Instructions/Amp Mods/Blues Jr. Made in USA Mods

Amp Mods · Blues Jr. Made in USA Mods

Fender Blues Jr USA Mod Kit Instructions

Steps47

Difficulty—

Time—

AuthorJohn Fromel

Thanks for purchasing the Blues Jr. USA Modification Kit. After these simple changes you amp should sound far better than it does now.

Optional Tools/Supplies:

Silicone glue.
9/16” nut driver or socket wrench.
1/2” nut driver or socket wrench.
Fine point Sharpie or similar permanent marker.
Masking Tape

Skills required:

Basic soldering skills .
Basic knowledge of electronics.
The Ability to follow instructions.

Resistor Guide

BLACK 0 K
BROWN 1 N
RED 2 R
ORANGE 3 O
YELLOW 4 Y
GREEN 5 G
BLUE 6 B
VIOLET 7 V
GREY 8 E
WHITE 9 W

If you ordered the Supreme kit your kit will also have the following:

22/25 RAD, QTY 2
22/500 AXIAL, QTY 3

NOTE: Capacitor voltage values may vary some from that specified.

Parts

33K Resistor 1/4W (Code: OOKRN)
270 pf MLCC CAP (Code: 270)
.1 uf WIMA 10mm box cap (Code: 0,1)
.015 uf WIMA 10mm box cap (Code: 0,015)
100 uf FILTER CAP (Code: 100UF/450V)
SW INPUT JACK
CST Reverb Mod (Code: Custom)
330 Power Resistor (Code: 330)

Tools

Temperature Controlled Soldering Iron and Solder
Solder Sucker and or de-soldering braid
Multi-Meter Capable of reading 500V DC
Phillips Screwdriver
Pliers with insulated handle
Wire Cutters
Safety Goggles
Alligator Leads

Step 1

WARNING: Performing these modifications WILL void the warranty on your amplifier. Working on amps can be deadly if you do not take proper precautions. Follow these instructions very carefully. If you are at all unsure of your ability to perform these modifications STOP and take your amp to a professional to be modified. You can also send your amp to me for modification.
No Warranty: This kit and the instructions included do not include any warranty of any kind. The modifications to your amplifier are at your own risk and you agree to completely hold harmless the seller of this kit against any and all claims. Tube amplifiers contain parts that operate at very high temperatures and deadly voltages. If you are not sure of your abilities to perform these modifications then do not perform them.

Prep the Amp

Instructions: Follow these instructions to the letter.
It’s a good idea to have all the parts and tools you will need laid out before starting in a well lit area.
Unplug the Amp and put on your safety goggles.

Step 3

Remove the chicken head control knobs and the washer holding the input jack with a 9/16” in nut driver or socket wrench.

Step 4

Remove the 6 screws holding on the back panel and remove the panel. If the back panel does not come off easily you can loosen the two chassis screws on the side of the amp about a half turn each.

Step 5

Drain the Filter Caps: The filter caps on your amp may be drained already but you must follow these steps to insure that any deadly voltage is removed from the amp before you begin work. Test the voltage across the filter caps. Set your multi meter to DC and measure the voltage across the positive and negative side of any of the filter caps. If the voltage reads higher than 10 volts, then you need to drain the caps.

Step 6

Using the insulated pliers to hold the power resistor touch one lead of the included 5W power resistor to the negative side of the filter cap and the other to the positive side, leave the leads in contact with the power resistor for 30 seconds and measure again. Keep repeating this step until the voltage reads below 10V.
Capacitors can recharge even when the amp is not plugged in!!!
After the filter caps are drained take an alligator lead and clip one end to the positive lead of the filter cap and the other end to the chassis and leave it there until you are done with your work. This will prevent the filter caps from re-charging.

Step 7

Clip all of the wire ties.

Step 8

Remove the LED from the top of the chassis, it should just unsnap when pulled while squeezing the white LED holder.

Step 9

Remove the nuts holding the jack for the speaker and FAT switch with a ½” nut driver or socket wrench. Note: The lock nuts for these jacks are inside the chassis between the jack and chassis, not between the nut and the chassis.

Step 10

Unplug all the power and output transformer leads by pulling firmly with steady pressure gently rocking the lead terminal from side to side. As you remove each lead mark the wire with masking tape and a Sharpie so you will know where to plug it back in when you are putting the amp back together. I just mark the terminal number on the lead.

Step 11

Remove all seven (7) circuit board screws.

Expose the circuit board

Pull gently on the board to make sure that it is not stuck to any of the standoffs that were holding the circuit board screws. Move the small board holding the jacks for the speaker and FAT switch out of the way, hanging over the edge of the chassis.

Step 13

Press the wires from the output transformer and reverb as far away from the board and as close to the chassis as possible. The board needs to come down toward the bottom of the amp before the pots will clear the top of the chassis as shown.

Step 14

Using gentle pressure on the top of the circuit board press the board down making sure it is not getting hung up on any wires until all the pots have cleared the top of the chassis then pull the board to you to expose the underside. DO NOT PUT PRESSURE ON ANY COMPONENTS INCLUDING THE FILTER CAPS WHEN MOVING THE BOARD.

