who fixes amps. You tried a tube swap and no dice.

An aside, your head cabs are very nice.


Uhh......


Mike, sounds like they burned out a power tube, right? So check the grid and screen resistors, and plate resistors on the ecc83.
I build and repair tube amps. http://amps.monkeymatic.com
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Colossal
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Re: Vox a30 needs some direction

Post by Colossal »

Mike,

It's very important to completely assess the voltages throughout every part of the amp, from mains entry to the input ground, before changing anything. Mark up a schematic with all of the voltages at every point and then sit back and assess. A blow part or tube will usually reveal itself.
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M Fowler
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Re: Vox a30 needs some direction

Post by M Fowler »

Mikey, after your voltage check as Dave suggest take a wood chop stick available from your favorite Chinese vendor and tap around that top channel components for bad solder or loose connections.

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andyfromdenver
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Re: Vox a30 needs some direction

Post by andyfromdenver »

[quote="xtian"][quote="andyfromdenver"]if you'll take "any" idea, I suggest taking it to someone who fixes amps. You tried a tube swap and no dice.

An aside, your head cabs are very nice.[/quote]



Uhh......


Mike, sounds like they burned out a power tube, right? So check the grid and screen resistors, and plate resistors on the ecc83.[/quote]

My opinion is not valid, to you xtian? these threads so commonly devolve into a bunch of wags with little info to go on. The op is toying with a friend's expensive amp.
I offered my opinion based on the very nature of the thread inquiry showing the op's level of experience, I meant no ill will to him whatsoever.
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xtian
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Re: Vox a30 needs some direction

Post by xtian »

Andy, Mike asked for advice, and you told him,
andyfromdenver wrote:I suggest taking it to someone who fixes amps.
The whole point of this forum is to help people learn to make and fix tube amps. This is where I learned, thanks to the patient advice of the membership.

I didn't say your "opinion is not valid"--those were your words.
I build and repair tube amps. http://amps.monkeymatic.com
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M Fowler
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Re: Vox a30 needs some direction

Post by M Fowler »

Mike is not a stranger to tube amps so with some patience he can fix this thing. :)

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rp
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Re: Vox a30 needs some direction

Post by rp »

Is that one of the Handwired ones? Not bad. Surprised Korg made the extra effort to use carbon comps. What brand are <C-Z> caps?
ec83 preamp was cooked
That's an odd failure. You really have to go through it.

BTW not sure if it already has one, but you might want to add a B+ fuse. An inline type wouldn't mod the amp.
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pdf64
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Re: Vox a30 needs some direction

Post by pdf64 »

Colossal wrote:Mike,

It's very important to completely assess the voltages throughout every part of the amp, from mains entry to the input ground, before changing anything. Mark up a schematic with all of the voltages at every point and then sit back and assess. A blow part or tube will usually reveal itself.
For amps in which the power tubes share a cathode bias resistor, it probably isn't a good idea to leave them idling for a prolonged time until all power tubes have been verified to be conducting normally.
eg if one tube is dead / has a blown screen grid resistor, it won't be passing any cathode current, so the shared cathode bias voltage will be low and the other tube/s will run hot
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ampmike
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Vox ac30

Post by ampmike »

Thanks Guys,Im still havent found the correct schematic or a layout,The amp seems to work great except when you switch the hot to cool toggle .It flubs out on the cool switch.This is located in the Top boost channel.Does anybody know a link for the ac 30 handwired schematic so I could check the voltages.The 5watt 150 resister is fried cant get a reading there.Im going to check all the sugestions you all posted and after that it will go to a repair guy in Nj.I really just thought they had a bad power tube and I could help out.
Thanks Guys
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