AkitikA GT-108 User manual

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GT-108 STEREO
AUDIO POWER AMPLIFIER
ASSEMBLY MANUAL
© 2022 AkitikA LLC
All rights reserved
Revision 1p05 December 23, 2022

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Table of Contents
Table of Contents ................................................................................................................ 2
Table of Figures .................................................................................................................. 3
Section 1: About This Manual ............................................................................................ 5
Who Should Attempt this Project? ................................................................................. 5
Tools You’ll Need........................................................................................................... 5
Project Overview ............................................................................................................ 5
Important Safety Notes ................................................................................................... 6
About Components ......................................................................................................... 6
Recommended Solder ..................................................................................................... 6
Warranty ......................................................................................................................... 6
Section 2: Building the Power Supply PCB ....................................................................... 7
Component Order............................................................................................................ 7
Install the Resistors ......................................................................................................... 7
Install the Diodes ............................................................................................................ 9
Identifying the glass body diodes ............................................................................... 9
Install the Last Resistor ................................................................................................. 11
Install the Small Capacitors .......................................................................................... 11
Install the Integrated Circuits ........................................................................................ 11
Install the Transistors .................................................................................................... 11
Install the Big Capacitors .............................................................................................. 12
Final Inspection of the Circuit Board............................................................................ 12
Power Supply Final Assembly ...................................................................................... 12
Section 3: Assembling the Amplifier Circuit Boards ....................................................... 15
Install the Resistors ....................................................................................................... 16
The following are 0.1% resistors .............................................................................. 18
Install the Diodes .......................................................................................................... 18
Small Capacitors ........................................................................................................... 19
Power Resistors ............................................................................................................. 20
Transistors ..................................................................................................................... 20
Install the Voltage Regulators ................................................................................... 21
Install the Speaker Coupling Capacitor ........................................................................ 21
Winding the Output Inductor ........................................................................................ 21
Install T8 and T9 ....................................................................................................... 22
Install the Polarized Capacitors ................................................................................ 24
Install Relay K1 ............................................................................................................ 24
Install Bias Pot .............................................................................................................. 24
Final Inspection of the Circuit Board............................................................................ 25
Install the Power MOSFETS and Mount the Amplifier Board to the Heatsink ........... 25
Section 4: Wiring it All Together ..................................................................................... 29
Attach the Power Supply Ground Wire ........................................................................ 29
Attach the Amplifier Module Power Wires .................................................................. 30
Attaching LEFT Module Power Wires ..................................................................... 30
Attaching RIGHT Module Power Wires .................................................................. 30
Prepare Input Shielded Cables ...................................................................................... 31

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Build the Ground Harnesses ......................................................................................... 31
Install the Feet ............................................................................................................... 32
Build the Ground Lifter ................................................................................................ 32
AC Power Wiring ......................................................................................................... 33
Install the RCA Jacks and Speaker Binding Posts .................................................... 35
Install the Speaker Binding Posts ................................................................................. 36
Connect the Speaker Binding Posts .............................................................................. 36
Amplifier Channels Final Wiring ............................................................................. 37
Installing Amplifier Modules into the Chassis ......................................................... 38
Transformer Preparation ........................................................................................... 38
Transformer Installation............................................................................................ 40
Testing the Power Supply ............................................................................................. 41
Final Amplifier Wiring ................................................................................................. 42
Wiring Amplifier Power ........................................................................................... 42
Fasten the Power Supply to the Chassis ................................................................... 42
Install the Wall and Fasten the Transformer Down .................................................. 43
Visual Inspection ...................................................................................................... 45
Section 5: Testing It Out, Setting the Bias, and Installing the Top .................................. 45
Power-Off Tests ............................................................................................................ 45
Power-On Tests ............................................................................................................. 45
Set the Bias Current .................................................................................................. 46
Install the Top ........................................................................................................... 47
Section 6: Using the GT-108 ............................................................................................ 47
Section 7: Specifications and Schematics ......................................................................... 47
Section 8: Theory of Operation ......................................................................................... 51
Power Supply ................................................................................................................ 51
Bulk DC Supply ........................................................................................................ 51
Pass Transistor .......................................................................................................... 51
Regulator Control Circuits and their Power Supply ................................................. 51
Current Limiting ....................................................................................................... 51
Start-up Circuits ........................................................................................................ 52
Amplifier Modules ........................................................................................................ 52
Reference Voltage Generator .................................................................................... 53
Gain Setting Circuits ................................................................................................. 53
Relay Control Circuit .................................................................................................... 53
Appendix 1 - Resistor Color Code .................................................................................... 54
Appendix 2 - Preparing a Shielded Cable End ................................................................. 55
Table of Figures
Figure 1-Component side of power supply PCB before loading ........................................ 7
Figure 2-Installing resistors ................................................................................................ 8
Figure 3-Form the cathodes of D4-D7 as shown (banded end denotes cathode) ............. 10
Figure 4-Pinout of U1, viewed from the top. Pin 1 is shown by either u-shaped mark or
circle .................................................................................................................................. 11
Figure 5-Note the rounded brackets used for the power supply heatsink and PCB ......... 13
Figure 6-Placing thermal compound on Q5 ...................................................................... 13

