The current production Vox AC15C1 by Korg is one of the most popular tube amplifiers being used among P&W guitarists today. At $599 USD for a new 1×12 combo it is undoubtedly one of the best bang-for-the-buck amplifiers readily available through major music retailers. Though these Chinese made PCB amplifiers have many strengths, they also have a few common weaknesses that are widely recognized among those who own them. Notably, poor bass response, grainy mids, and ice pick highs. In my experience working with AC15C1 and AC15CC owners to help them improve their tone I happened across a non-invasive formula that takes these little amps from good to great. Every one of these AC15 owners has thanked me for helping them bring out the mojo in their amp. I recommend trying these upgrades in the order listed. If you find a point where you are completely satisfied with the results, stop. Continue to the next step if you decide you want to take it to the next level. We usually don’t use our articles to sell product. However, a couple of the items recommended in the article are items we offer. We encourage you to purchase these products from whomever you please and support local businesses whenever possible.
1. Don’t clip the bright cap yet! ($0)
Most guys don’t realize that the Chinese Alnico Blue or Greenback is the reason for the shrill top end on these amplifiers. Clipping the bright cap removes the chime that made the original AC15 famous.
2. Install a Bad Cat Celestion Speaker ($189) *minus whatever you sell the original speaker for.
There are many speaker options that will yield a sonic improvement in these amps, and the Bad Cat Celestion is a proven example. So much so, my customers and friends have deemed it “The AC15 Saver”. This custom designed speaker is the secret weapon for balancing the tone on these amps. Developed for Bad Cat by Celestion to combine the percussive low end of a G12H30 with the creamy cone response of vintage Bulldog alnico, this 65w/ 100 db speaker gives the AC15C1 and CC solid bass, pulls the mids together, and puts a smooth layer of cream on the top end. Once this speaker is installed you may be glad you didn’t touch that bright cap. If it’s still too bright for your taste after this speaker upgrade, then you may want to give the bright cap mod a try. It’s an easily reversible mod if done correctly. 
$189.00 USD
3. Replace the Speaker Cable & Plug ($15)
Believe it or not that short little cable can make a slight difference. If you decide to order your Bad Cat speaker from the “Buy Now” option above or our website http://tonesmiths.com/collections/amp-lounge/products/12-celestion-speaker we will solder on a premium speaker cable with a heavy duty 1/4″ pancake plug or 1/4″ female jack for absolutely free upon request. This is the same cable, plug, and jack used by Bad Cat. This addition will ensure strong solder joints, proper phase connections, and save you time on the install too.
4. Install a New Set of Tubes ($60-$75)
There are many brands to choose from when your looking to retube your amp. JJ, Electro-Harmonix, and Groove Tubes are the most popular since they’re easily available at most music store chains. But there’s a brand heralded by hi-fi enthusiasts that will absolutely change your life. These tubes are produced by legendary tube designer Peter M. Watson’s company PM Components. The crazy thing is they’re only a couple dollars more than JJ. I recommend their 12AX7HG for the V1 position, 12AX7A for reverb (not needed for the CC), a 12AX7LM for the phase inverter, and a matched pair of their amazing EL84s. All I can say is prepare to be awed at the increase in output and fidelity these premium tubes provide. They’re so good I dedicated an entire Tonesmiths Report to them The Best 12AX7 on the Market. PM Tubes are only available through a couple US sources. My friend Israel at http://www.tubeboutique.com is the fastest and most reasonable. He offers matched tube sets for every version of the AC15, has great prices, and ships fast.
Here’s a YouTube demo of an: AC15C1 with PM Tubes
5. Upgrade the Output Transformer and Choke ($260 parts/ $200 labor)
This is the most costly and labor intensive upgrade. At this point you will be into your amplifier a total of $850 if you purchased it new at $599. Upgrading the OT and choke to a Mercury Magnetics drop-in replacement will push your investment into the $1,300 range if you have to pay a professional to install them. You could buy a used boutique amp or an AC15HW for that! I would only recommend this upgrade if you are 100% convinced that you need more bass and touch sensitivity after the upgrades and you are planning to keep this amp for a very long time. *Note: With the exception of the transformer upgrade these mods have also been performed on the AC15HW with great results.
