Blues Junior Output Transformer Upgrade

Written by
Logan Tabor
Published on
January 22, 2021 11:33:10 AM PST January 22, 2021 11:33:10 AM PSTnd, January 22, 2021 11:33:10 AM PST
Welcome back, everyone. Today we’re going to be performing a little experiment on an extremely common amplifier, the Fender Blues Junior.  This amp has become one of the most widely-used amps in the world, and has a sound that pretty much every guitarist is familiar with, at least to an extent.  
There are a number of mods which can be made to the Blues Junior circuit, but today we’re going to be talking about the amp’s output transformer.  A lot of people think that changing an amplifier’s POWER transformer will have a huge effect on the overall sound of the amp.  And while upgrading a power transformer can yield some tonal benefits, it doesn’t have nearly as much to do with the amplifier’s signature tonality as the output transformer.  Mojotone offers a replacement output transformer for the Blues Junior; this transformer is hand-wound in the United States by Heyboer Transformers and is truly a quality product.  
While we will be lightly covering the steps involved in the transformer swap from a technical standpoint, today’s exercise is more about demonstrating the effects of this modification on the overall sound of the amp.  So to begin, we should hear how the amp sounds before the modification is made.  Below are two sound clips of my original, un-modded, Blues Junior amp: 
This first clip has the treble and bass controls on 6 with a slight bump in mids, the ‘Fat’ button engaged, no reverb, master volume on 6, and channel volume on 2…

This second clip carries over all the same settings except, here, the master volume is on 2, and the channel volume is on 7 (for a bit more overdrive)...

Now that we know what the original circuit sounds like, more or less, it’s time to make the modification.  Here’s a quick list of tools and materials you’ll need:
SolderSoldering Iron, Needle-nose Pliers, Screwdriver or electric drillDigital MultimeterAlligator ClipsMojotone Chassis StandWire Cutters and/or Wire StrippersMojotone Blues Jr. Style Upgrade Output Transformer

The first thing we need to do, as per usual, is remove the chassis from the amplifier.  We’ll need to disconnect the speaker from the amp’s output jack, disconnect the reverb tank wires from the reverb tank itself, and then remove the upper back panel from the cabinet.  In the case of the Blues Junior, the chassis is mounted directly to the back panel, so after the back panel is removed from the cabinet, it will then need to be removed from the chassis itself. See Image 1A below, wherein the screws circled in red indicate those which attach the back panel to the cabinet, and where those circled in green indicate those which attach the chassis to the back panel. First remove the red screws and remove the back panel (along with the chassis) from the cabinet. Next, remove the green screws and pull the back panel away from the chassis.

Once this is done, we have a chassis that is just about ready to be worked on. Since we’ll be dealing with the high voltage lead on our output transformer (the red lead), there is the potential for an electrical charge to be held where we’re working.  In order to make sure we’re safe, we need to drain the amp’s filter caps.  Take a look at Image 2A below, and make note of the red-encircled capacitors, as these are the filter caps…

In the case of the blues junior, draining the large grey filter cap will drain the rest, so we really need to turn our focus to this large grey cap. To properly drain these, without letting too much voltage out too quickly, we will need to connect one end of our alligator clips to the metal chassis, and the other end to R3 (Resistor 3) at the top left side of Image 3A below…

It is important that we attach this end of the alligator clip to the R3 lead that is closest to the tubes. Once the connection is made, as it is in Image 4A below, the filter caps should all be leaking voltage…

To test for this, we’ll need to grab our digital multimeter, turn it to the DC Voltage setting, place the black probe on our metal chassis, and the red probe on the positive lead of the large grey filter cap (Image 5A). You should be able to watch the voltage drop continuously until it reaches a reading of 0 volts.  

It should take a few minutes for the reading to reach 0 volts, but once it does, the amplifier is safe to work on. Leave the alligator clips in place throughout the rest of the procedure, as otherwise phantom charges can potentially occur within the capacitors.  
Now it’s time to disconnect the old output transformer’s leads from the amp’s circuit board.  Notice that the output transformer’s leads are most likely connected to the board via spade connectors (Image 6A); this is nice, because removing them just requires grabbing your needle-nose pliers and wiggling the spade connectors back and forth a bit.  Before removing them, take a picture of the circuit, so that later you can reference which color wire went to which connection point on the board.  There are 5 wires you’ll need to account for: red, green, blue, black, and brown.  The Mojotone replacement transformer will have all these same leads, and they’ll need to be connected just as they were with the old transformer...

Once the leads have been disconnected, it’s time to flip the chassis over, remove the mounting screws from the old transformer, and pull the transformer out of the chassis (Image 7A). At this point, you’ll likely discover that it’s necessary to use your wire cutters to remove the spade connectors from the current transformer wires in order to pull them through the holes in the chassis and successfully remove the transformer.

Once the old is out, it’s in with the new!  Feed the green and black wires through the hole closest to the control pots, and feed the remaining three wires through the hole closest to the tubes (Image 8A). 

After you’ve fed the lead wires through their respective holes, secure the transformer to the chassis using the same screws you removed from the old chassis.  Next, we must wire the new leads into their proper places.  Go ahead and pull up the reference photo you took earlier and make sure you get the right color wire into the right place.  Since we don’t have spade lugs on the end of these new wires, we will just insert the wire through the holes in the terminals on the PCB, and solder them in (Image 9A). 

After getting each wire seated in its proper position, simply solder the connection in and move on to the next one.
Pro tip: When it comes to the black and green wires, definitely work on the black wire first and then move on to the green wire.  This will result in the use of the least amount of audible swear words.
Once everything is soldered in, you should have something resembling the chassis in Image 10A below…

We’re essentially done with the modification.  Before we call it, however, let’s trim any excess lead wire that’s left sticking too far out, and remove our alligator clips from R3 and chassis ground. Then, prior to reinstalling the chassis onto the back panel and into the cabinet, we should briefly connect the speaker to the amp and turn the amp on for a few minutes to make sure we don’t hear anything obviously wrong with the amp. If all is well, the only thing left to do is reattach the chassis to the back panel, and then install the back panel (and chassis) in the cabinet.
I’m actually pretty pumped about this mod. I immediately notice a much more smooth sound coming from the amp.  Both clean and dirty settings have a nice, smooth (but not overbearing) compression.  The mids and lows are tighter, whereas before I felt like they were a bit out of control.  While recording sound clips of the amp with the new transformer in it, I can actually SEE that the sound waves are smoother and tighter.  Here are the new sound clips, with the first one being the aforementioned clean settings, and the second being the aforementioned gritty/overdrive settings:

We love this mod, and we sure hope those of you following along enjoyed it as well.  This is just one of many things you can tweak on a Blues Junior; and who knows, maybe we’ll dive deeper into Blues Junior mods in the near future.  Until then, keep tinkering, everyone!