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Figure 7-Installing Q5 on the heatsink and to the PCB .................................................... 13
Figure 8-Remember to solder Q5's leads .......................................................................... 14
Figure 9-Assembled Power supply mounted on heat sink ................................................ 14
Figure 10-Empty the amplifier components into a soup bowl .......................................... 15
Figure 11-Close-up of bare Amplifier PCB ...................................................................... 15
Figure 12-Note flat side of LED package sits under number 1 and 2 in LED1 and LED2
........................................................................................................................................... 19
Figure 13-Capacitor Identification guide .......................................................................... 19
Figure 14-Vertical resistor mounting ................................................................................ 20
Figure 15-Starting to wind the output inductor................................................................. 22
Figure 16-finishing step for the output inductor ............................................................... 22
 Figure 17-Spreading thermal compound on driver transistors ................................. 23
 Figure 18-Do you have a 273 or a plain heatsink? ................................................... 23
Figure 19-273 style mounting (left side) and plain mounting (right side) ........................ 23
Figure 20-Installing T8 and T9 on the circuit board ......................................................... 24
Figure 21-Mounting assembled PCB to heat sink ............................................................ 25
Figure 22-mounting the PCB to the heat sink ................................................................... 26
Figure 23-Spread a thin film of thermal compound on Q4 (IRFP9140), but keep it off the
leads .................................................................................................................................. 26
Figure 24-threading the screwdriver to Q4’s mounting screw ......................................... 27
Figure 25-Q5 with 6-32x1/2" mounting screw and insulator ........................................... 27
Figure 26-first amplifier channel completed and installed ............................................... 28
Figure 27-from left to right, #6 lug, #10 lug, RCA connector ground lug ....................... 29
Figure 28-power supply with attached ground wire ......................................................... 29
Figure 29-Right and Left Channel Designations .............................................................. 30
Figure 30-shielded cable for left channel. Right channel has overall length of 11 inches.
........................................................................................................................................... 31
Figure 31-Channel Grounding Harness ............................................................................ 31
Figure 32-Install the feet here ........................................................................................... 32
Figure 33-Ground Lifter PC Board ................................................................................... 32
Figure 34-Ground Lifter PCB with ground wires connected............................................ 33
Figure 35-FASTON connector ......................................................................................... 33
Figure 36-Wiring IEC power connector line and neutral ................................................. 33
Figure 37-Power wiring with IEC connector installed into chassis .................................. 34
Figure 38-Installing power wires on the switch ................................................................ 35
Figure 39-Installing RCA jacks ........................................................................................ 35
Figure 40-Install both left and right binding posts............................................................ 36
Figure 41-I/O Connector wiring ....................................................................................... 37
Figure 42-Wiring toroidal transformer for 120 Volt operation ........................................ 39
Figure 43-Western Union splice has diameter less than the insulated wires and no sharp
points ................................................................................................................................. 40
Figure 44-connecting up the power transformer ............................................................... 40
Figure 45-Showing active and storage locations in the fuse drawer ................................ 41
Figure 46-Grounding Stud Detail ..................................................................................... 43
Figure 47-Routing the wires thru the slots in the wall ...................................................... 44
Figure 48-Toroidal Transformer Mounting ...................................................................... 44

