Epiphone Plexi SE PCB Manual

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Plexi SE PCB Valve Junior Conversion Version
Build Manual 3 August 009
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Plexi SE PCB
Valve Junior Conversion
Build Manual

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Plexi SE PCB Valve Junior Conversion Version
Build Manual 3 August 009
of 34
1. Introduction
This manual contains instructions to convert an Epiphone Valve Junior into a 5
watt single ended tube amplifier modeled after the legendary Marshall 1 5
Super Lead (Plexi). The heart of the amp is the Plexi SE PCB, which
conveniently lays out the circuit in a printed circuit board, PCB, package.
This manual provides instruction for the assembly of version 1.0 of the Plexi SE
PCB.
To simplify things,
• The Epiphone Valve Junior will be referred to as EVJ, and
This manual guides you through the conversion which includes:
• removing the stock EVJ chassis from its cabinet,
• extracting the stock EVJ pcb,
• drilling the chassis,
• assembling the Plexi SE PCB,
• installation and hookup of the Plexi SE PCB into the EVJ chassis,
• initial power-up and testing, and
• reinstalling the chassis back into the EVJ cabinet to finish up the job.
Here’s a look at the Plexi SE PCB if you haven’t purchased yours yet.
Version 1.0

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2. Electrical Shock Warning
Building tube amplifiers involves working with, or around, high voltages. Working
inside a tube amplifier can be dangerous if you don't know the basic safety
practices. Building, modifying, or repairing tube amplifiers should only be
performed by trained personnel.
3. Disclaimer of Liability
GuitarAmplifierPCBs.com assumes no liability or responsibility, under any
circumstance, for personal injury or damage to property or personal property.
GuitarAmplifierPCBs.com reserves the right to make design changes or
improvements without obligation to revise prior versions.
All specifications are subject to change without notice.

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Build Manual 3 August 009
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Table of ontents
1. Introduction ............................................................................................................................. 2
2. Electrical Shock Warning........................................................................................................ 3
3. Disclaimer of Liability .............................................................................................................. 3
4. Project Overview..................................................................................................................... 5
4.1 Here’s what you’ll need: ................................................................................................. 5
4.1.1 Parts: ..................................................................................................................... 5
4.1.2 Tools: ..................................................................................................................... 5
4.1.3 Supplies: ................................................................................................................ 5
5. Let’s get started…................................................................................................................... 6
5.1 Remove the EVJ chassis ............................................................................................... 6
5.2 Remove the EVJ’s Stock Components .......................................................................... 7
5.2.1 Remove the Tubes and Tube Retainers ............................................................... 7
5.2.2 Remove the Output Transformer from the Chassis .............................................. 7
5.2.3 Clean Up the Power Transformer.......................................................................... 8
5.2.4 Remove the Volume Potentiometer ......................................................................
5.2.5 Remove the Stock EVJ Circuit Board....................................................................
6. Prepare the Chassis ............................................................................................................. 11
6.1 Layout and Drill ............................................................................................................ 11
6.1.1 Layout the Plexi SE PCB for Drilling ................................................................... 11
6.1.2 Layout the Faceplate for drilling .......................................................................... 12
6.1.3 Drilling the Chassis.............................................................................................. 13
7. Plexi SE PCB ........................................................................................................................ 13
7.1 Assemble the Plexi SE PCB ........................................................................................ 13
7.1.1 PCB Standoffs ..................................................................................................... 14
7.1.2 Tube Sockets....................................................................................................... 14
7.1.3 Spade Connectors............................................................................................... 15
7.1.4 Diodes.................................................................................................................. 16
7.1.5 The “Plexi Preamp”.............................................................................................. 17
7.1.6 The “Cascade Preamp” ....................................................................................... 18
7.1.7 Wiring the Heaters............................................................................................... 1
7.1.8 Filter Capacitors .................................................................................................. 21
7.1. Bypass Capacitors............................................................................................... 22
7.1.10 Install the Remaining Components ..................................................................... 23
7.1.11 Input Jack ............................................................................................................ 24
7.1.12 Control Leads ...................................................................................................... 25
8. Putting it All Together............................................................................................................ 26
8.1 Install the New Components ........................................................................................ 26
8.1.1 Hookup the Plexi SE PCB ................................................................................... 26
. Turn on the Power ................................................................................................................ 2
.1 Check Your Work ......................................................................................................... 2
.1.1 Take a Break ....................................................................................................... 2
.1.2 Visual Inspection ................................................................................................. 2
.2 Power Up without Tubes .............................................................................................. 30
.2.1 The First Power Up.............................................................................................. 30
.3 Power Up with Tubes ................................................................................................... 32
.3.1 The Real Test ...................................................................................................... 32
10. Finish Things Up............................................................................................................... 34
10.1 Re-Assemble the Amp ................................................................................................. 34

