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GUYTRONIX
www.guytronix.com
Assembly Manual for the Gilmore Jr
Guitar Amplifier Kit
Written by: Richard Guy
v.4.mm
2
Abstract
This manual is designed to aid in the assembly of the Gilmore Jr. Guitar Amplifier Kit. A
complete and clear manual is necessary for novice builders. Instructional Information is
presented in the form of general assembly practices; specific instructions for this kit;
drawings, pictures, schematics, parts lists, and glossaries. Also included, is a
recommended reading list, and helpful Internet links. Following the direction of this
manual, and the suggested resources, a user will achieve a successful build of the
Gilmore Jr. amplifier.
************************ WARNING! *************************
It is essential that you be familiar with standard, high-voltage
electronics safety precautions before working on any high voltage
electronics device, ESPECIALLY TUBE AMPLIFIERS like the
Gilmore Jr. Once powered up, TUBE AMPLIFIERS PRODUCE
HIGH VOLTAGES, AND ENOUGH CURRENT TO
PERMANENTLY INJURE OR KILL! They also store energy in
their capacitors and WILL SHOCK YOU EVEN AFTER BEING
TURNED OFF AND UNPLUGGED!
******************************************************
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Table of Contents
1. Before Beginning
1.1. Introduction 1.2. Using these instructions 1.3. Work area 1.4. Unpacking the kit 1.5. Tools and supplies
1.5.1. Minimum required tools for building with the optional Gilmore Jr. chassis: 1.5.2. Recommended tools 1.5.3. Additional tools for home-brew chassis
1.6. Safety precautions 1.7. Personal Injury/property liability Disclaimer
2. Chassis Preparation
2.1. Gilmore Jr. chassis 2.2. Home-brew, after-market or donor amp chassis tips
3. Install Chassis Mounted Parts
3.1. Transformers 3.2. Switches 3.3. Potentiometers 3.4. Input and output jacks 3.5. Fuse holder 3.6. IEC socket (Line cord socket) 3.7. Tube sockets
4. Install Wires and Parts on the Board
4.1. Attaching wires to turret board 4.1.1. Tips for easy assembly 4.1.2. Connect the wires to the turret board 4.1.3. Board jumper wires
4.2. Installing the electronic components on the turret board 4.2.1. Tips for installing components onto the turret board 4.2.2. Component polarity
5. Final Wiring
5.1. Wire routing and dressing tips 5.2. Line (power)
5.2.1. IEC socket, Line (cord) socket 5.2.2. Fuse holder 5.2.3. Power on/off switch 5.2.4. Standby switch 5.2.5. Power on indicator light
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5.3. Power transformer 5.3.1. Primary wires 5.3.2. Secondary wires
5.4. Output transformer 5.4.1. Primary wires 5.4.2. Secondary wires
5.5. Tube sockets 5.5.1. V1 (Pre amp tube) 5.5.2. V2 (power amp tube)
5.6. Volume and tone controls 5.6.1. Volume control (VR2 1 Meg) 5.6.2. Tone control (VR1 250k ohm)
5.7. Input jack 5.8. Output jack 5.9. Star Grounds
5.9.1. About star grounds 5.9.2. Section grounds 5.9.3. Main star ground lugs
6. Testing and Tryout
6.1. Testing B+ voltage 6.2. Install tubes 6.3. Turn the amp on, and watch the tubes 6.4. Turning the amp on/off
7. Glossary of Symbols
8. Glossary of Terms
9. References
10. Appendixes
A. Suggested reading and Internet links (webliography)
B. Parts List for Basic Gilmore Jr. Kit
C. Additional Parts in Complete Gilmore Jr. Kit
D. Schematic, with list of symbols
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List of Figures
Photographs
Photo 1: Minimum required tools
Photo 2: Recommended tools
Photo 3: Additional tools for home-brew chassis work
Figures
Figure 1: Chassis mounted parts detail
Figure 2: Turret board wire hookup points
Figure 3: Turret board component layout
Figure 4: Input Jack wiring detail
Figure 5: Parts List for Basic Gilmore Jr. Kit
Figure 6: Additional Parts in Complete Gilmore Jr. Kit
Figure 7: Gilmore Jr. Schematic
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1. Before Beginning
1.1. Introduction
Congratulations on your choice of building the Gilmore Jr. Kit. The Gilmore Jr. is designed for
easy assembly, yet it is a fully functional and thoroughly satisfying guitar amp. It is particularly
well suited for use in the recording studio or as a solo practice amp.
A great deal of thought, care and effort has gone into making the Gilmore Jr. Kit as easy as
possible to understand and assemble. This manual is designed to provide all the information
needed to assemble the kit, but it also provides educational information in the form of parts
descriptions and their function within the amplifiers design. General amplifier construction tips and theory are provided also.