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Figure 49-Measuring bias voltage, left channel shown .................................................... 46
Figure 50-Power Supply Schematic.................................................................................. 48
Figure 51-Amplifier Module Schematic ........................................................................... 49
Figure 52-Overall Wiring (power transformer is wired for 120 VAC) ............................ 50
Figure 53-demonstrating the resistor color code .............................................................. 54
Figure 54- Shield wire end prep completed ...................................................................... 56
Section 1: About This Manual
This manual gives the information you need to build Akitika LLC’s GT-108 Stereo
Power Amplifier. The GT-108 delivers a total of 120 Watts in the form of 60 Watts per
channel into 8-Ohm loudspeakers.
Who Should Attempt this Project?
You can build this kit if you can:
1. solder (using normal rosin core solder and a soldering iron).
2. use simple hand tools like screwdrivers, wire cutters, and pliers.
3. read and follow directions.
It helps if you:
1. know a bit about electronics, or
2. have a friend who knows a bit about electronics
3. can get to YouTube to watch a few helpful videos about the assembly process
Tools You’ll Need
You’ll need the following tools:
1. Phillips screwdriver (#1 and #2)
2. pliers or nut drivers suitable for #6 hardware (5/16” nut driver or hex wrench).
Note that the 5/16” nut driver is also the correct size for the speaker binding post
nuts.
3. needle nose pliers (helpful, but not strictly necessary)
4. pencil type soldering iron of 25 to 50 Watts (no huge honking soldering guns or
blowtorches)
5. wire cutters and strippers
6. multi-meter (strongly recommended to check resistor values!)
7. magnifying glass, if you’re over 42!
Project Overview
The project consists of the following steps:
1. Build the Power Supply Regulator Board
2. Build left and right channel amplifier circuit boards.
3. Install and wire the circuit boards, switches, and connectors into the chassis.

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Important Safety Notes
By purchasing, using, or assembling this kit, you have agreed to hold AkitikA, LLC
harmless for any injuries you may receive in its assembly and/or use. To prevent
injuries:
 Wear safety glasses when soldering to prevent eye injuries.
 Always unplug the power before working on the amplifier.
 Large capacitors hold lots of energy for a long time. Before you put your hands
into the amplifier:
o Pull the AC plug!
o Wait 2 full minutes for the capacitors to discharge!
 Remove jewelry and rings from your hands and wrists, or anything that might
dangle into the amplifier.
 If working in the amplifier, keep one hand in your pocket, especially if you’re
near the power supply or power supply wires. This can prevent serious shocks.
 Build with a buddy nearby. If you’ve ignored all the previous advice, they can
dial 911 or get you to the hospital.
About Components
We reserve the right to make design and/or component changes at any time without
prior notification.
Recommended Solder
The kit must be assembled with 63/37 (tin/lead) Rosin Core solder1. The recommended
diameter is 0.031 inches. Kester p/n 24-6337-8800 solder is a very good choice.
Warranty
With the exception of fuses, Akitika will replace for free any parts of a correctly
assembled GT-108 that fail within one year of the date of purchase when the amplifier
has been used in home stereo applications. It is the responsibility of the kit builder to
install the replacement part(s). This warranty applies to the original purchaser only. It
does not apply to units that have been physically or electrically abused, modified without
prior factory authorization, or assembled with other than 63/37 Rosin Core solder.
Akitika LLC’s liability shall in no event exceed the cost paid to Akitika LLC for the kit.
1 https://en.wikibooks.org/wiki/Practical_Electronics/Soldering has entries for both 60/40 and 63/37 solder
blends. This reference calls out a preference for 63/37 because it is a eutectic solder, which it says is less
prone to cold solder joints.

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Section 2: Building the Power Supply PCB
This section details the process of building the power supply circuit board. We start with
an overview on this page. The specifics you need to start building begin on the next page.
The bare power supply PCB is shown in Figure 1.
Figure 1-Component side of power supply PCB before loading
Begin by carefully emptying the contents of the envelope marked “GT-102 PSU Module”
into a broad soup bowl, as shown below. In general, you’ll start with the components that
lay closest to the board, working your way towards the taller components. You will:
1. Install the resistors (all but one
power resistor, left for later)
2. Install the diodes
3. Install the capacitors
4. Install the integrated circuits.
5. Install the transistors
6. Install the pass transistor Q5 as
you mount the PCB to the heat
sink.
Component Order
You’ll notice that the component designations in the directions don’t go exactly in order.
We have grouped them so that all components with the same value appear together. This
makes assembly easier. You’ll find in the parts kit that similar parts, e.g. 3 1K resistors,
are typically (though not always) taped together.
Install the Resistors
In general, you install the resistors by placing the body on the silk screen side of the
board, and the leads through the indicated holes. Bend the leads over on the back of the
board to keep the resistors from falling out until your solder them in place. Try to bend
the leads in a direction that won’t lead to solder bridges between traces that should
remain disconnected.

