VOX AC15C1 Schematics

Completed amps from Fender, Orange, Hiwatt, Vox, etc.

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songsmith1950
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Re: VOX AC15C1 Schematics

Post by songsmith1950 » Thu Sep 20, 2012 9:03 pm

Another question, my friend. In the normal channel isn't that 22 nf coupling cap in the normal channel a good bit on the small side. Seems to me it would surely have both a lot of impedance as well as setting up the output very heavy on the treble side. Would help with the Vox jangle for sure, but couldn't even a slightly larger cap such as a .01 mf warm it up a good bit. That may perhaps give it both a great top boost channel and a really nice full range normal channel as well.

Thanks

Tom

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inkomodo.dragon
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Re: VOX AC15C1 Schematics

Post by inkomodo.dragon » Fri Sep 21, 2012 6:40 am

Yes, it is a bit small but its' not that much smaller than it's supposed to. It's a matter of taste anyway. If you want a more vintage correct response you could try modding the entire channel back to original spec, I did that on this post.

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Re: VOX AC15C1 Schematics

Post by IanL » Thu Oct 18, 2012 12:36 pm

Many thanks for spending the time to reverse engineer and document this, great job!

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Re: VOX AC15C1 Schematics

Post by jusa1982 » Mon Oct 22, 2012 7:46 am

Hi there,

Thank you for your awesome job and all your effort. Great instructions and all!! :)
I was wondering about the OP-AMP bypass mod:
It seems like the resistor between R22 and R23 is not a 220k because it departs from the other 220k resistors you ha ve put in for example R5 and R6?? Please explain me, i'm confused with that? :)

Other concern is why my tonestack mod doesn't seem to work anymore? I've put a jumper between the top boost channel bass pot ground and the ground of the C67 which I have taken off completely in terms of the bright cap mod which I've done earlier??
I would really appreciate if you could help me with these riddles??

Thank you very much!!

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Re: VOX AC15C1 Schematics

Post by inkomodo.dragon » Tue Oct 23, 2012 10:25 am

You're welcome!

Most of the resistors in this amp are metal film type (blue ones), the one I used between R22 & R23 is carbon type. If you look at the color bands you'll see they are the same value. The metal film resistors are red-red-black-orange-brown, the carbon one is: red-red-yellow-brown. Check the color codes here

As for the Tone stack mod, from what I could gather your mod is basically jumping R32 and the effect it has is scooping the mids. If that's the case, even with the opamp bypass mod, It should still work.

Be aware that my mod changes the impedance that the tone stack see and that can have some impact on tone. In case I didn't mention it earlier, there's bleed between channels due to the shared cathode of V1 so it's a good idea to kill the gain of the channel not being used to prevent interactions.

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Re: VOX AC15C1 Schematics

Post by jusa1982 » Tue Oct 23, 2012 3:42 pm

Thanks again!!

For clearance this is a link to the page where I found the mods earlier: (http://acapella.harmony-central.com/sho ... tion-Clips" onclick="window.open(this.href);return false;)

The tonestack mods just not affecting anymore, no matter if the switch is on or off?? Perhaps I should just take the tonestack mod out of there or play it much louder and try to hear the difference again. I just don't get it why it doesn't work anymore but I hope that link helps you to understand something I don't.. :) I've done all those mods at that link, and then afterwards your mods. The op-amp bypass is great! By the way does some of all these mods I have made overrule each other?? And the last sentence didn't reach me perfectly: "to kill the gain of the channel not being used to prevent interactions"??

Thank you very much again!! :rockon:

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Re: VOX AC15C1 Schematics

Post by jusa1982 » Tue Oct 23, 2012 3:47 pm

Hello again,

I forgot to mention that I've also taken the C68 away because it's not in the original AC15. I heard it has something to do with the upper middles.

Thanks!!

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Post by Bug2222 » Tue Dec 25, 2012 3:38 pm

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Last edited by Bug2222 on Mon Mar 03, 2014 7:26 am, edited 4 times in total.

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Re: VOX AC15C1 Schematics

Post by jusa1982 » Wed Apr 24, 2013 1:01 am

Hello!!

Nice job you've done there! I was wondering why did you change the R9 from 33K to 56K? Meaning what is the benefit for that?
Also womdering what would happen if you do the "op-amp bypass mod" but leave the plate resistors R5 & R6 in tehir original values 100K? Less gain yes but does it have any other impact on sound?


Thank you guys!! :)

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Re: VOX AC15C1 Schematics

Post by robert » Wed Apr 24, 2013 7:21 am

Hi Dragon,

wow, what a nice piece of work!
Is it possible for you to add the AC- voltages e.g. from the transformer secondaries also?

Regards

Robert

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Post by Bug2222 » Wed Apr 24, 2013 8:26 am

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Last edited by Bug2222 on Mon Mar 03, 2014 7:27 am, edited 7 times in total.

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Re: VOX AC15C1 Schematics

Post by jusa1982 » Wed Apr 24, 2013 11:14 pm

Hi Bug2222!!
Thank you for your answers! I was wondering why not also change the R20 which is the same "grid stopper resistor" in the normal channel? Aren't they all 68K in the old vox amps?


Greetings!

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Post by Bug2222 » Thu Apr 25, 2013 2:33 am

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Re: VOX AC15C1 Schematics

Post by inkomodo.dragon » Thu Apr 25, 2013 1:12 pm

Hey Bug2222, just read your comment and there's a couple of things I would like to point out.
I've tried the Fryer AC30 mod and did not like it (way too much gain). It will get you closer to Brian's current tone but further from his early tones. It's clearly designed for lower output treble boosters.

Most of the Queen Era tones are easier to get from the standard normal channel circuit. The only improvement I recommend is raising the filtering of the preamp stages a bit to tighten up the bass (like Cornish did on Brian's amps).

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Post by Bug2222 » Thu Apr 25, 2013 2:51 pm

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Last edited by Bug2222 on Mon Mar 03, 2014 7:28 am, edited 2 times in total.

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