The following is a statement from the designer of the Gilmore Jr. Mr. Gary Gerhart, about its
theory of operation:
The Gilmore Jr. is an all tube watt guitar amplifier using one 12AX7 dual triode tube for two preamp stages, and one 6n1p dual triode tube configured as a self-split push pull
power amp.
The guitar signal enters the amplifier via the input jack, J1 and the first stage of
amplification takes place within one section of the 12AX7 tube, V1a. The amplified
signal from V1a is next routed through a network of passive components that comprise
the control section of the amplifier. C2, VR1, and C3 form the tone control and act to
attenuate high frequencies, while R5 and C4 provide for a fixed low frequency response.
Finally, VR2 is the volume control.
The second preamp stage begins with R6, the grid stopper resistor for V1b, which is
bypassed by C5. The bypass capacitor, C5, allows some high frequency signal to bypass
the resistor, R6. The signal passes through V1b, the second preamp stage, where it is
further amplified to a suitable drive level for the power amp.
The self-split push pull power amp is comprised of both sections of the 6n1p dual triode.
The first section, V2a, is grid driven in a conventional manner and the drive signal for the
second section, V2b, is derived from the antiphase signal with exists in the cathode. The
cathodes of both sections are DC coupled, and the second section, V2B, operates in
grounded grid, or cathode drive mode. The two out of phase signals are then combined in
the primary, just as in any push pull amplifier. (End quote)
Technical jargon aside, it comes down to a great sounding little amplifier, of ingenious and
simple design, that incorporates a push-pull power amp section (a rarity in very low powered
amps).
1.2. Using these instructions
Read through the instructions before beginning, and then keep them handy for reference while
building the kit. These instructions include- a schematic, directions, layout graphics, descriptive
parts list, procedure list, and glossary. So, for convenience and clarity, the information you need
may be presented in more than one place and more than one form.
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1.3. Work area
A clean, well-lighted area with room for the amp chassis, staging area for parts, tools and
instructions is essential. A bench, desk or table that allows you to work while seated is very
desirable. Having your schematic, parts list, or other graphics taped, or pinned to a wall within
easy view is also very handy. A proper work area will help make the kit build faster, safer and
more enjoyable.
1.4. Unpacking the kit
Remove all of the kit parts from the shipping box and lay them out on your worktable. Do not
throw the box and packing materials away just yet. Check to be sure all parts on the parts list (See
Appendix B) have been included. If anything appears to be missing, check the packing materials
to be sure everything was removed.
1.5. Tools and supplies
1.5.1. Minimum required tools and supplies when using the optional Gilmore Jr. chassis
(See photo 1)
Photo 1 Minimum required tools
1. 30-40 watt soldering iron
2. Rosin-core 60/40 solder
3. Adjustable wrench 4-6 4. Needle nose pliers
5. Wire strippers
6. Wire cutters
7. #1 Philips head screwdriver
8. 3/16 flat blade screwdriver 9. 5 of #20 gauge wire (in at least three colors) and 18 of shielded (microphone) wire
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1.5.2. Recommended tools
Photo 2 Recommended tools
10. Digital multi-meter (that can read at least 500VDC and 500VAC)
11. Solder removal tool (suction device or wick)
12. Wire ties
13. Flashlight (its best to use a plastic light around electrical circuits) 14. Magnifying glass
15. Fine point indelible marker (Sharpie)
1.5.3. Some additional tools and supplies suggested for home-brew chassis work
Photo 3 Additional tools for home-brew chassis work
16. Flat, half-round and round files for hole dressing and de-burring
17. 3/8 drill of adequate power to drill/saw needed holes 18. Taps for #4, #6, and #8 machine screw holes
19. Assorted drill bits from 1/16 1/2" 20. 7/8 hole saw (for tube socket holes) 21. Calipers (for parts, hole and tool measuring)
22. Tape measure
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1.6. Safety Precautions
********************* WARNING! ***********************
It is essential that you be familiar with standard, high-voltage electronics safety precautions
before working on any high voltage electronics device, ESPECIALLY TUBE AMPLIFIERS like
the Gilmore Jr. Once powered up, TUBE AMPLIFIERS PRODUCE HIGH VOLTAGES,
AND ENOUGH CURRENT TO PERMANENTLY INJURE OR KILL! They also store
energy in their capacitors and WILL SHOCK YOU EVEN AFTER BEING TURNED OFF
AND UNPLUGGED!
Some basic safety rules
Always know whether or not the amp is plugged in or turned on! Its easy to forget, so every time you decide to do something different to the amp, CHECK AGAIN!