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Figure 2-Installing resistors
We recommend the following procedure:
1. Insert all the resistors of the same value, e.g. R2, R3 and R4.
2. Bend the leads as described above.
3. Solder the leads on the back of the board.
4. Clip the leads.
Track your progress by placing a check-mark in the done column as you install each
resistor. Check resistor values with a meter, and by reading the color code (See
Appendix 1). Orient the resistor with the fat brown band on the right, then you can read
both the Color Code column and the resistor from left to right.
The GT-108 is based on parts from the GT-102, with the addition of a number of
parts that are unique to the GT-108. You will therefore see that there are two power
supply envelopes. One marked, “GT-102 PSU Module Rev 6”, and the second
marked “GT102=>108 PSU Delta Rev 1”. The parts marked with an asterisk come
from the Delta parts bag.
¼ Watt, 1% resistors (if you use a lead
-
bending jig, use the 0.45” width)
Designation
Value
Color code
Done? (

)
R2
1K
Brown, Black,
Black, Brown, Brown
R
3*
6K81
Blue, Gray, Brown, Brown, Brown
R
4
1K
Brown, Black, Black, Brown, Brown
R6
10K
Brown, Black, Black, Red, Brown
R26
10K
Brown, Black, Black, Red, Brown
R27
10K
Brown, Black, Black, Red, Brown
R8
20K
Red, Black,
Black, Red, Brown
R9
20K
Red, Black, Black, Red, Brown
R1
*
1
6
K
2
Brown,
Blue, Red, Red
, Brown
R5
1
4
0K
Brown,
Yellow
, Black
,
Orange, Brown
R7
5K76
Green, Violet, Blue, Brown
, Brown
R1
0
26K1
Red, Blue, Brown, Red, Brown
R21
3K48
Orange,
Yellow, Gray
, Brown, Brown
R13
3M01
Orange, Black, Brown, Yellow, Brown
R22
120
K
Brown, Red, Black, Orange, Brown
R23
100
Brown, Black, Black, Black, Brown
R24
9
5K3
White, Green, Orange, Red, Brown
R2
5
This location remains empty
We will defer installation of R12, a 0.1 Ohm power resistor, for a bit later in the assembly
process.

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Install the Diodes
Now install the diodes. Be careful to observe the polarity markings on the diodes. You’ll
notice that one end of the diodes has a band. That band indicates the cathode of the diode.
Match the banded end of the diode with the banded end of the silk screen. The following
information should help you identify the diodes.
Identifying the glass body diodes
The glass body diodes have the following identifying marks. If your vision is like mine,
you may need good light and a magnifying glass.
 1N4148, D8-D11, has the number 48 visible
 BZX55B33B, D1 and D14, has the number 33 visible
 BZX79-B10, D2, has the number 10 visible
Also, typically when these diodes are packed, you can preliminarily identify the types
because:
 The 4 1N4148 diodes will typically be taped together
 The 2 BZX55B33B diodes will typically be taped together
 The 1 BZX79-B10 diode will be by itself
D4-D7, opaque
black plastic
package
D12, D13,
package type
DO-41, opaque
black plastic
package
D1, D2, D8-
D11, D14,
package type
DO-35, glass
package
Keep track of the diodes as you install them using the following table. Remember to
watch the polarity of the diodes, matching the banded end of the diode to the banded end
of the silk screen.

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Designation
Type, Package
Description
Done? (

)
D1
BZX55B
33
, DO
-
35
33
Volt 2% zener
diode
D1
4
BZX55B33, DO
-
35
33 Volt 2% zener
diode
D2
BZX
79
-
B10, DO
-
35
10 Volt 2% zener
diode
D8 1N4148, DO-35 0.2 A, 100 PIV, switching
diode
D9 1N4148, DO-35 0.2 A, 100 PIV, switching
diode
D10 1N4148, DO-35 0.2 A, 100 PIV, switching
diode
D11 1N4148, DO-35 0.2 A, 100 PIV, switching
diode
D12 1N4004, DO-41 1A, 400 PIV, rectifier
diode
D13 1N4004, DO-41 1A, 400 PIV, rectifier
diode
Warning: Don’t cut the leads of D4-D7 until after the leads have been formed and the
diodes have been soldered into the PCB.
Form the leads of D4-D7 as shown in Figure 3. Solder one lead of each diode while
attempting to keep the body perpendicular to the plane of the board. Ideally, the anodes
of D4-D7 should sit about 1/8” to 1/16” off the board.
Figure 3-Form the cathodes of D4-D7 as shown (banded end denotes cathode)
Designation
Type, Package
Description
Done? (