The Tonesmiths Report 
Thanks! I’ll leave the JJ’s in the AC15C1 for now and order the PM’s when they are back in stock.
I’m using a load box with my amp to record silently since I live in an apartment and can’t turn the amp up volume-wise. Is there anything else I can do to mod the AC15C1? Doesn’t make much sense to replace the speaker since I don’t use it.
Thanks,
Steve
A lower wattage speaker with a sensitivity rating of 100 DB and no doping will perform better at lower volumes since it will move more freely and breakup faster. I recommend the 15 watt Blue Dog alnico speaker with no doping from Ted Weber Speakers for your application. Here’s a link to his site. http://www.tedweber.com/blue12a
Hi Tonesmith, thanks for the recommendation but I am bypassing the speaker altogether and using a load box with speaker simulations so I can record silently. I’m still getting the preamp and power tube saturation via the AC15C1 head, but that’s about it. Why would I need a new speaker? Unless I am missing something here?
Thanks again,
Steve
The bright cap (i.e. ‘ice-pick’ sound) is starting to get to me. Is there a way to do the bright cap mod on the AC15C1 without permanently disabling it?
Thanks,
Steve
Yes. Using sharp snips cut the lead on the cap as close to the circuit board as possible. That way it can easily be reattached should you change your mind. Have you already installed the Bad Cat speaker?
Have not replaced the speaker yet, but, I just did the ‘bright cap’ mod per your instructions and all I can is, “Wow!!!” My AC15C1 is a beast now! What have I noticed? Those annoying ice-pick highs are gone. The amp takes effects much much better. The tone controls are way more responsive. And, I can drive the amp more. In short, I am simply blown away by this simple mod!
Thanks!!!!
Steve
Hi Tonesmiths,
So I just read that one can put a switch in the AC15C1 to engage/disengage the bright cap in the AC15C1. How would one go about doing this? It would give the amp more versatility sound-wise, I think.
Thanks,
Steve
All your tech would need to do is place a mini toggle switch between output on the cap and where it attaches to the PC board. The toggle can be mounted on the faceplate.
Thanks. I kind of wanted to do the switch mod myself. What do you think?
I got a Celestion G12H30 to swap out my Greenback with. Your post mentions upgrading the speaker cable & plug.
Do you have any recommendations for a quality speaker cable & plug? Brand – gauge – length – etc.?
Thanks,
Dave
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I’m getting ready to install the Bad Cat Celestion and I noticed that the Green back I’m removing is a 16ohm speaker. Can I really just replace a 16ohm speaker with an 8ohm?
You just need to slide the ohm selector to 8 ohm. It’s global, so it controls the ohm load to any and all speakers connected internally or externally.
I noticed the Bad Cat speaker has a Vintage 30 sticker on the magnet. Same speaker, or is there a difference?
I have an AC4C1-12 and the amp was a bit of a disappointment. I was also constantly disappointed with the pedals I was using with it. I just couldn’t get it sounding right….until I clipped the tone cap by the volume pot. Amazing difference – It’s a new amp now and the pedals sound “right”.
Thoughts on buying a 12au7 PM tube for the ac15? I use pedals and would love to get a bit more clean headroom out of the amp. I’ve heard those tubes do the trick! Just nervous about actually doing it haha.
I really don’t recommend it for this amp. You can accomplish the same thing by just running your input volume about 30% lower. The PM 12AX7HG is the perfect solution for increasing headroom in these anps. It is designed to provide 20% more headroom before break up without any compromise in power
Hey Tonesmith,
It seems like when I read up on the mods people are playing strats or telecasters on this amp. I am playing a Godin Archtop. I play in Open D a lot and play slide with the gain around 1 o clock. Is upgrading to the PM tubes necessary if the tubes already sound great so far? I have the greenback speaker in there and I’m satisfied but would it be worth it to change to the badcat? Thanks for any help!
Also, my reverb tank recently died. I’m replacing it with the Accutronics 8EB2C1B based off of a recommendation. Good idea?