Large capacitors can hold deadly charges for a long time after the power is removed, so discharge
them properly through a resistor and jumper wire. Probe the high voltage capacitors with a volt-
meter to be sure they drained properly. (If you are unfamiliar with this procedure, you MUST
learn how to do it correctly to work on amps after they have been plugged in. (Contact an
electronics technician if necessary.)
When probing a live circuit, never touch the amp with both hands. This is the one hand rule and it can save your life! (Using only one hand prevents a shock from traveling through your heart
and STOPPING IT!)
Never replace a fuse with one larger than is called for in the circuit, doing so is dangerous to
people and equipment. If proper sized fuses continue to fail, the circuitry must be repaired.
Keep other people, especially children, away from your work area! Pets can be electrocuted just
as easily as humans, so keep them away from your work area as well.
Wear proper clothing for the task- soldering in short pants is a bad idea. DONT WORK ON LIVE CIRCUITS BAREFOOTED; wear safety glasses when soldering, sawing, drilling.
Always read and follow the manual for your tools! Especially read the sections on safety.
A complete guide to electronics safety is beyond the scope of this document. If you are unfamiliar
with high voltage safety techniques it is your responsibility to learn about it before attempting to
build or repair any electrical/electronic device, especially a tube guitar amplifier.
If you are in any way uncertain of your ability to work safely with potentially deadly
electronics circuits, take your equipment to a qualified amplifier technician.
1.7. Personal Injury/property liability Disclaimer
This kit, and its instructions, are designed for educational/informational purposes, and as such are
intended for use by, or under the direction of a qualified electronics technician. Gerhart
Amplification, nor the manual writer, will be responsible for personal injury or property damage
resulting from unqualified persons attempting to build this kit. Working on high voltage electrical
devices (such as tube amplifiers) can cause injury or death if done improperly (even when turned
off and/or unplugged)! Read and follow standard electronics safety procedures and rules. If you
are uncertain about your qualifications to build this kit, you should contact a qualified
technician to work with you, or build it for you.
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2. Chassis Preparation
2.1. Gilmore Jr. chassis
The optional Gilmore Jr. Chassis is ready to use and complete with needed hardware for
mounting the turret board, transformers and tubes.
The chassis is supplied with a chemical film coating per MIL-C-5541 Class 3. This is the
standard first coating for aluminum avionics enclosures, and provides limited corrosion
protection, excellent electrical conductivity, and an outstanding base for subsequent finishes.
While it is perfectly acceptable to use the chassis as supplied (much better than bare aluminum),
you may want to paint it in the color of your choice. If you choose to paint the enclosure, mask
the inside of the chassis before paint. Some overspray will not harm anything, but masking first
makes for a neater job. In any event, the areas surrounding the two ground studs on the inside of
the chassis should be completely free of any organic coating to ensure proper ground bond. In
addition, to ensure as low a noise level as possible, the faying surface between the upper and
lower halves of the enclosure should be free of paint. For best results, apply masking tape to the
two surfaces of the upper enclosure half where the bottom cover attaches. There is no need to
paint the bottom cover, and it is best left in its as-shipped condition.
After paint install the supplied rubber grommets in the transformer lead feed-through holes before
continuing with the rest of the build. Whatever graphics you may wish to decorate the chassis
with are most easily applied now, but can be applied at any time if you choose.
Take pride in your work! Make it your own.
2.2. Home-brew, after-market or donor amp chassis tips
On request, GUYTRONIX will supply your turret board for either tubes on top or tubes hanging under chassis designs. A mirror image turret board makes for cleaner wire routing, and fewer crossing wires in home-brew amps that have the tubes hanging under. The standard turret
board is tubes on top, as used with the optional Gilmore Jr. Chassis.
Depending on the chassis chosen, you will need to locate and drill holes in it for: volume and
tone controls, on/off and standby switches, input and output jacks, transformer mounting, wire
pass-through, and the line cord jack.
Care should be taken when locating the parts on the chassis to avoid running wires that carry high
voltage or current with small signal wires. Example: Dont run the shielded wire from the input jack adjacent to the VAC line (wall voltage) wires. The earlier in the gain stages a wire is, the
more susceptible to interference it is.
Consider the ultimate implications of the physical layout to the wire routing scheme. The pre amp
and power tube should be mounted at least a couple of inches apart. Flying wires from the controls and tubes to the board should be kept reasonably short (as should any small signal
carrying wire). Avoid crossing wires, but if necessary, try to cross them perpendicular to one
another. The power and output transformers should be located to minimize any possibility of
magnetic interference with each other. It is standard practice to mount them at opposite ends of
the chassis, and some prefer to mount them at a 90 degree (or other) angle from one another.
(Chassis layout and lead dressing are nearly sciences unto themselves, and every possibility cant be covered here, but these tips probably cover some of the most important points.)