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4. Project Overview
You’ll find that this is a fun project that can be completed by an inexperienced
builder in a day or two. We at GuitarAmplifierPCBs.com wrote this manual
specifically for the novice builder. So, anyone can do it. Really, it’s simple and it
doesn’t require expensive tools or complex equipment.
4.1 Here’s what you’ll need:
4.1.1 Parts:
• The Plexi SE PCB
• Misc. parts and components (refer to the Plexi SE PCB parts list
document for a full listing of what parts are needed and where you
can buy them)
4.1.2 Tools:
• Phillips head screw driver
• Standard screw driver
• Needle nose pliers
• Adjustable wrench
• Power drill (corded or cordless)
• 3/16”, 5/16”, 7/16”, 1/2" Metal cutting drill bit
• Unibit or punch (one 1¼ in. holes)
• Soldering iron (30-40 watt power rating)
4.1.3 Supplies:
• Fluxed solder
• Roughly 6 ft. 600v rated wire (22-18 AWG)

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5. Let’s get started…
OK. So you have all the parts and are ready to start. Let’s get that EVJ chassis
out of the cabinet.
5.1 Remove the EVJ chassis
Take off the back panel by removing the seven (7)
screws using a Phillips head screw driver.
Be careful pulling off the back panel.
They are typically stuck onto the chassis.
Slowly pull from each side to prevent the
tolex from lifting off the panel.
Locate the screw caps on the top of the EVJ. Using
a standard screw driver, the smaller the better, pry
up the caps until you can pull them out of their
sockets.
Remove the four chassis screws and pull out the
chassis.

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5.2 Remove the EVJ’s Stock omponents
A few of the stock EVJ parts have to be replaced. The tubes, tube retainers, and
stock circuit board need to be removed.
5.2.1 Remove the Tubes and Tube Retainers
It’s as simple as:
• lift the tube retainer off the power tube,
• turn the retainer shield until the spring lifts the shield off the per-amp tube,
• pull the tubes out, and
• unclip the power tube retainer.
These items can be discarded. They will not be used later in the build.
Now that the tubes and their hardware have been
removed we can take a look inside. This is a version 2
EVJ. You can tell by the green board and the rectified
DC heater supply. Version 3s have a black board.
5.2.2 Remove the Output Transformer from the hassis
One mounting location for the circuit
boards falls near the edge of the stock
output transformer. So, the output
transformer needs to be unscrewed
from the chassis and pushed aside.

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5.2.3 lean Up the Power Transformer
The stock EVJ’s power transformer provides leads for various mains supply
voltages. The extra leads are fastened to dead posts on the stock circuit board
for storage. We are going to clean up the inside of the chassis by pulling any
unused power transformer leads outside of the chassis cavity.
Disconnect these three (3) leads from
the stock EVJ circuit board.
Version 1 and 2 EVJ will
have 12v secondary taps
tied off and tucked inside the
chassis. These need to be
pulled outside and secured
as well.
After the leads are all pulled outside
the chassis, bundle them together and
tie them off.

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5.2.4 Remove the Volume Potentiometer
Unscrew the fastening nut from the volume
potentiometer.
Pull the volume pot back into the chassis. Leave it
connected to the stock EVJ circuit board.
The volume pot will be glued to the
chassis. This part will NOT be used later
in the build, so use whatever force you
need.
5.2.5 Remove the Stock EVJ ircuit Board
Disconnect these four (4) leads from
the stock EVJ circuit board.
Lift the ground wire from the star
ground post.
Unplug the input jack wire at the circuit
board.
Remove the six (6) screws fastening
the circuit board to the chassis.

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Remove the stock circuit board and
pull all of the power transformer leads
to the rim of the chassis.
Salvage the ground wire from the
stock circuit board for use later.
Unscrew the input jack and remove it
from the chassis. Set it aside for use
later.

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6. Prepare the hassis
6.1 Layout and Drill
The Plexi SE PCB, and control potentiometers require some drilling in the
chassis.
6.1.1 Layout the Plexi SE P B for Drilling
Since the Plexi SE PCB was designed to use the stock EVJ chassis standoffs,
the best method to mark the additional chassis holes is to install the board.
Lay the circuit board in the chassis, align the stock mounting studs, and install
two to three screws to fasten the board. With the board aligned to the chassis,
mark the four additional drill locations as shown.