There are some videos on my facebook page here if you want to listen to the amp with my Godin. https://www.facebook.com/DevinTheDerailer/
The PM 12AX7HG is designed to allow 20% more gain before breakup and the Bad Cat Celestion is extremely efficient at nearly 100 db with more low end than Greenback. This combination will give you the headroom needed for open D tuning.
If I follow your tube advice will the amp need to be rebiased?
I already installed a Celestion cream and the tone is spectacular. I’m thinking the tube swap will complete it.
Thanks!
AC15 CC/C1/C2 amps are all cathode bias meaning they adjust automatically
I sold my AC15 and got a Budda SD18 head. Best amp I’ve ever had!
Thanks for a great “article”!
I run an ac15 and blues junior in stereo and really like the combo of the sounds in the amps. I’ve modded the BJ with an eminence swamp thang and it plays really loud with alot of headroom and now to my question… Is it possible to get more headroom out of an AC15?
Best regards /Jimmy
The PM 12AX7HG is designed to allow 20% more gain before breakup and the Bad Cat Celestion is extremely efficient at nearly 100 db. This is primary reason I recommend this winning combination.
When I first got my AC15C1, I was a little disappointed with the shrillness/harshness unless the tone cut was maxed and the treble was dialed way out. The problem is that when you dial out the harshness, the amp starts to sound muffled, like someone threw a blanket over it. The first thing I did was put in a bad cat celestion speaker. The most immediate difference I noticed was that the tone cut and treble controls started working the way I thought they should – that is, I could dial out the harshness without having to max out the tone cut and dial back the treble to its lowest setting. That made a huge difference. Huge. No more harshness or having to live with a muffled sound. Then I changed out the stock tubes for a set of PM tubes. I can’t say that my untrained ear can hear a big difference, but I can say that now that I’ve had the speaker and tubes in for a couple of months, the tone I’m getting out of the amp is 1,000% better than when I bought it. It sounds great with both Fenders and Gibsons and I can control the sounds much better than I could with the stock speaker. I can’t say what accounts for that difference in a technical sense, but I can definitely hear it. I would highly recommend both changes, but especially the bad cat speaker.
Fantastic article and just what I was looking for. Iv already ordered a set of the recommended PM tubes and can’t wait to get them installed. I still have the original greenback installed in my AC15C1 and was thinking about fitting a celestion blue or gold until I happened across your article. Unfortunately the reccomended speaker doesn’t seem to be readily available in the UK and with the obscene import charges I am left looking for other possible alternatives easily available from UK retailers such as the Celestion range. I love the vintage tone of the blues but I love the clarity of the green for rythm and would love a compromise between the two somehow, whilst balancing the amps issues in the same way the recvomended speaker does. Any help or advice would be appreciated.
Glad you got a chance to they the PM tubes. Coda Music is a big dealer for Bad Cat in the UK. You can order a speaker from them.
That’s great, just what Iv been searching for, thanks. I notice that the bad cat is an 8ohm speaker whereas the default green back is a 16ohm speaker. What difference will this make, if any?
You will need to adjust your amp to 8 ohm mode. There won’t be any noticeable difference in sound between the different ohm speakers
Is there anywhere online where I can listen to a bad cat celestion speaker. Iv searched YouTube with little avail. Fantastic guide, the PM tubes have already made a difference but the green back has to go as it is far to shrill for my tastes.
Absolutely love the PM tube upgrade. It completely changed the tone of my amp and I am definitely noticing a much longer sustain which fades quickly but mellow at the same time. I’m guessing this is down to the increased gain from the 12AX7HG.. I have however noticed that when I play a single coil guitar that I get quit a bit more humming than I ever noticed with the default tubes. Would this be down to the increased gain in the preamp too? I
I have the AC15CC. Can anyone confirm that this only takes 2 pre and 2 power tubes? I am seeing that some VOX 15’s take 5 tubes (AC15C1). I just want to confirm that mine only needs 4. Thanks!
The CC only has two 12AX7 and two EL84. I recommend a PM 12AX7HG for the input/tonestack buffer, a PM 12AX7LM for the phase inverter and a matched pair of their EL84s.