Vox AC4C1 cab build. Mod ideas


Silver Supporting Member
Hey gents,

My buddy (also a TGPer) and I have been fiddling around with an AC4C1 with a 12" speaker.
We found ourselves wondering into our local GC on a Saturday morning a couple months ago and played around with this little AC4 for about 2hrs. You know the story, didn't need it but had to have it. It arrived in the mail a week later. We were still impressed with it but thought that a couple simple tweaks would help it out a bit.

We decided to build a bigger cab for it, mostly out of curiosity and because us musicians are on that never ending quest for THE tone we hear in our heads. So into the shop we went to make some simple tweaks, BUT put two gearheads with know how together and simple plans become grandiose aspirations to change the face of amp building forever!....lol ok, not that extreme, but you get the point.

So far to date this is what we've done

New Cab
Finger Jointed 12 ply birch 19"x20"x9.5"
3/4" sides 1/2" baffle and back panels

5 or 6 speaker swaps
Stock speaker, Alnico Blue, G12H30, ET65
(8ohm and 16ohm), and a couple WGS

Retubed with JJs

Bright Cap Mod

This is what we are going to do in the next week or so

Replace 1/2" Baltic birch baffle with 3/4" one

More efficient (102.5db) Eminence speaker (8ohm)

Upgrade to Mercury Magnetic Trannys

Add vox cream corner protectors

While this has become quite the undertaking it has been extremely rewarding and one of the best learning experiences and besides I get to hang with my buddy and chase tone! What's not to love.

Just wanted to enlist the help of the TGP "think tank" and see if there are any of you that have had luck modding your AC4C1.

We are hoping to squeeze a little more head room out of this amp and hopefully tighten up the low end.

Also wondering what everyone's experience has been after doing the bright cap mod

After doing the bright cap mod (cut two caps) the treble knob acts like a funky "cut" knob. Has this been everyone else's experience?

I'm sure my buddy will jump in and add some more insight.

Thanks in advance for any help or insight you are able to offer.

Here's some pics

Last edited:


Did you follow AC30's height and depth dimensions and how did that affect the bass resonance of the amp?

And to add to that how did different speakers you tried affected the bass response? I have alnico blue in there and think of putting Greenback55 in there. It just seems to miss some of the bass in there and when I put a larger amp through the same C1-12 cab it sounds much better, when I put C1-12 amp through larger AC30 box it still doesn't have enough bass...I don't need anything drastic just to sound a little more serious and if the speaker would give me that it would be the easiest mod. And if that doesn't improve the bass response at least a little, I am going for better OT.

Other than that frankly I'd like it more if you had just left it in its wooden form without the pink color...


Mine is still in the original cabinet, and my mods have been limited so far. I replaced the OT first. This made the bass a little more prominent. Not huge but noticeable. Recently I also snipped the bright cap that sits on the volume pot (C20). Subtle but a little smoother treble I think.

I a surprised that you went taller rather than wider with your cabinet. The AC10C1 has a wider cabinet, and I think that helps the bass response a bit (as does the push pull output section of course).

I've noticed that my amp oscillates when fully cranked on both gain and volume. Not sure if this is somehow the result of my OT mod, or if it was thee before as well (I had never cranked it like that before so I don't know).

One thing you might try is to lift one leg of the cathode cap on the last triode stage - should clean things up a bit. I have not tried this yet, but I plan to.


Using the 3/4" ply for the baffle is a good move. The baffle should always be the strongest panel in your cab.

It looks like you have room to get two 10" speakers in there. That might be something else to consider. The 10's can sometimes tighten up the bass response considerably if you can put your hands on drivers of the correct impedance. It's best to vertically align them, but you would probably have to fit them in your cab diagonally. Shouldn't be a problem.

Two 10's can move a little more air, and often sound bigger than one 12, all other elements being equal.

Good luck!


I suspect the floppy bass is because the low end goes too deep for the amp to handle. Try changing a coupling cap to 1/10 the present value and see what happens.

Trending Topics