Vox AC4-C1 power scaling / schematic?

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Vox AC4-C1 power scaling / schematic?

Postby mike_lawyer on Wed Aug 21, 2013 1:59 am

Does anyone have a schematic for the new Vox AC4C1-BL? I know there was a schematic floating around for the original AC4, but the new blue model I'm looking to mod.

Basically, what I have found is that the master volume is really only usable between 9 a.m. and noon on the dial. The amp is stock except for some new JJ tubes, and I upgraded the speaker to a Celestion Vintage 10 inch. I'd like to install power scaling instead of the current master volume, or install a better master volume circuit. Thoughts on what would be more optimal and/or easier would be appreciated.
Last edited by mike_lawyer on Sun Aug 25, 2013 10:57 pm, edited 1 time in total.
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Re: Vox AC4-C1 schematic?

Postby Masco on Wed Aug 21, 2013 5:52 pm

How about putting up a clear hi-res photo of the circuit bd. Can't be much in there.
What are you trying to achieve exactly? Looks like a little cruncher, kind of a novelty eh?
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Re: Vox AC4-C1 schematic?

Postby mike_lawyer on Wed Aug 21, 2013 7:05 pm

I am mainly trying to upgrade the master volume circuit on this amp. I have a more expensive Marshall clone that has much better low volume control than the AC4-C1. My suspicion is that this is a fairly basic circuit that could be upgraded. I also might add power scaling to get better low volume control.

I will try and get some photographs this evening. Does anyone know what type of MV circuit the AC4-C1 has stock?
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Re: Vox AC4-C1 schematic?

Postby Masco on Wed Aug 21, 2013 7:41 pm

Well, its a SE amp w/ no phase inverter. Most likely a potentiometer before the grid of the EL84.
Have you thought about building an attenuator like the one that's built into the AC4 from a few years ago?
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Re: Vox AC4-C1 schematic?

Postby mike_lawyer on Wed Aug 21, 2013 8:01 pm

My thought is instead of an attenuator like the AC4TV, to install one of the London Power Scaling kits. I think this would preserve tone better than an attenuator.
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Re: Vox AC4-C1 schematic?

Postby mike_lawyer on Thu Aug 22, 2013 2:15 am

Here is an inside gut shot that I found on the web. I may need to take a closer picture, but this should give you an idea of the layout.

Image
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Re: Vox AC4-C1 schematic?

Postby mike_lawyer on Thu Aug 22, 2013 12:54 pm

Several other good gut shots of the AC4-C1 are contained at the link below:

http://www.voxshowroom.com/uk/amp/ac4c1bl_hood.html
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Re: Vox AC4-C1 schematic?

Postby Masco on Fri Aug 23, 2013 3:55 pm

mike_lawyer wrote: but this should give you an idea of the layout.

Image


Yup, pictured something like that in my head...
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Re: Vox AC4-C1 schematic?

Postby mike_lawyer on Fri Aug 23, 2013 4:09 pm

Masco -

I was able to find a schematic of the Vox AC4TV head online. That amp only has two tubes, while the AC4-C1 has three tubes. I'm not sure how different the rest of the schematic is.

It looks like the master volume is simply a pot in a circuit. I'm thinking of replacing the master volume with a power scaling circuit to scale everything down. The problem I am having is that you really cannot get low volume saturated tones because it appears that the master does not scale down the power section, only tames the preamp (I might be mistaken on this, so let me know!).
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Re: Vox AC4-C1 schematic?

Postby mike_lawyer on Sun Aug 25, 2013 7:17 pm

I'm still looking for a schematic for the AC4C1-BL. I'd like to add power scaling to allow some power tube distortion at low levels. Really the only distortion I can get is preamp distortion and then dial back the master.

Any thoughts on adding power scaling to this circuit?
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