)
D4 6A4, R-6 6A, 400 PIV, rectifier
diode
D5 6A4, R-6 6A, 400 PIV, rectifier
diode
D6 6A4, R-6 6A, 400 PIV, rectifier
diode
D7 6A4, R-6 6A, 400 PIV, rectifier
diode

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Install the Last Resistor
R12 should be air-mounted, with the bottom of its body about 1/8” above the PCB.
2 Watt, 5% resistor
Designation
Value
Marking
Done? (

)
R1
2
0.1
0.1
Install the Small Capacitors
Now install the small capacitors:
C1, C2, and C3 are polarized, showing a minus sign (-) on the negative end of the
capacitor. Make sure that the minus sign faces away from the plus sign (+) marked on
the silk screen for each of the capacitors.
Designation
Value
Description
Done? (

)
C1 10 µF 100V electrolytic (polarized), cylindrical
shape
C2 10 µF 100V electrolytic (polarized), cylindrical
shape
C3 10 µF 100V electrolytic (polarized), cylindrical
shape
C4
100 nF
50V, Z5U, +/
-
20%, marked 104
C5 10 nF 400V, film, 20%, box shaped, marked
10nk400
Install the Integrated Circuits
Install the integrated circuits.
Orient U1 so that pin 1 on the chip matches pin 1, the square pin, on the PCB.
Orient U2 to match the silk screen outline.
Designation
Type
Description
Done? (

)
U1
LM258 or LM358
8 Pin DIP
U2
TL431, TO
-
92
Programmable shunt
regulator
Be careful! Don’t confuse U2 with the transistors. Make sure you see “431” on U2.
Figure 4-Pinout of U1, viewed from the top. Pin 1 is shown by either u-shaped mark or circle
Install the Transistors
You may have to spread the outside leads a bit to make it easier to insert them into the
board. Do not install Q5 now. It will be installed later.
Orient the transistor so its body shape matches the silk-screen outline. Leave the top of
the transistor about ½” off the board! The lead length prevents stress on the body and
keeps the transistor safe from too much heat during the soldering operation.

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Designation
Type
Description
Done? (

)
Q2
2N3904
, TO
-
92
60 V NPN
bipolar transistor
Q3
2N3904
, TO
-
92
60 V NPN
bipolar transistor
Q4
2N3904
, TO
-
92
60 V NPN
bipolar transistor
Q6
2N5401
, TO
-
92
150 V PNP
bipolar transistor
Q7
2N5401
, TO
-
92
150 V PNP
bipolar transistor
Don’t use too much solder on the transistor leads. This is one place where the spacing
is close enough that extra solder might cause short circuit between two leads on a
given transistor. Note that Q4’s collector and base are connected by the circuit board.
Install the Big Capacitors
Install C6 and C7.
C6 and C7 are polarized. Be careful to get the polarity correct. Double check their
polarity before you solder them in place. They have large terminals, so it will take a
fair amount of heat and solder.
Designation
Type
Description
Done? (

)
C6
10,000 µF/80V
Electrolytic capacitor
C7
2200
µF/1
6
0V
Electrolytic capacitor
Cauti
on: C6 and C7 are not identical!
Be sure to put each in its correct place!
Final Inspection of the Circuit Board
After you’ve taken a break, look over your completed circuit board one more time.
Looking on the component side, double check:
1. polarity (banded end) of diodes matching the banded end on the silk screen.
2. polarity of C1, C2, C6, C7 and C7 (minus sign away from the indicated plus sign
on the silk screen).
If you get one of these polarities wrong, there is a good chance that the power supply
won’t work, or there will be damage when you power it up.
Look at the solder side of the board. Make sure that:
1. All component leads are soldered (it’s easy to forget one or two, and that will
cause either unreliable operation, or no operation at all!)
2. There are no solder bridges between pads which should be isolated. Solder
bridges may stop the power supply from working correctly.
Power Supply Final Assembly
In this section, you’ll install Q5 while you mount the power supply circuit board on its
heatsink. Some kits have a silver colored finned heatsink. Other kits have a black-
anodized heat sink. In either case, the power supply heatsink is about 5 3/8” wide. The kit
has one other silver colored double wide heatsink that that will be used for both amplifier
modules2.
2 Some kits may have instead been supplied with two separate black 4” wide heatsinks, one for each
amplifier module.