Removing the Old Components

Be careful that your soldering iron is not too hot. The solder pads on the PCB are prone to curling and separating from the PCB, so remove the components with care, never using force. It is best to clip the component leads on the front of the PCB then remove the remainder of the lead and solder from the back side carefully with a solder sucker and de-soldering braid. This is an example of a pad that has come off the board during de-soldering. The trace next to the pad is carefully scraped to expose the copper and the lead will be bent and soldered to the trace.

Step 16

Remove the Bass Cap C11.

Step 17

Remove the Mid Cap C13

Step 18

Remove the Treble Cap C25, this one is difficult to get to.

Step 19

Remove the bias resistor R31

Step 20

Remove C20, just under the fat switch, R40 and R56

Step 21

Remove C15, it’s the largest cap on the board.

Step 22

Remove the Input Jack. This part is snapped into the PCB then soldered, I find it easiest to remove the solder from the back side first then remove the jack. You can also clip the leads on the solder side of the PCB after the solder is removed to aid in removing the input jack.
CAUTION: The circuit board is very fragile and the solder pads can easily become separated from the circuit board if extreme care is not used. Carefully examine each solder pad after removing the component to make sure the solder pad is intact. If the solder pad is damaged you will need to carefully scrape away the solder mask of the trace next to the hole, bend the lead to the trace and solder directly to the trace. Remember to always check continuity with your multi-meter to insure that you have a good connection between components.

Adding the New Components

The leads of the components are pre-bent to match the holes in the PCB. The orange drops caps and bias resistor do not have polarity so it does not matter which lead is installed in which hole. The filter cap and input jack must be installed exactly per instructions.

Step 24

At R29 which is the Mid Tone Pot, bridge pins 2 and 3 which are the hole closest to the master volume pot and the center hole. Use any clipped lead for this function. You can also bridge the pins on the solder side of the board.

Step 25

Install the prewired reverb mod which consists of C20 with a lead to R40 (430K resistor) joined to R56 (470K resistor). Install the kit as shown with the lead to C20 installed in the right side of where the old component used to be, the other leg of C20 will have a lead to the right side of R40 and the leg of R56 on the left side. R40 gets replaced as it was before and the open leg of R56 goes in the top hole of where it used to be.

Step 26

At C25 install the 270pf MLCC cap.

Step 27

At C11 install the .1 uF WIMA box cap.

Step 28

At C13 install the .015 uF WIMA box cap.

Step 29

At R31 install the 33K bias resistor.

Step 30

At C15 install the 100uf/450v filter cap, minding the polarity.

Step 31

Solder the wires included in your kit to the jack as shown. The green ground wire should bridge the two eyelets on the left. The input jack should be wired with the ground (green) wire in the hole closest to the top of the board and the lead wire at the hole closest to the bottom.
Ground
TIP
Ground Lug also connected to Switch Lug

Step 32

If you purchased the complete mod kit you can skip to the next page putting the amp back together.
If you purchased the supreme mod kit replace all the electrolytic caps as indicated below.
Position Value
C3 22/35
C4 22/35
C17 22/500
C18 22/500
C10 22/500
To the left is the chart for all the caps and where they go with position numbers for both the USA and Mexican made amps. Below is the board layout for each amp. Make sure you mind the polarity of the caps, they must be inserted the right way or your amp will have problems which could be serious.
The circuit board will have a “+” sign closest to the hole where the positive lead of the cap goes. The axial caps have an indented ring on the positive side and a “-” sign in an arrow pointing to the negative side. For radial caps the longer lead is the positive side and there is a “-” sign in a band closest to the negative terminal.
NOTE: Capacitor voltage values may vary some from that specified.

Step 33

Install the radial 22uf / 50v capacitors by bending the leads to fit and minding polarity, the longer lead being the + Positive lead and the shorter being the - Negative.

Made in USA amps

100/450
22/500
22/500
22/500
22/35
22/35

Putting the Amp Back Together

Make sure that you have clipped all the excess leads from the solder side of the board and that all your new solder joints are secure before continuing.

Step 36

Carefully put the PCB back into place making sure that the pots, fat switch, and input jack all line up with the control panel.

Step 37

Replace all 7 of the PCB mounting screws.

Step 38

Re-install the LED

Step 39

Make sure all the ribbon cables have a smooth path with no kinks or sharp turns in the cable.

Step 40

Dress the Transformer leads!!! The poor lead dress is a large part of the noise in this amp. Instead of using a new cable tie to put the wires back like we found them we are going to use standard wiring techniques for proper tube amp design. Twist the green heater wires and position as shown. All other lead pairs should cross each other at 90 degree angles to avoid noise, hum, and oscillation.

Step 41

Re-install the small board with the Speaker and FAT switch jack, remember that the lock washer goes on the inside of the chassis between the chassis and the jack. Do not tighten the nut too much on the jacks or you will strip out the jacks.

Step 42

Make sure all the ribbon cables have a smooth path with no kinks or sharp turns in the cable.

Step 43

Glue ALL the caps to the board as well as the lead wire for the reverb. You can omit this step if you want but securing all the caps to the board will decrease the likely hood that they will get shaken loose in travel or by vibrations from the speaker.

Step 44

Use cable ties as necessary to make the cables nice as tidy.

Step 45

Plug the speaker back into the speaker jack.

Step 46

Re-install the control knobs and install the washer and nut on the new input jack.

Step 47

Replace the back panel and make sure to tighten the chassis screws if you loosened them.

Finishing up

ENJOY YOUR NEW AMP

Download the original PDF instructions