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3. Install Chassis Mounted Parts: (See figure 1)
Figure 1: Chassis mounted parts detail
3.1. Transformers
The transformers are mounted to the chassis with #8-32 machine screws. Feed the transformer
wires through the chassis holes/grommets as you locate them. *Dont mount the transformers and then try to feed the wires through their holes! In general, take care when handling the
transformers. Do not to pull or twist the wires where they enter the transformer bell. The internal
connections to the transformer may be damaged if mishandled.
3.2. Switches
Remove the mounting nut and on/off plate from the switch. The on/off switch goes in the far right
hole on the front panel of the Gilmore Jr. Chassis. The standby switch goes in the next hole to the
left. Install them through the holes from the rear (inside the chassis) put the on/off plate and the
nut on from the outside, orient the switches with the ON position of the switches to the top (up), and then tighten the mounting nut.
3.3. Potentiometers
Remove the mounting nut and ONE washer from the potentiometers. The one Meg ohm volume
control installs into the second hole, and the 250k tone control in the third hole in the front panel
(counting left to right). Insert the control shaft through the hole from inside the chassis. Orient the
potentiometer with the solder lugs to the top, bottom, or side depending on your chassis, decide
which position will allow best wire routing, and soldering access. Install the washer and mounting
nut and tighten finger tight. You may wish to remove the pots for easier wiring after you have
determined proper wire lengths. Be sure to fully tighten the pots when wiring is finished.
3.4. Input and output jacks
Remove the mounting nut and ONE washer from the jack. The input jack goes into the furthest
left hole in the front panel of the Gilmore Jr. Chassis. Insert the jack through the hole from inside
the chassis. Orient the jack with the solder lugs to allow best wire routing and soldering access.
Install and tighten the washer and mounting nut. Be certain to mount R1 (1Meg) directly to the
input jack (J1) before installing in the chassis. (See figure 4)
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3.5. Fuse holder
Remove the plastic nut from the fuse holder. Install the fuse holder into third hole, (counting left
to right) on the rear panel of the Gilmore Jr. Chassis. Insert the holder through the hole from
outside the chassis. Orient the holder with the solder lugs to allow best wire routing and soldering
access. Install and tighten the mounting nut.
3.6. IEC socket (line cord socket)
From outside the chassis, press the IEC socket (line cord socket) into the pre-cut 1 x 3/4 hole at the far left on the rear panel of the Gilmore Jr. chassis.
3.7. Tube sockets
The pre amp tube socket mounts from the top into the leftmost socket hole in the Gilmore Jr.
chassis (looking at the chassis from the front), and has its mounting tabs permanently attached to
the socket. Use two #4-40 machine screws to hold it in place. The power amp tube socket mounts
in the hole to the right of the pre amp tube socket and uses a separate mounting ring to clamp it to
the chassis. Use two #4-40 machine screws to hold it in place. Both tube sockets should be
oriented so that the gap between pins #1 and #9 is toward the circuit board (front of chassis).
4. Install Wires and Parts on the Turret Board
4.1.Attaching wires to turret board
(See figure 2) Yellow lines are under-board jumper wires, blue dots are turrets, white dots are wire holes, and turrets are numbered 1-19.
Figure 2: Turret board wire hookup points
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4.1.1. Tips for easy assembly
Install the star ground wires, control potentiometer wires, and wires to the tube sockets to the
turret board first (before other components). The wires should be attached to the bottom level
of the turret and other components on the top level.
4.1.2. Connect the wires to the turret board
Note: This is done with the turret board out of the chassis. The star ground wires, control
potentiometer wires, and wires to the tube sockets should be added to the turret board
beforehand. The transformer wires are long enough to reach their connections on the turret
board in most applications, and will be soldered to their turrets just before mounting the
board into the chassis. The Turret board will be installed into the chassis in a later step. Strip
approximately 3/8 insulation from each wire, and tin the end (heat the stripped end an allow solder to flow onto the exposed end before making a connection). From the bottom of
the board, thread the wire through the hole adjacent to the turret it will solder to. Using
needle-nosed pliers pull the tinned end of the wire around its turret. It should wrap tightly
around the turret and if done just right the end of the wire will almost meet the insulation after wrapping. If two or more wires share the turret, they should wrap around the turret
separately above/below each other. (Reminder: wires go on the bottom level.) Solder the
connection and move to the next.
4.1.3. Board jumper wires
There are two jumper wires that cross under the turret board. One from turret #1 to turret
#19 and the other from turret #11 to turret #16 (see figure 2). They are pulled up through the
provided holes, wrapped and soldered in place like any other wire.
4.2. Installing the Electronic Components on the Turret Board
(See figure 3) White dots on this view are standoff mounting holes. Component descriptions and
functions are listed on parts list (Appendix B).
