23 Comments

mygif
Ian Tan Said,
June 27th, 2008 @9:07 am  

Hi Dave, great article! Are you able to post a picture showing the location of the valves please? ie. which valve is V1, V2, etc.

mygif
June 27th, 2008 @9:32 am  
Good point Ian. I’ll update the article with more detail on the valve placement.

I can only get photos next time I have the chassis out. The placement of the valves is to hard to photograph whilst still in the amp.

mygif
July 10th, 2008 @1:50 pm  
Had the chassis out to change the power tubes, so took a picture of the chassis and added it to the article.
mygif
July 26th, 2008 @1:03 pm  

Hi Dave, thank you for the great article. You were right ON the weakest points of the VOX AC15 in your mods, so now this is my Bible for the following weeks. Perhaps, here is one more mod I am working currently on. I have a project to remove the amp from the speaker cabinet, make it shorter (18-3/4″ across) and place it into my gear rack right under digital effect processor. So, I have full control on sound right under my fingers. The remaining cabinet from the amp I use as a speaker cabinet but modified also in order to make it closed for tighter sound. If you are interested, I can share a picture. In fact, dimentionally the shassis is very close to rack dimensions and even removing both PCB was no nessesary!
1 hour work to move some wires inside and another hour to cut out the exessive metal from the sides. I even mounted the amp to the rack through holes on sides of the front panel. Just made front wider, about 19-1/2″. Now no more microphonic issue with valves as it was before even under moderate volume of AC15 combo.

Best Regards,

Alex, Toronto, CANADA

mygif
July 26th, 2008 @11:27 pm  
Hi there Alex,

Thanks for stopping by and posting a comment.

I think the only mods left, to fully iron out the weak spots for AC15, is to change all the coupling caps to Sozo’s, and to replace the output transformer with a Mercury Magnetics. After that I think the amp would be right up there tone wise, although right now I’m very happy with how my amp is sounding.

It’s a very interesting project you’ve got going on there, I would really like to see some pictures of the finished result. Send them through to dbuntsma@gmail.com if you could, that would be great.

How do you find the heat dissipation with the chassis mounted inside the rack unit? Does the heat dissipated affect the digital processors sitting above, or were you able to design it so the majority of the heat is dissipated through the front and back?

mygif
July 27th, 2008 @3:06 pm  

Hi Davivd, you’re right to the point! All these issues are concerning me too. First of all I was going to cover open side of the chassis with a heavy gage metal plate to prevent any interference with other equipment in my rack. Do you think that aluminium foil will work as much as effective?

Still concerned though, because when I removed the amp from the cabinet my guitar began producing more feedback noise. I do really worry about its behaviour when the amp intalled into the rack. I have finished all re-wireing to keep everything within rack size, relocated input jack on opposite side, installed 8/16 Ohm switch on the front panel using previous Input jack hole and ready to cut the chassis with a grinder. Pictures are on me!

Another question though. As soon as all resistors are accessible (i.e. visible), can I just cut them out with the side cutters and replace them without removing PCB at all? I know, it’s not pretty,but I had already bad experience with overheating PCB while resoldering the elements.

Cheers from Toronto!

Alex Smirnoff (Vodking)

mygif
July 28th, 2008 @3:38 am  

Hi Dave, here is the answer on your question. I have installed the amp under the digital processor and it’s dead quiet! The heat dissipation problem doesn’t seem big yet, but I’l keep my eye on this.
Another tip to all. I use microphone preamp by ART with variable impedance on front of VOX. It gives me extra gain stage and more variety in distortion and clean sound. I am more than happy with the sound, because due to preamp I can make it sound darker like ORANGE or more sustained like MARCHALL on any volume level. That was the reason why I have finally chosen this CHEAP chinese amp for my project. The amp itself does not impress me much.

Cheers,

Alex

mygif
July 28th, 2008 @5:07 pm  
Hi Alex,

Good to hear it’s working out well.

Although not the most ‘elegant’ way of doing things, you can go about swapping resistors the way you mentioned. Although I’d be careful that when you re-solder the resistors, the connections are good and solid. You shouldn’t have any problems overheating the PCB though, if you’re using the correct iron at the correct temperature. I use a 40W temperature controlled soldering station set at about 330 degrees and haven’t had any problems yet. I bump the temperature up more, when de-soldering the ‘lead free’ solder that they use, as lead free solder usually has a slightly higher working temperature.

Aluminium foil will work in trying to eliminate interference. Maybe go over a couple of times to give a thicker layer and see how it works out.

Dave

mygif
July 29th, 2008 @1:01 pm  

Thanks for the valueble tips! I’ve got even better idea! If the goal is to reduce the resistor value from 220K to 180K, how about to leave the current resistor “as is” and to add another variable resistor in parallel? At this point less work involved and you will never go higher than factory parameter.

I have another question for you, though… I play through a speaker cab with 16Ohm Blue Celestion in it. Amazing speaker, BTW! So, I have found that when amp set on 8 Ohm output the cab sounds better, brighter, louder, tighter, whatever, compare to “obviously correct” switch setting on 16Ohm. Could you double check or at least theoretically explain my observation ( I would think in oposite way, that’s why I need the second opinion)? Another question is can I actually leave this switch on 8 Ohm as my prefered sound setting while using 16 Ohm amp? I know, it’s not healthy for tubes and tranny, but can it really bust them up and kill the amp after all?