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Figure 5-Note the rounded brackets used for the power supply heatsink and PCB
1. Use two 6-32x3/8” screws to fasten a pair of mounting brackets to the heat sink,
placing the screw through the 9/64” clearance hole in the bracket. Note that one
hole in the mounting bracket is a 9/64” clearance hole, the other hole is threaded
to accept a 6/-32 screw. Don’t tighten the screws yet. Make sure you use the
brackets with the rounded edges to mount the power supply PCB.
2. Use two 6-32x1/4” screws to fasten the PCB to the mounting brackets and heat
sink, see Figure 9. Don’t tighten the screws yet.
3. Place a thin film of thermal compound on the metal tab surface of Q5 as shown in
Figure 6. Inset Q5 into the circuit board as shown in Figure 7. Secure it in place
with a #6-32x1/2” screw, just finger tight for now. The side with the thermal
compound must attach to the heat sink.
4. The previous step uses just a small amount of the thermal compound in the
squeeze tube. Put the squeeze tube in a safe place, as you may choose to use the
rest of its contents when you Fasten the Power Supply to the Chassis.
Figure 6-Placing thermal compound on Q5
Figure 7-Installing Q5 on the heatsink and to the PCB

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Figure 8-Remember to solder Q5's leads
5. Line up the PCB, brackets, and Q5 so everything is square and fits comfortably,
then tighten the 5 screws:
a. 2 that hold the brackets to the PCB
b. 2 that hold the brackets to the heatsink
c. 1 that holds Q5 to the heatsink.
6. Solder the three leads of Q5 on the solder side of the board, and clip the leads.
Inspect your work from both the top and the bottom of the board to make sure
there are no solder bridges between the leads.
Once the power supply/heat sink module is completed, set it aside and proceed to the next
section, where you will build the amplifier modules.
Figure 9-Assembled Power supply mounted on heat sink
You may have 1 or 2 fuse clips left over at the end of this section. Put them in your
spares cupboard for a future project.

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Section 3: Assembling the Amplifier Circuit Boards
This section details the process of building the amplifier module circuit boards. We start
with an overview on this page. Begin by carefully emptying the contents of one of the
envelopes marked “GT-102-108 Amp Delta Kit” and one “GT-102 Amplifier Module”
into a broad soup bowl, as shown in Figure 10. We’re doing this as the 108 is a delta built
upon the 102.
In general, you’ll start with the components that lay closest to the board, working your
way toward the taller components. You will:
1. Install the resistors
2. Install the small capacitors
3. Install the diodes
4. Install the LEDs (watch polarity)!
5. Install the medium size capacitors
6. Install the transistors
7. Install the big capacitor
8. Wind and install the output
inductor.
9. Install the relay.
10. Install the power transistors
Figure 10-Empty the amplifier components into
a soup bowl
Figure 11-Close-up of bare Amplifier PCB

Page 16 of 56
Install the Resistors
In general, you install the resistors by placing the body on the silk screen side of the
board, and the leads through the indicated holes. Bend the leads over on the back of the
board to keep the resistors from falling out until your solder them in place. Try to bend
the leads in a direction that won’t lead to solder bridges between traces that should
remain disconnected.
We recommend the following procedure:
1. Insert all the resistors of the same value, bending the leads as described above.
2. Solder the leads on the back of the board.
3. Clip the leads just a bit beyond the end of the top of the solder joint.
To track your progress, place a checkmark in the done column as each resistor is
installed. Check resistor values with a meter and by reading the color code. Orient the
resistor with the fat brown band on the right, then you can read both the Color Code
column and the resistor from left to right.
Assembly begins with installing the small (1/4 Watt) resistors and will progress to the
larger (higher wattage) resistors.
Important Notes:
1. The GT-108 is a dual channel amplifier. You will build two identical channels.
We’ve provided two Done columns, Done1 for the first channel you build, and
Done2 for the second channel. We recommend that you build the channels one
at a time, completing the first channel, then returning to this point to build the
second channel.
2. With the exception of two resistors, a 1K 0.1% and a 20K 0.1% resistor, all of
the ¼ W resistors have 1% tolerance. The two 0.1% resistors are listed at the
end of the table.
3.
S
ave time and make
it
neater if you use the 0.4” setting on a lead
-
bender.
Designation
Value
Color Code
Done 1
Done2
R22
24.9
Red, Yellow, White, Gold, Brown
R34
49.9
Yellow, White, White, Gold, Brown
R51
49.9
Yellow, White, White, Gold, Brown
R16
88.7
Gray, Gray, Violet, Gold, Brown
R10
100
Brown, Black, Black, Black, Brown
R11
100
Brown, Black, Black, Black, Brown
R14
100
Brown, Black, Black, Black, Brown
R15
100
Brown, Black, Black, Black, Brown
R52
100
Brown, Black,
Black, Black, Brown
R21
200
Red, Black, Black, Black, Brown
R28
332
Orange, Orange, Red, Black, Brown
R29
332
Orange, Orange, Red, Black, Brown
R18
499
Yellow, White, White, Black, Brown
R3
1K
Brown, Black, Black, Brown, Brown