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6.1.2 Layout the Faceplate for drilling
We worked with BNP Lasers to supply a professional grade faceplate for the
Plexi SE PCB. The information is on our web site. We highly recommend you
buy this faceplate and use it to layout the front panel drill locations. It’s much
more accurate to use the actual faceplate than it is to use a drill plan printout.
If you have a faceplate, align the faceplate with the front of the chassis and trace
each drill location.
If you must used a printed copy of the drill plan…
Always verify you are using the latest version of the drill plan posted on
GuitarAmplifierPCBs.com.
If you can not print onto 11x17” paper, you can take an electronic
version of the drill plan to Kinkos. They will print it professionally for
under $1.
Print out a copy of the VJ Conversion Drill Plan. Make sure it is printed to scale.
Cut away the unused portion of the
drill plan as shown. The mounting
hole near the back of the power
transformer is optional. It’s very close
to the rubber grommet. It’s not used in
the picture shown.
Align the drill plan to the chassis using
the alignment holes indicated on the
plan.
Affix the plan to the chassis with a
generous amount of white glue or tape.
Note: This is not a picture of the Plexi SE PCB drill plan. Instead, it is a
drill plan of a similar build. But it adequately represents the front panel of
the Plexi SE PCB. Since we used a BNP Lasers faceplate to mark the
front panel, we did not photograph the printed drill plan method.

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6.1.3 Drilling the hassis
Start by setting the center of each hole to be drilled. A center punch is often
used. Use the appropriate drill bit size as prescribed by the drill plan.
Deburr or countersink each hole to ensure no sharp edges will cause injury or
component failure.
7. Plexi SE P B
7.1 Assemble the Plexi SE P B
The anticipation has been killing you, hasn’t it? Finally, the assembly of the Plexi
SE PCB begins.
Did you check you make sure you have all of the
parts you’ll need to complete the assembly?
I wish I did.
The graphics used in the assembly of this circuit board will NOT be an
exact reflection of what your board will look like. Various parts will have
changed. The order of installation may vary from the picture and this
manual. Pictures were taken during various stages of the first production build.
This manual reflects the lessons learned during that build.
Don’t use the pictures to compare the progress of your build. Follow the
instructions at each stage of the build and compare your work when the manual
calls out a comparison check.

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7.1.1 P B Standoffs
Install the three (3) PCB standoffs on the back side of the board.
7.1.2 Tube Sockets
The tube sockets get installed on the back side of the board, too.
Insert the tube sockets as
shown. The noval ( -pin)
sockets only go in one way
(on the back side). However,
the octal (8-pin) socket
needs to be properly
oriented with the circuit
board. Position the notch in
this socket so that it is
pointing toward the two
noval sockets.
Solder them into the board.

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7.1.3 Spade onnectors
DO NOT install ANY male spade connectors in board.
The use of spade connectors with version 1.0 of this board may result in
arching between the spade connectors and the ground plane. This may
result in charring of the circuit board, poor amplifier performance,
transformer damage, and may pose a safety and/or fire hazard.
Again, DO NOT install ANY male spade connectors in board.
Instead of using spade connectors, the wire leads should be soldered directly to
the board in location marked above. Don’t worry about this now. It will be cover
later when the board is installed in the chassis.

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7.1.4 Diodes
Install the four (4) rectifier diodes.
Orient the diodes as indicated on the circuit
board.
Bend the leads over, insert them into the
board, and bend them to the side as shown.
Solder them to the board.
Trim the leads off at the top of each
solder pool, like this.

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7.1.5 The “Plexi Preamp”
To configure the Plexi SE PCB to have a 1 5 Super Lead “Plexi” preamp, install
the various components as shown in the picture below. Determine the
component value from the schematic and/or chassis layout drawing.
Notice how the leads of the 470K resistor and 250pF capacitor are inserted in the
Plexi solder pad locations.
Wiring the control potentiometers is discussed in a later section.

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7.1.6 The “ ascade Preamp”
To configure the Plexi SE PCB to have a Cascade (or JCM800) preamp, install
the various components as shown in the picture below. Determine the
component value from the schematic and/or chassis layout drawing.
Notice how the leads of the 470K resistor and 250pF capacitor are inserted in the
Cascade solder pad locations.
Wiring the control potentiometers is discussed in a later section.

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7.1.7 Wiring the Heaters
Cut two 12” lengths of wire and twist them together as shown.
Cut this twisted pair in half.

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Twist the wires together as shown.
Strip ¼” of the insulation off of the ends of the wires. Insert the leads into the
solder pad locations on the middle tube socket labeled “HTR” for heaters.