VOX AC15 Bright Cap Mod

Sly Fox

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Greetings,

Been dabbling with vox for a while, rented them several times...when juicing the front end and cranking them I've gotten really satisfying lead tones...of course problem, is I dont practice at those volumes and hence willing to do the bright cap mod on top boost channel.

Is it really as easy as using some toenail clippers or scissors and cutting one leg off those two caps as pictured in this thread?

https://www.thegearpage.net/board/index.php?threads/just-bought-a-vox-ac15c1.1198981/

Maybe a dumb question but if I clip one leg or both, wouldnt the circuit be opened and create noise or other problems?
 

PelliX

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Hi,

"yes", you can clip it. It would be much better to desolder it. There is a chance that with one leg clipped the remainder might add some noise to the input. Not much, probably, but for example when using the Normal input, leaving the gain on the Top Boost channel open can pick up some noise.

The AC15 doesn't have to be pushed that hard to get good lead tones, either, fortunately.
 

Pete Farrington

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juicing the front end and cranking them I've gotten really satisfying lead tones

I dont practice at those volumes and hence willing to do the bright cap mod on top boost channel
So you’re setting channel volume high and controlling the sound pressure level with the master volume?
If so, just turn the master volume down.
The bright cap only affects the tone when channel volume is set low to medium. If set high, it has a negligible effect. Hence removing the bright cap C8 from the circuit would be pointless.
The benefit would be when using fuzz or distortion pedals, and setting the channel volume control low to medium. Which is what that TGP thread is all about.

Whatever, it seems a bad idea to remove C67, as it prevents the amp becoming unstable at high settings, and should have negligible effect on its tone. FYI, in case you’re not sure, an unstable amp sounds really bad.
Instability might only manifest at extreme settings, eg channel volume, treble and master volume all set high. Which isn’t how the amps in TGP thread were being used. So they might never have noticed a problem with removing C67, especially if the valves were a bit weak.
AC15C1 schematic
 
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PelliX

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Hmmm, upon checking the schematic, Pete is right. I was ass-uming that it was one of the caps straight at the input. Here's the service manual I used when fixing my AC15 up after purchase, FYI.


EDIT: Ugh, deleted. If anyone wants the AC15C1 service manual, drop me message.
 
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Sly Fox

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I do wish to use a distortion pedal and OD combination such as a rat and tubescreamer type pedal in front so it seemed no matter what there was a fizz and thin edgy harshness, really on both normal and top boost channels.

I clipped one leg off of each of those bright caps for top boost and experimented with different channel volume and MV ratios and found much easier to get something more satisfactory.

I also took a few moments to crank the hell out of the amp and did not notice anything particularly unstable. Can you elaborate what exactly you mean by unstable?

If it's something that might manifest itself I would probably solder the one leg back on that specific cap.

Since I've found the best combination is blending the channels with an AB box, I am still finding that harsh edginess present in the normal channel. Is it worth clipping that bright cab or is it possible it is the bright cap on the master volume since I am still running it fairly low MV...suppose 7:00 to 10:00?

For the record, I'm okay giving up some clean top end as I'm playing an ES345 with the caliberated T pups so there's plenty of mids and high end to go around.
 

Pete Farrington

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Can you elaborate what exactly you mean by unstable?
Oscillation. It can result in note decay fizzling out, kinda like this https://www.marshallforum.com/threads/2204-fade-out-issue.129063/#post-2311549

If it's something that might manifest itself I would probably solder the one leg back on that specific cap.
I suggest to reconnect C67, it’s not a bright cap, dunno why the function of C67 got lumped in with C8. C67 is there for good reason, regardless of whether a bright cap is in circuit.

I am still finding that harsh edginess present in the normal channel. Is it worth clipping that bright cab or is it possible it is the bright cap on the master volume since I am still running it fairly low
C4 is the normal channel bright cap.
It’s fine to disconnect bright caps if you dislike their effect.
The master volume doesn’t have a bright cap, it’s not feasible to add them to that type of control.
Rather there’s a tone cut control across it already.
 

Sly Fox

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Thanks for your reply and clarification, much appreciated. In other threads it doesn't seem c67 had created any issues for those that removed it. I may attempt to solder the leg back on if I think I can do it cleanly. I will probably snip C4 as well...if I like the overall sound afterwords with the blended channels maybe I will try to meticulously desolder the legs on the bright caps completely for them to be gone for good. I might eat them once they're out, in fact.
 

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