Vox AC4C1-BL amp.

My son bought this amp off ebay. The guy decided to make some changes and decided to unload it. My son got it pretty cheap, but it's pretty messed up. The po lifted a bunch of traces and lifted parts at one end, and supposedly eliminated one of the preamp tubes?

So I am going to try and put it back to stock. At least he left most of the parts connected at one end. Here are a couple pictures showing the aftermath.

25055733566_bb0c02e5b6_b.jpg


24714407539_8d7449d652_b.jpg


I can't find a good schematic. There is one out there, but I think it's from the guy who tore up the board. I guess I could trace it out, and get most of it.
 
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couple of schematics to look at

These Vox amps are not very much different from each other on the inside. Kind of like the Marshall amps, the signal path is pretty much the same. Extra tube stages are used for reverb, fx buffers, tremolo/vibrato, etc. Depends on what options your model has. The vintage AC's had a pentode first stage, which give you that Jimmy Page overdrive sound.

VoxAC4_V101_schematic.jpg

The newer models are more like the clean channel of a marshall. 12ax7 instead of pentode, 2 stages instead of 1 pentode, gain knob in between stages. Next comes boost / cathode follower / tone stack / master volume knob. If it has an fx loop / reverb / tremolo, it will be combined with the tone stack recovery stage. Then comes the output stage. The newer models tend to sound a bit more fizzy or chimey than the old design. Rolling preamp tubes and/or upgrading signal path caps in the newer models makes them change character dramatically.

Here's a print of a basic model without any options:
218275d1391392951-vox-ac4c1-schematic-mod-ideas-ac4c1-schematic-jpg
 
My son actually bought that very amp that the guy on the Tele site modified. Right down to the broken mounting hole on the corner of the pcb.

He replaced the OT, but appears to have put the original Vox OT back in.

He also left it with a real crappy looking old speaker, but those are the kind my son likes?
 
Wow, what a co-inky-dink! I didn't read through the whole thread to know how it ended up...

Sometimes you just have to wonder why people do what they do. All that time invested tweaking and testing, ordering and installing parts. Not to mention material costs. Just to halfway it back to stock and cut the cord? Not my style.
 
Thanks. It's actually my oldest son's amp. I will get a lot of satisfaction getting it to work. It is cute little amp. It will be a bit of a challenge, but that just makes more satisfying when you do succeed.

His last "project" that I worked on was an old Fanon FMA-20 PA amp. It would work and then would have issues. I finally figured out the bias adjust pot was intermittent, so it would go open circuit and try to run with zero volts on the grid. I never saw a plate glowing red before. I guess they don't call it redplating for nothing.
 

Leopoldo

Member
2018-05-14 10:22 pm
Hello friends! Please, I need a big help...!
I have the same amplifier (Vox ac4c1-bl) and recently I have burned the metal film resistor R24.
As I dont know the values of it (since it has been burned completely) I would like to ask you if "Any of you will have the specipic values of the R24?"
Thank you very much already!

R24_board_Vox_AC4c1_bl.jpg
 

TequiaKez

Member
2016-11-03 12:57 am
Hello friends! Please, I need a big help...!
I have the same amplifier (Vox ac4c1-bl) and recently I have burned the metal film resistor R24.
As I dont know the values of it (since it has been burned completely) I would like to ask you if "Any of you will have the specipic values of the R24?"
Thank you very much already!

R24_board_Vox_AC4c1_bl.jpg

Btw, Leopoldo, what issues does your amp exhibit with a burned R24?
 
Hello friends! Please, I need a big help...!
I have the same amplifier (Vox ac4c1-bl) and recently I have burned the metal film resistor R24.
As I dont know the values of it (since it has been burned completely) I would like to ask you if "Any of you will have the specipic values of the R24?"
Thank you very much already!

Hi,

My schematic shows R24 as 5.6k. If it's the correct one for your amp, it'll be the grid stopper on the EL84, and will connect the centre lug of the volume pot to pin 2 of the valve. I've also seen schematics listing the value as 6.8k, but having a brief flick through the versions I've collected, 5k6 is far the more common.

Hope that helps,

Matt.
 

TequiaKez

Member
2016-11-03 12:57 am
Hi,

My schematic shows R24 as 5.6k. If it's the correct one for your amp, it'll be the grid stopper on the EL84, and will connect the centre lug of the volume pot to pin 2 of the valve. I've also seen schematics listing the value as 6.8k, but having a brief flick through the versions I've collected, 5k6 is far the more common.

Hope that helps,

Matt.

Hi Matt, thats awesome, thanks a bunch.
Seeing as there's 2 examples of this guy burning out, would it be wise to fit a higher wattage replacement? 1/2W maybe?
 
Hi Matt, thats awesome, thanks a bunch.
Seeing as there's 2 examples of this guy burning out, would it be wise to fit a higher wattage replacement? 1/2W maybe?

Hi,

Definitely, although I'd guess it's be 1/2W already. I use 1W as a minimum in all my valve projects, which is often far more than strictly neccesary but it simplifies things, and gives a little margin for error in areas of a circuit which might otherwise be sailing a little close to the wind with a 1/2W part.

Matt.