The Vox AC15CC, or AC15 Custom Classic is a nice amplifier – despite being manufactured in China, it has plenty of character and tone. Still, there are plenty of people on the internet willing to invest time into performing modifications on this amp to take it a step further. Having done quite a few modifications myself, I thought I would write an article to try and consolidate those that I have performed, both to document my journey and also to produce an easy access guide, so that you don’t have to scour the net looking for resources.
Operating voltages of valve amps are lethal and can remain lethal, even after the amp has been turned off. There is a serious risk of electrocution and possible death when working on valve amps.
The author of this article and GuitarPug.com will not be held responsible for any damage to gear, or injury to persons, as a result of reading this guide.
If you are uncertain in anyway, have the work done by an amp tech.
Additionally, please note that performing any any of these modifications will, in all likeliness, invalidate the manufacturer’s warranty.
If this is the first modification project you’ve embarked on, or you haven’t already read our guide on Must Have Tools For Modding and B.Y.O Kits, I suggest stopping by there as a first port of call.
Before you start any work on any valve amp, you MUST first check the voltages across the filter caps and discharge them if necessary. These filter caps are capable of storing up to 400 volts, even after the amp is turned off.
Another point, DO NOT power your amp up without a speaker load connected, you risk damaging your amplifier.
It is also wise to print out a schematic of the amp you are working on and document any changes you have made. That way, if you do need to take the amp to an amp tech, you can give them a schematic that correctly reflects the circuit. The schematic for the AC15CC is provided below.
The first step before working on your amp, is getting the chassis out. This is not hard and is worth attempting, even if you aren’t planning on modding your amp, as it gives easy access to the tubes for changes and other projects.
First, remove the back panel. Remove the six screws on the back panel, the panel should then slide off. These screws are phillips head #2 size.
While you are at the back, disconnect the speaker from the chassis. If you are planning on fully removing the chassis to work on it, remove the reverb tank from the black pouch at the bottom of the cabinet. Remove the two crews on the left hand side, and the reverb tank will slip out. If you are just changing tubes, then removing the reverb tank is not required, as there is plenty of slack in the cable to place the chassis on the floor.
Next, on each side of the cabinet is two screws (phillips head #3). Remove the 4 screws in total. These are the screws that hold the chassis to the cabinet. Don’t worry about the chassis falling when you remove the screws, as it sits on wooden rails. The chassis will then slide out. It is normal if you have to use a bit of force to slide it out. The chassis also has a bit of weight in it, so be prepared as you are sliding it out.
This is extremely easy, straightforward to do and well worth the effort of learning to do it yourself. Knowing how to change your valves will save you money, instead of having to take it to an amp tech (unless of course, you suspect something else is wrong with the amp). The AC15CC has four valves; 2 x 12AX7 preamp tubes and 2 x EL84 power tubes. The valves are labeled V1, V2, V3 and V4. When looking at the back of the amp:
- V1 is the Top Boost channel valve – Furthest valve on the left, next to the output transformer.
- V2 is the phase inverter – Valve sitting to the immediate right of V1.
- V3 and V4 are the two power valves – The two valves sitting next to the power transformer, on the right hand side of the chassis.
V1 and V2 can be changed easily without removing the chassis, as you can reach around and access them. The tube protectors on V1 and V2 are spring loaded, just push down slightly and they’ll twist off. V3 and V4 are protected by a heat shield, which needs to be removed first. It is easiest to remove the chassis when replacing these valves, as the heat shield is screwed into the chassis.
As the AC15CC is a cathode biased amp, it does not require the bias to be set every time you change valves. As long as you purchase a matched pair of EL84 valves, you can whack them in and rock away! The 12AX7’s don’t have to be matched or anything, although some people like using a 12AX7 with balanced halves for V2, the phase inverter. Feel free to experiment with different valves to find your tone.