Page 17 of 56
R13
1
K
Brown, Black, Black,
Brown
, Brown
R12
1K
Brown, Black, Black, Brown, Brown
R20
1K
Brown, Black, Black, Brown, Brown
R31
1K
Brown, Black, Black, Brown, Brown
R42
1K
Brown, Black, Black, Brown, Brown
R53
1K
Brown, Black, Black, Brown,
Brown
R54
1K
Brown, Black, Black, Brown, Brown
R37
1K5
Brown, Green, Black, Brown, Brown
R48
1K5
Brown, Green, Black, Brown, Brown
R36
8K
06
Gray, Black, Blue, Brown, Brown
R50
8K
06
Gray, Black, Blue, Brown, Brown
R2
10K
Brown,
Black, Black, Red, Brown
R5
10K
Brown, Black, Black, Red, Brown
R26
10K
Brown, Black, Black, Red, Brown
R45
10K
Brown, Black, Black, Red, Brown
R61
10K
Brown, Black, Black, Red, Brown
R63
10K
Brown, Black, Black, Red, Brown
R6
4
12K7
Brown, Red, Violet, Red, Brown
R24
15K
Brown, Green, Black, Red, Brown
R39
15K
Brown, Green, Black, Red, Brown
R41
15K
Brown, Green, Black, Red, Brown
R43
15K
Brown, Green, Black, Red, Brown
R44
15K
Brown, Green, Black, Red, Brown
R23
20K
Red, Black, Black, Red, Brown
R60
20K
Red, Black, Black, Red, Brown
R9
24K9
Red, Yellow, White, Red, Brown
R17
40K2
Yellow, Black, Red, Red, Brown
R19
40K2
Yellow, Black, Red, Red, Brown
R27
40K2
Yellow, Black, Red, Red, Brown
R46
40K2
Yellow
, Black,
Red
,
Red
, Brown
R55
49K9
Yellow, White, White, Red, Brown
R62
49K9
Yellow, White, White, Red, Brown
R25
68K1
Blue, Gray, Brown, Red
, Brown
R4
100K
Brown, Black, Black, Orange, Brown
R6
100K
Brown, Black, Black,
Orange, Brown
R47
100K
Brown, Black, Black, Orange, Brown
R59
100K
Brown, Black, Black, Orange, Brown
R1
*
14
3
K
3
Brown,
Yellow
,
Orange
, Orange, Brown
R49
1MEG
Brown, Black, Black, Yellow, Brown
R56
1MEG
Brown, Black, Black, Yellow,
Brown
R
65
1MEG
Brown, Black, Black, Yellow, Brown
3 Sets output quiescent voltage to 38 Volts DC, which is correct for nominal 76 Volt rail.