N.B. Make very sure that no wires have been mashed between
the turret board and the stainless steel threaded PEM nut
standoffs when the turret board was screwed down into the
chassis. This has happened more than once!
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Figure 3: Turret board component layout
NOTE: Capacitors C7 and C8 are significantly smaller in size as compared to the illustration.
4.2.1. Tips for installing components onto the turret board
For convenience, use a fine point indelible marker (Sharpie) to number the turrets on the board
from 1-19 (as shown in figure 2). Using the turret board layout graphic (See figure 3), and the
parts list (see appendix B), as a guide, start at one end of the board and work your way to the
other. Wrap the lead from each component that goes on a turret around the post a little more than
halfway around and solder each turret as you go. When more than one component shares the post,
wrap the lead above/below each other. Be sure all leads are on a post before soldering it.
(Reminder: component leads go on the TOP level of the turrets.)
4.2.2. Component polarity
Polarity is critical in only five components of the kit- the three electrolytic capacitors, C1, C7, C8,
and the two diodes D1, D2.
CAUTION: BE SURE THAT THESE PARTS ARE INSTALLED CORRECTLY!
You can tell the polarity of the capacitors by the +/- markings or the indention around one end of
the canister. (See figure 3)
The Diodes are polarity marked by a stripe around one end. (See figure 3)
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5. Final Wiring
5.1. Wire routing and dressing tips
Avoid running wires that carry high voltage or current close to small signal wires.
The earlier in the gain stages a wire is, the more susceptible to interference it is.
Avoid crossing wires, but if necessary, try to cross them perpendicular to one another.
Tube heater wires from the power transformer to the tubes are traditionally twisted together as a
pair and either routed high above the other wires, if they must cross others or flat against the
chassis if they dont cross other wires.
Some builders run other wires as twisted pairs, the power transformer primary load and neutral
(black and white) and output transformer primary (blue and brown) may be run as twisted pairs.
Fortunately, the Gilmore Jr., when built in its optional chassis, has been designed to have a
minimum of potential for wire routing/lead dress problems.
5.2. Line (power) wiring
5.2.1. IEC socket, (line cord socket):
1. Terminal marked L (load) connects to the terminal on the back (end) of the fuse holder.
2. Terminal marked N (neutral) connects to the neutral wire on the primary side of the power transformer and one wire from the power on indicator light.
3. Terminal marked G (ground) connects to the chassis, either under a power transformer bolt or to a mounting place (screw) near the power transformer. A ground stud is
provided next to the socket on the Gilmore Jr. Chassis. Firmly tighten the ground stud
nut
5.2.2. Fuse holder
1. The back (end) terminal is connected to the line socket terminal marked L.
2. The side terminal is connected to one side of the on/off switch.
5.2.3. Power on/off switch
1. One terminal is connected to the side terminal on the fuse holder.
2. The other terminal connects to one wire of the power on indicator light, and the White wire on the power transformer primary.
5.2.4. Standby switch
1. One terminal connects to the red w/yellow stripped wire from the power transformer secondary (B+ Center Tap).
2. The other terminal connects to the star ground termination point
5.2.5. Power on indicator light
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1. One wire connects to the power on/off switch on the same side as the power transformer primary.
2. The other wire connects to the IEC socket at the terminal marked N.
5.3.Power Transformer Connections
5.3.1. Primary wires
The Primary side of the provided Mercury Magnetics GG-P50-A-DP Power Transformer is wired to
accommodate the various World-wide AC Voltage choices.
1. For 110 to 120VAC operation, tie the Black and Brown wires together (2wires). Then tie the White and Brown/White wires together (2wires). These are your two 110 to 120VAC
leads. Cap off and float the Black/White wire.
2. For 100VAC operation, tie the Black and Brown wires together. Then tie the White and Brown/White wires together. Your two 100VAC leads are the tied together Black and
Brown wires and the single Black/White wire. Cap off and float the tied together White
and Brown/White wires.
3. For 220VAC operation, tie the Black and Brown/White wires together and cap them off and float them. Your two 220VAC leads are the Brown wire and the Black/White wire.
Cap off and float the White wire.
4. For 230 to 240VAC operation, tie the Black and Brown/White wires together and cap them off and float them. Your two 230 to 240VAC leads are the Brown wire and the
White wire. Cap off and float the Black/White wire.
5. IEC terminals as viewed from Rear of IEC Connector (Power Cord Jack Connector)
L N
G
5.3.2. Secondary wires
1. Both solid green (heater) wires connect to the tube sockets, at pins # 4 and # 9 on V1 (pre amp tube), and pins #4 and #5 on V2 (power amp tube). Note: These two wires should
be twisted together before soldering to the tube socket pins. To avoid damaging the
transformer, care should be taken to not twist them at the point where they enter the
transformer bell. Once they are connected to the tube closest to the transformer, it is
necessary to make a twisted pair of jumper wires to connect to pins on the other tube.