Regards,

Alex Smirnoff (Vodking)

mygif
July 29th, 2008 @1:06 pm  

Sorry, just one correction to the last post. You have to read: “…. while using 16Ohm speaker?”. :-)

Cheers!

mygif
July 29th, 2008 @1:39 pm  

Currently I am adding channel switch to my “racked” VOX. After that I’ll send you the photos. Waiting for your reply!

Alex Smirnoff (Vodking)

mygif
July 29th, 2008 @4:51 pm  
Hi Alex,

Yes…In fact it is very common to solder another resistor in parallel to drop the resistance down, to save pulling the PCB out. This is also done with capacitors to change values.

Regarding your speaker set up. It’s fine to run the amp set at 8-ohm into a 16-ohm load, no problems there. You can run into problems though if you go the other way, say a 16-ohm tap into an 8-ohm speaker.

mygif
ShinnickRelay Said,
July 30th, 2008 @1:56 am  

First off - great page. I’ve been searching for mods for my AC15 and this has been a godsend! I performed the Bright Cap mod the day I found this page and really loved the results - definitely more pedal-friendly.

I’m about to attempt to install the tone cut pot you describe and I have a few questions:

1 - Will a 250k pot do the trick?

2 - You describe replacing the reverb pot with the tone cut pot - could you go into a bit more detail about that? Sorry - still new at working on amps…

Any help would be much appreciated!

mygif
July 30th, 2008 @12:30 pm  

Hi David, I’ve sent the latest photos of the “racked out” VOX AC15 to your e-mail. Feel free to share those pictures on your web site. Cheers!

Alex Smirnoff (Vodking)
Toronto, CANADA

mygif
July 30th, 2008 @1:53 pm  
Hi there ShinnickRelay,

Thanks for dropping by and checking out the article.

The bright cap mod really does work wonders doesn’t it. What guitar are you using? I’m currently experimenting with different capacitor values for the bright cap. At the moment I’ve got a 56pF in there and find it works really well when I’m playing clean on the neck pickup of my Les Paul. I’m still undecided about whether I’ll leave it in there or not as I’m not sure if I like the tone when I use pedals. Although not harsh like the amp is stock, there is an ever so slight ‘fizzyness’ in the top end that I’m hearing. Maybe I’ll try a slightly lower value.

Regarding the tone cut pot, a 250K pot will work fine there, just make sure you get a logarithmic one. They are labeled with an A in the front, so the pot will say A250K on it.

If you don’t use the reverb at all, you can replace the reverb pot with the 250K pot, for the tone cut control. What this means is that you’ll have to remove the pots board from the chassis and then de-solder the reverb pot from the board. Then mount the 250K tone cut pot in the hole that the reverb pot used to sit in, and run the wires around to where the tone cut resistor was on the main PCB board.

To remove the pots board from the chassis, you’ll need to remove all the chicken head knobs from the control knobs and then remove the little nuts that fix the pots to the chassis.

This takes a bit more work, but then you’ll have a dedicated tone cut control. I was planning on doing this before, but am happy with the 180K resistor swap I did, so I think I’ll leave it out for now.

Let us know if you need any more clarification
Dave

mygif
ShinnickRelay Said,
August 2nd, 2008 @2:30 am  

Thanks a bunch for your help thus far! I’m actually enjoying working on this thing. I’m halfway done with the tone cut - just gotta put everything back together now…

While I’ve got the board out and whatnot - you had mentioned changing the coupling caps over. What values did you change to (or did you change them)? Where are they located on the board?

Thanks again!

mygif
August 4th, 2008 @7:38 pm  
Hi there ShinnickRelay,

The coupling caps don’t change in value. They remain as per the schematic.

The change is to swap them out for better quality caps, as the coupling caps do play a role in the tone of the amp.

The brand that you should look for is Sozo.

Dave

mygif
ShinnickRelay Said,
August 12th, 2008 @1:11 am  

Forgive the newbie…

but I’m not quite sure how to wire up the 250k pot for the tone cut. Does Pin 1 go to ground and the other two to the spots on the board where R23 was? Thanks again for all of your help!!

mygif
August 12th, 2008 @7:39 pm  
Hi there,

what you want to do when installing the 250k Pot, is to wire one of the pads to Leg 3 and the other pad to the wiper (middle leg) of the pot. The other leg can be left untouched. These are the pads from R23.

This should give you 250K Ohms when you have the pot at minimum, which is giving you zero cut. As you turn the pot up, the resistance will decrease, giving you more cut, or less high end.

If you’re unsure which leg is which, get a multimeter and wind the pot all the way down and measure which legs give you 250K Ohm resistance.

It doesn’t matter too much if you get it around the wrong way. All it means is that the Cut control will work oppositely, it won’t damage the amp in any way.

Hope that helps,
Dave

mygif
ShinnickRelay Said,
August 14th, 2008 @2:08 am  

You are a stud. Final question: the coupling caps - correct me if I’m wrong: but you would replace C9, C11, C13, and C14?

mygif
August 14th, 2008 @9:34 am  
Yeh….those are the caps to change out. Basically you want to change the caps that are directly in the signal path.

I would also change out C8 (for ‘completness’ since your changing out C9) and C15 (the cap that sits in the tone cut circuit).

C5 and C10 are also in the signal path and could possibly also benefit from being swapped out, but you won’t find Sozo caps in those values. You could experiment with Silver Mica caps in those positions though.

Whether you hear a ‘night-and-day’ difference is hard to say, but all these changes are things that cumulatively work together for the better.

Dave

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