Page 18 of 56
The following are 0.1% resistors
Designation
Value
Color Code
Done 1
Done2
R8 1K,
0.1%
Brown, Black, Black, Brown, Violet;
In some kits, the resistor may have
numbers: 1K 0.1% on a brown body
R7 20K,
0.1%
Red, Black, Black, Red, Violet
In some kits, the resistor may have
numbers: 20K 0.1% on a brown body.
Install the Diodes
Now install the diodes. Here’s what the 1N4004 diodes look like (not to scale):
And the rest of the diodes, with the exception of the LEDs, look like this:
Be careful! Diodes have a polarity. Make sure the band on the diode aligns with
the banded end of the silk screen!
Designation
Value
Marking
, type
Done1
Done2
D1
1N4004
4004
, 1 amp 400 V diode
D
9
1N4004
4004, 1 amp 400 V diode
D12
TZX6V2C
6V2, 6.2 V Zener diode
D2
BZX79
-
10
B10, 10 V Zener diode
D6
BZX79
-
10
B10, 10 V Zener diode
D7
BZX79
-
10
B10, 10 V Zener diode
D
8
BZX79
-
10
B10
, 10 V Zener diode
D
11
BZX79
-
10
B10
, 10 V Zener diode
D4
BZX79
-
C15
C15, 15 V Zener diode
D
5
BZX79
-
C15
C15,
1
5
V Zener diode
D3
1N4148
4148, switching diode
D10
1N4148
4148, switching diode
D13
1N4148
4148, switching diode
D
1
4
1N4148
4148, switching diode
Next, we install the LEDs (Light Emitting Diodes) – the green light from these LEDs will
be of medium brightness, and thus visible in a normally lit room.
Be careful! Light Emitting Diodes have a polarity, also! Read carefully to make
sure you’re putting the diodes in the right way!
 For all LEDs, if the leads are uncut, the shorter of the two leads is the
cathode.
 Viewed from the top, the circular outline has a flat side. That flat side is
the cathode.
 Look at Figure 12. It shows that the flat side of the LED lines up with the
number 1 or number 2 in LED1 and LED2.

Page 19 of 56
Designation
Value
Package
Done1
Done2
LED1
LED
, green
T1 LED package
LED2
LED, green
T1
LED package
Figure 12-Note flat side of LED package sits under number 1 and 2 in LED1 and LED2
Small Capacitors
Designation
Value
Marking, Type
Done 1
Done 2
C3
0.1µF@100V
µ1J100, small blue box shape, film
C12
0.1µF@100V
µ1J100, small blue box shape, film
C6
0.1µF@100V
µ1J100, small blue box shape, film
C1
220pF
COG
221, radial, COG
, typically brown
4
C11
100
p
COG
101, radial, COG
C8
100
p
COG
101, radial, COG
C2
1uF@63V
film
1J63, white box,
film
Figure 13-Capacitor Identification guide
The kit may contain 47 pF capacitors, but do not use them.
4 There may be an extra 220 pf cap, but it will be axial. Once again, put it in your spares cupboard.

Page 20 of 56
Power Resistors
The following resistors are power resistors, having a larger body than the ¼ Watt
resistors installed in the previous section. These resistors are all mounted vertically as in
the examples shown in Figure 14.
Figure 14-Vertical resistor mounting
R32
0.1@2W
, 5%
Written on body
R33
0.1@2W
, 5%
Written on body
R30
10@2W
, 5%
10 Ω J
R35
10@
1
W, 5%
Brown, Black, Black, Gold.
R38
10@
1
W, 5%
Brown, Black, Black, Gold.
R58
3
K
3
@1W
, 5%
Orange, Orange
, Red
, Gold
R40
1K@2W, 5%
Brown, Black, Red
,
Gold
Transistors
Install the small signal transistors. Spread the leads a bit to make it easier to insert them
into the board. Double check their marking before you install them.
Orient the transistor so its body shape matches the silk-screen outline. Leave the top of
the transistor about ½” off the board! The lead length prevents stress on the body and
keeps the transistor safe from too much heat during the soldering operation.
Q2
2N3904
NPN, 40 volts
Q3
2N3904
NPN, 40 volts
Q6
2N3904
NPN, 40 volts
Q7
2N3904
NPN, 40 volts
Q8
2N3904
NPN, 40 volts
Q10
2N3904
NPN, 40 volts
Q11
2N3904
NPN, 40 volts
Q14
2N3904
NPN, 40 volts
Q15
2N5401
PNP, 150 volts
Q16
2N5401
PNP, 150 volts
Q17
2N5401
PNP, 150 volts
Q18
2N5401
PNP, 150 volts
T3
2N5401
PNP, 150 volts
T6
2N5401
PNP, 150 volts
T
7
2N3906
PNP, 40 volts
Q12
2N5551
NPN, 150 volts
Q13
2N5551
NPN, 150 volts
Q20
2N5551
NPN, 150 volts