2. Both solid red wires (B+ high voltage) connect to the turret board. One at turret # 8, and the other at turret # 9.
3. The red with yellow stripe wire (B+ Center Tap) connects to one terminal of the standby switch.
4. The green with white stripe wire (Heater Center Tap) connects to the star ground termination point.
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5.4.Output Transformer
5.4.1. Primary wires
1. The blue wire connects to pin #6 (plate) on V2a (power amp tube).
2. The brown wire connects to pin #1 (plate) on V2b (power amp tube).
3. The red wire (center tap) connects to turret #1 (+ side of C8 20uf/450V)
5.4.2. Secondary wires
1. The black wire (common) connects to the output (speaker) jack sleeve terminal.
2. The gray wire connects to the output (speaker) jack tip terminal.
5.5.Tube Socket Wiring
5.5.1. V1 (preamp tube)
Pin #1: V1b (plate) connects to turret #6
Pin #2: V1b (grid) connects to turret #14
Pin #3: V1b (cathode) connects to turret #13
Pin #4: V1 (heater) connects to a green heater wire
Pin #5: V1 (heater) connects to pin #4 on V1 (solder a jumper between the pins)
Pin #6: V1a (plate) connects to turret #10
Pin #7: V1a (grid) connects to: R2 solders directly on the pin and is connected to the center
conductor of the shielded wire to the input jack
Pin #8: V1a (cathode) connects to turret #12
Pin #9: V1 (heater) connects to a green heater wire
5.5.2. V2 (power amp tube)
Pin #1: V2b (plate) connects to the brown wire from output transformer secondary
Pin #2: V2b (grid) connects to turret #7 (star ground connection point)
Pin #3: V2b (cathode) connects to turret #18 (jumper across to V2 pin #8 also)
Pin #4: V2 (heater) connects to a green heater wire
Pin #5: V2 (heater) connects to a green heater wire
Pin #6: V2a (plate) connects to the blue wire from the output transformer secondary
Pin #7: V2a (grid) connects to: R9 solders directly on the pin and is connected to turret #17
Pin #8: V2a (cathode) connects to V2 pin #3
Pin #9: no connection
5.6.Volume and Tone Controls
5.6.1. Volume control (VR2 1 Meg)
1. Terminal side that reads -0- ohms, with a DMM, when volume control is in full on position, connects to turret #4, and jumpers to the center (wiper) terminal of the tone
control (VR1)
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2. The other side terminal connects to the star ground at turret #2, the sleeve terminal of the input jack and one end of C3 (C3 is connected directly to this terminal and one side
of the tone control)
3. The middle (wiper) terminal connects to turret #3
5.6.2. Tone Control (VR1 250k ohm)
1. Terminal side that reads -0- ohms, with a DMM, when tone control is in full on position, connects to turret #5
2. The other side terminal connects to one end of C3 (C3 is connected directly to this terminal and one side of the volume control)
3. The middle (wiper) terminal connects to one side terminal of the volume control (as described in 5.6.1 1)
5.7.Input Jack
1. Solder R1 (1Meg ohm) as follows: (Figure 4)
Rear View of INPUT Jack
2. The tip terminal connects to the center conductor of a shielded wire that is connected to V1 pin #7 through R2
3. The sleeve terminal connects to the shielding conductor of the shielded wire described above in (5.7. 2), and also jumpers to the terminal on the volume control (VR2), that is connected to the turret #2 (star ground point)
5.8.Output Jack
1. The ring terminal connects to the black wire (common) from the output transformer
2. The tip terminal connects to the gray wire from the output transformer
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5.9.Star Grounds
5.9.1. About star grounds
The purpose of using a star grounding design is to keep unwanted noise (hum and static) to a minimum. In a nutshell, the star grounding design does this by grounding the most
sensitive sections of the amplifier to the same point. This eliminates ground loops A ground loop happens when there is more than one path to ground, and the separate paths
have a different electrical potential to ground. The difference in potential causes current to
flow in a circle around the loop, and induces hum or other noise into the circuit. Since the star grounding scheme brings all of the separate section grounds together to one final
ground point, they all have the same potential and only one path to ground (no ground
loops, less noise). All ground lugs need to be firmly tightened.
5.9.2. Section grounds
The Gilmore Jr. has star grounding points for sections of the circuitry at turrets #2 and #7 on
the turret board. Turret #2 gathers the ground of several board components with the ground
from the volume control (and the input jack connected to it). Turret #7 gathers the grounds
from several board components with the ground from Pin #2 V2 (grid). Both turret #2 and
#7 are connected to the main star ground, termination lug. Tighten the ground stud nut
firmly.