The speaker in your amp has a massive influence on your tone. So if you are tone hunting, it’s worth considering a speaker upgrade. The AC15CC comes with two speaker choices; the Wharfedale or the Celestion Alnico Blue. The Celestion Alnico Blue is the better sounding speaker of the two. Not that the Wharfedale sounds bad, it just doesn’t sound like how a Vox should. Of course, that is a very subjective statement, but most people are chasing that vintage Vox tone that the Alnico Blue produces.
A good idea is trying to pick up an AC15CC with a Wharfedale speaker second hand, which will then enable you to experiment with different speakers. Apart from the Celestion Alnico Blue, the Webber Blue Dog is also a good choice.
Remember that new speakers need time to break in. So if your amp is new, give the speaker time to break in before you make any decisions about upgrading.
Accessing the PCB Board
With the amp lying face down on the floor, or the chassis removed, look inside. If you have removed the chassis, have the chassis sitting on its transformers with the control knobs facing towards you.
When you look inside you’ll see two long PCB boards. One board sits along the bottom of the chassis, this is the main PCB board. The other PCB board is mounted vertically, behind all the control knobs and is called the ‘pots board’ as all the potentiometers are mounted to it.
To access the main PCB board do the following: (Each procedure will relate to the picture below it, with components that need to be removed circled in white. Click on the pictures to enlarge them)
- Remove all the screws, the red and white plug (reverb send/return) and the input jack PCB board. Remove the input jack PCB board by removing the plastic nut that holds the input jack in the chassis. The board will slide out.
- Remove all the screws and disconnect the two connector plugs circled. Leave the small connector that goes to the input jack PCB board, this does not need to be removed.
- Remove all the screws and the connector plug circled. You’ll notice a white arrow pointing to a green wire, follow the wire to the chassis and remove the screw and nut that holds the wire to the chassis. Remove the foot-switch PCB board which is circled at the bottom of the picture, using the same method as the input jack PCB board. Tuck the foot-switch PCB board down, vertically, along the chassis. De-solder the wires from the standby switch.
- Remove all the screws. This picture shows the power transformer. Along the wires are cable ties that neatly bundle the wires together, cut these cable ties to free the wires. Note: Yours will not look the same as the picture, as I have fitted an after-market power transformer, however it should be similar. Your cable ties will most likely be white in colour. The white arrow pointing to the green wire is the same wire as described in the previous step.
- Now, carefully grasp the PCB board with one hand at each end and slowly manoeuvre it to the right a little bit, to clear the power switch. Now gently rotate the PCB board away from you. You should be able to rotate the PCB board enough so that it is sitting vertically. Remember that input jack board? You should be able to hang it off the other side of the chassis now, out of the way. I use some sticky tape at each end and in the middle, to hold the board while I work on it. This should now give you enough access to the solder side of the board to de-solder components etc.
Bright Cap Mod
If you’ve ever thought that your AC15CC sounds very ‘harsh’ and ‘brittle’ with overdrive pedals, this mod is for you. Basically the ‘bright cap’ is a capacitor that sits across the Top Boost volume control and lets high frequencies through at lower volumes. As the volume is increased it has a lesser effect. The stock value is 120pF, far too high in a Top Boost circuit. 68pF is considered a good all round value, or most people remove it completely.
If you are more adventurous, you can install a switch to toggle between a few values. I’ve removed mine completely and find it works well, as you can still dial in plenty of highs with the treble control. The good thing too is that this mod is probably the easiest to perform, as you don’t even have to pull the chassis to do it.
With the amp lying face down on the floor and the pack panel removed, look inside the chassis. The bright cap is located on the ‘pots board.’ Next to the input jack is the Top Boost volume control, the bright cap sits across this. Circled in the picture below, is the two pads where the capacitor is soldered too. De-solder these two points and the capacitor will drop out from the other side, you don’t have to remove anything else. Easy.
