NC3015 Output Transformer Swap

Written by
John Manning
Published on
January 22, 2021 11:58:44 AM PST January 22, 2021 11:58:44 AM PSTnd, January 22, 2021 11:58:44 AM PST

Welcome back to another exciting episode of Fix It Friday! A lot of our tech-savvy customers have asked about experimenting with alternative output transformers in some of our kits, so we decided to outline the procedure and capture the sonic changes that would occur with a modification like this. In today’s example we’ll be experimenting on our NC3015 kit, which is a spin on a Vox AC15 style amplifier.


This kit typically utilizes our 18 Watt output transformer, but today we will be implementing our Supro S6424 output transformer.  There is one more thing we need to note here, however. Although our NC3015 kit does come stock with our British Style 18 Watt Output Transformer, the kit we're using in today's experiment had already been modified to include our Vox Style AC15 Output Transformer. Once we demonstrate the process of swapping the Vox Style transformer for our Supro S6424 Transformer, we will feature sound clips of our NC3015 Amp Kit utilizing the stock 18 Watt OT, the Vox Style AC15 OT, and the new Supro S6424 OT, so you can see how each version affects the overall tone of the amp.


For those following along at home, here’s a list of tools and parts you’ll need:

Before we dive in, as always, we need to drain our filter caps for the sake of safety. If you are unfamiliar with this procedure please read our article on Filter Cap Discharge Procedures before moving on to the next part. If you ARE familiar with this process, please drain your filter caps now and then meet us at the next paragraph!  


Let's get started! First thing to do, is to unsolder the existing leads from the primary and secondary wires on the output transformer. In this case, there are three primary wires and four secondary wires. Take a look at Image 1A below for reference…

Now, one at a time, heat up these solder joints with your soldering iron and use your needle nose pliers to remove the wire from the joint (Image 2A). 

Once our leads are disconnected, we can now remove the transformer from the chassis. Use your Phillips head screwdriver to remove the mounting screws from the outside of the chassis (Image 3A).

After the mounting screws have been removed, carefully pull the transformer away from the chassis, but be mindful of how those lead wires are moving on the inside of the chassis; we don’t want to damage any other internal connections by pulling the transformer out too quickly and aggressively.  


As you’re removing the old transformer, you’ll notice there are two holes in the chassis through which the wires have been routed. One side is for the primaries, the other for the secondaries. Make note of which side is which, so that upon inserting the new transformer, you’ll be able to route the proper wires through the proper holes. Image 4A below shows the similarity in wire orientation between the two transformers.

Now we need to install the new Supro output transformer. To start, route the lead wires through their respective holes in the chassis. From the inside of the chassis, you’ll need to guide the wires to their approximate final locations. This can be frustrating as these wires are coming up from under the component board inside the chassis. Just take your time and be mindful of the other delicate components in the amp circuit.  


Once everything has been carefully routed to its approximate final location, the output transformer should be able to sit fairly flush with the outside of the chassis. You’ll notice that, if you align one of the mounting holes from the transformer with one of the mounting holes on the chassis, the other set of mounting holes does not align (Image 5A).

This is where our sharpie, electric drill, and drill bit come into play. We need to drill a new mounting hole, but just ONE. On the side of the chassis closest to the amp’s front panel, align the mounting hole on the transformer with the mounting hole on the chassis. On the other mounting hole (the one that does not align with the chassis), use your sharpie to mark where your new mounting hole needs to be drilled (Image 6A). 

Now that you’ve marked the location of the new hole, you just need to drill it. Using your electric drill and your drill bit (we are using a step bit in this example but you can also use a standard 3/16” bit) drill the new hole through the chassis (Image 7A).

Now we can mount our new transformer. Using the screws you removed from the original transformer, insert one screw into one mounting hole. Then tip the chassis on its side while holding said screw in place. With a pair of needle nose pliers in your free hand, grab one of the locking nuts you removed from the old transformer and affix it to the threaded side of the mounting screw on the inside of the chassis, and tighten it down. Refer to images 8A and 9A below…

Repeat this process with the second mounting screw and locking nut. It’s a tricky process and can also be frustrating, but just take your time. You’ll get it.  


Phew! We’ve removed the old transformer, inserted, drilled for, and mounted the new know what that means? We’re on the home stretch! Time to wire in the new transformer leads.


In this case, the color coding of the wires on both of these transformers matches, so we can just reference our photo from the beginning (10A below) to see which wires need to go where. If for any reason, in your particular project, the color coding doesn’t match up, you’ll need to reference your transformer diagram and amplifier diagram to make sure your primary and secondary leads are going to the correct places. 

As we do with so many of our projects, we need to cut these leads to length before we strip back the wire insulation and solder them in. So pick the lead you want to start with and route it all the way the point at which it will be soldered. Then, if there is any excess wire, trim the wire to length using your wire cutters. Now you can strip back a small length of insulation with your wire strippers, and solder the connection in place…

Now repeat this procedure with each of the leads until everything is soldered in properly. You should have something that looks similar to this…

You’re ready to put this bad boy back in its cabinet and let it rip! We had fun jamming around with our amp (before and after the swap), but we also noticed some tonal and tactile changes. The amp felt different to play (hopefully that makes sense), but it also did have some new tonal characteristics brought out by the Supro transformer; which makes sense considering this transformer has a different output impedance and different construction than the original. Here are some sound clips of our amp. The first is with the old Vox Style AC15 Output Transformer, and the second is with the new Supro S6424 Output Transformer after we made this switch.  The final clip is of the NC3015 Amp Kit with our British Style 18 Watt Output Transformer (the transformer that comes with the NC3015 Kit


Hopefully you had as much fun with this as we did. And maybe you learned something too. At any rate, we were happy to have had you join us on this wild adventure and we can’t wait to see you next time!