5.9.3. Main star ground lugs
The main star ground lugs should be located near the power transformer mounting and is
attached to the chassis with a #6-32 machine screw. A ground stud is provided on the
Gilmore Jr. Chassis. Connected to the lugs are; the ground wires from turrets #2 and #7; the
ground wire from one side of the standby switch, and the green with white stripe wire from
the power transformer secondary (heater center tap). Always tighten the ground lug nut
firmly.
6. Testing and Tryout
6.1. Testing B+ Voltage
1. Once building is complete, visually double-checked all connections.
2. With the tubes OUT of the amp, connect the red test lead of a DMM (Digital Multi Meter) to turret #19 and the black lead to ground.
3. Set the meter to its highest DC voltage range.
4. Plug the amp in and turn it on.
5. The meter should read between 300 and 350 VDC.
6. Providing the B+ voltage checks OK, you are ready to install the tubes and try the amp out! If the B+ is not OK, there is probably something connected wrong in the power supply and
every connection should be checked from point to point again.
20
6.2.Install the Tubes
Unplug the amp! The tube marked 7025 / 12AX7 installs into the socket with the metal shield
base. Take care that the gap between pins #1 and #9 of the tube is aligned with the similar gap in
the tube socket. Grip the tube firmly between thumb and your bent forefinger and push it firmly
into the socket. These are brand new sockets, and the tube pins fit rather tightly into the sockets pin holes. A slight circular rocking motion may help, but be careful because the tube pins bend
easily and are not easily straightened. With both tubes installed, the amp is ready to turn on.
6.3.Turn the Amp on, and Watch the Tubes
The first time the amplifier is turned on, no instrument is necessary, but it is vital to have a
speaker/cabinet of 4-16 ohms attached. Watch the tubes closely, (dimming the room lights
may help). Only the tubes filament, at the center of the tube, should be glowing red. If any other internal tube parts glow red (plates in particular) there is something wrong with the circuit,
and you should turn the amplifier off immediately and trace the problem. Continued operation
with any tube part other than the filament glowing red will lead to premature tube failure in a matter of seconds or minutes, and possibly damage the amplifier!
Blue glow in the tubes is normal, so dont worry about it.
6.4.Turning the Amp On/Off
1. Plug the amp in and plug an instrument into the input jack.
2. Plug a speaker/cabinet into the output jack. (Caution: running any tube amp without a speaker load can damage the amplifier.)
3. With the standby switch in the off position, move the on/off switch to on. Wait a few minutes for the heaters to pre heat the tubes (this extends tube life) then move the standby
switch to the on position and PLAY! Put the amp on standby for a few minutes before turning the main power switch off. This gives the tubes a chance to cool down progressively
and promotes long tube life. Also, dont move the amp while it is on, or while the tubes are still hot. Hot tubes break easily, and can also break if moved into the cold while still hot.
4. The Gilmore Jr. is ready to play!
7. Glossary of Symbols:
List of symbols found on the schematic:
Resistor Potentiometer Capacitor Polarized Fuse 117V plug
Capacitor
Neon Lamp Tube, triode Switch, SPST Phone Jack, Transformer, Transformer,
2 conductor, tapped secondary tapped primary
Interrupting
Chassis ground
Schematic symbols from: http://library.thinkquest.org/ Adapted for this document by Guytronix
21
8. Glossary of Terms:
B+ - the high voltage supply in a tube amplifier. The name is a holdover from the old days of battery-
powered radios, which had an "A" supply for the filaments, a "B" supply for the high voltage, a "C" supply
for the bias, and a "D" supply for the screen grids, if a separate supply was used. The conventions held
when radios switched over to rectified AC supplies.
Capacitor - a device consisting of two parallel plates separated by an insulator, called the "dielectric". The
capacitance is proportional to the area of the plates, and inversely proportional to the distance between
them. Capacitors are used to block DC while passing AC. They are frequency-dependent devices, which
mean that their capacitive reactance or "effective resistance" to AC increases as the frequency gets lower.
This makes capacitors useful for tone controls, where different frequency bands must be passed, or for
bypassing AC signals to ground while passing DC through for filtering purposes.
Cathode - the "current generating" element of an electron tube. The heater heats the cathode to a very high
temperature, causing it to emit electrons, which are then collected by the anode, or plate, which has a high
positive voltage, which attracts the negatively charged electrons from the cathode.
Chassis - the metal box that encloses the amplifier parts. It is usually made of steel, but occasionally
aluminum is used. The transformers and choke are usually mounted on top, while the passive components
are usually mounted inside the chassis.
Control grid - a wire mesh element located between the cathode and plate of an electron tube which
controls the flow of electrons between the two elements. The control grid draws no current, and as such,
presents high impedance to the driving circuit. Voltage variations on the control grid, with respect to the
cathode, cause variations in plate current, which is the basis of amplification within the tube.