Tone Cut Mod
The AC15CC does not come with a tone cut control installed. Instead, it has a fixed value resistor which is set at 220K. This value simulates the tone cut control set at ‘no cut’ so the maximum amount of top end is allowed through. In my opinion, a Top Boost circuit needs a tone cut control to help tame the top end. You can install a pot so that you have a full tone cut control like an AC30. I have dropped the value of the tone cut resistor to 180K, which simulates the tone cut control set at about 10-11 0’clock, roughly. I find that with this setting and the tone knob on my guitar, I can keep the top end in check.
To perform this mod you need to follow the ‘Accessing the PCB Board’ guide above, as you need to access the main PCB board. Locate the resistor R23, circled in diagram below, and remove it. Replace it with the new resistor value of 180K, or lower if you desire. I used a carbon film 1 watt resistor here. If you decide to use a pot instead, solder the wires from the pot here in place of a resistor. As the reverb on this amp is quite woeful, removing the reverb pot and replacing it with your tone cut pot is a good idea, if you don’t want to drill new holes in your chassis.
V1 Plate Resistor Mod
This mod is basically swapping a resistor out for the value that was used in the original Vox amps. It gives the amp a little more fuller/ warmer sound, a bit more bite and a smoother breakup into overdrive.
This requires access to the main PCB board, so follow the ‘Accessing the PCB Board’ guide above. Once inside locate the resistor R6, circled in picture below, and remove it. Replace R6 with a 220K resistor. I used a carbon film 1 watt resistor here.
Note: The picture below shows both the R6 and R7 resistors circled. Only the R6 resistor should be swapped out for a 220K resistor.
The AC15CC uses a 1K resistor just after the rectification stage as a current limiter, instead of a choke. Benefits of a choke include hum reduction, better intermodulation in the signal, reduction in ghost notes as well as changes to the response of the amp. A high end choke costs about $30, so it’s not an expensive mod to perform, although it does require drilling into the chassis to mount the choke.
This requires access to the main PCB board, so follow the ‘Accessing the PCB Board’ guide above. Locate resistor R71, it is a big white resistor. Remove it, the wires from the choke will now take its place. Mount the choke on the chassis, run the wires through and solder them in place of R71 (highlighted in picture below). Unfortunately I did not take pictures of the mounted choke. Next time I have the chassis out I will take some and post them. There is plenty of room on the chassis to mount the choke, so finding a spot is easy. Just take care when drilling through the chassis, so that you don’t damage anything.
Note: Swapping the 1K resistor with a choke will result in a higher B+/HT voltage. If you swap in a choke, you should also consider increasing the values of the screen/grid resistors on the power valves, otherwise they will have too high a voltage on them. This will result in decreased lifespan of your power valves.
‘Standard/ Custom’ EQ Mod
This mod basically replicates the ‘Standard/Custom’ EQ switch, that is found on the AC30CC. ‘Standard’ mode makes your tone controls more interactive, as opposed to ”Custom’ mode, which makes your tone controls less interactive. Both modes are ‘correct’ depending on what era and what model Vox you look at. In the stock form, the AC15CC is like having the switch set on ‘Custom’.
This mod involves the ‘pots board’ and can easily be performed without having to remove anything at all, as you can make the connection on the solder side of the board, which you can easily access.
All that his mod requires is linking the unused lug of the bass pot to ground. The easiest way to do this, is to use the ground lug from the volume pot of the Top Boost channel. You can easily identify the ground lug of the volume put by using a multimeter, and doing a continuity test to ground. You should also be able to see which lug of the bass pot is unused, as there are no tracks leading to/from this lug.
A good idea is to fit a toggle switch, allowing you to swap easily between both settings, just like the AC30CC.
Thus concludes the mods that I have currently completed on my Vox AC15CC. There are many more mods that could possibly be done and I will continue to update this article as I perform them. I hope this has been useful to you – have fun working on your amp to improve your tone. If you have any mods of your own, or have any interesting experiences with modding your amp, do leave a comment and let us know.