Coupling capacitors - capacitors that are used between stages in a guitar amplifier. They block the DC
plate voltage of the previous stage, while passing the AC guitar signal on through.
Current - The term given to electron flow. The unit of current is the "amp", or "ampere", and indicates a
current flow of one coulomb per second. A coulomb is a unit of electron charge.
Decoupling - the process of isolating one stage of an amplifier from another. This; is usually done by
adding a resistor in series with the power supply to a gain stage and a large value electrolytic capacitor
from the supply to ground after the resistor. Decoupling prevents oscillations and other noises that may
occur due to unwanted feedback through the power supply connections. It also provides further filtering of
the power supply to reduce ripple, producing a cleaner DC supply for the low-level preamp stages.
Decoupling capacitor - the large electrolytic capacitor used to filter the power supply after the decoupling
resistor.
Decoupling resistor - the series resistor used to isolate one stage of an amplifier from another.
Diode - a two-element device, that passes a signal in one direction only. They are used most commonly to
convert AC to DC, because they pass the positive part of the wave, and block the negative part of the AC
signal, or, if they are reversed, they pass only the negative part and not the positive part. This allows them
to be used to generate a positive or negative DC supply. There are both solid-state and tube diodes. Since a
diode will pass current in only one direction, they can also be used to "clip" the top or bottom part of a
signal. Diodes are also commonly called "rectifiers" because they rectify the AC voltage, however, the term
"rectifier" is usually reserved for diodes used in the power supply section of an amplifier, while "diode" is
generally used in small signal, or low power applications, such as clippers.
Electron Tube- the device used to make guitar amplifiers sound good! Actually, this is the name given to
the amplifying devices in some guitar amplifiers. They consist of a glass tube containing several elements
which are brought out to pins on the base of the tube. All of the air inside the tube is evacuated at time of
manufacture, which keeps the filament from rapidly burning up.
22
Filament - the heating element in an electron tube, also called the "heater". The filament heats the cathode
to a very high temperature, which "boils off" electrons, which are then collected by the plate. The filament
can be seen as the glowing element through the holes in the plate of most tubes.
Filter caps - Filter capacitors. The term used for the large capacitors used to filter out the residual AC
ripple in the power supply. The rectifier converts AC to pulsating DC, since it just allows current to flow in
one direction. The output of the rectifier is a series of "humps", which must be "smoothed out" to become
flat, ripple-free direct current. The filter caps store up the voltage on the positive rise of the pulsating
rectified AC waveform, and hold it there while the rectified waveform goes down to zero. This charge,
hold, charge, hold, etc. behavior is what smooths out the ripple. In general, the larger the capacitor, the less residual ripple there will be.
Fuse - a component designed to protect electronic circuits, usually made of a thin piece of metal mounted
in a glass or ceramic tube with metal end caps that is designed to safely burn in two if the current passing
through it exceeds the rated maximum.
Grid - the "control element", in a vacuum tube. The grid is normally biased negative with respect to the
cathode. As the grid is made less negative with respect to the cathode, more current will flow from the
cathode to the plate. As the grid is made more negative with respect to the cathode, less current will flow
from the cathode to the plate. It usually only takes a relatively small grid voltage swing to control the plate
current over its entire range. Since the grid element controls of the current flow in the tube, it allows the
tube to be used as an amplifier to take a relatively small input signal on the grid and generate a relatively
large signal swing at the plate. The amount of signal voltage at the plate is equal to the current flowing
through the tube multiplied by the resistance connected to the plate.
Grid leak resistor - a very large resistor from the grid of a tube to ground, which is used to generate the
bias voltage for the tube See "grid leak biasing" for an explanation of how this works. This term is
sometimes incorrectly used when referring to the grid-to-ground resistor in a cathode biased configuration,
which is used to provide a DC ground reference for the grid circuit.
Grid resistor - the term usually given to a series resistor connected to the grid of a tube, also called a "grid
stopper", but sometimes used to refer to the resistor connected from the grid of a tube to ground, which is
also sometimes called a "grid leak" resistor.
Grid stopper - a resistor connected in series with the grid of a tube, usually right at the pin of the tube. It is
used to prevent parasitic oscillations and reduce the chance of radio station interference by forming a low
pass filter in conjunction with the input capacitance of the tube.
Ground - The common "reference" point for the circuit. This is usually also connected to the chassis, but
there can be independent circuit grounds and chassis grounds
Heater - the heating element in an electron tube, also called the "filament".
Jack - the input or speaker output connector on a guitar amplifier.
Ohm - the unit of resistance, or impedance.
Output transformer - a transformer used to match the low impedance of a speaker voice coil to the high
impedance of a tube output stage. Output transformers consist of at least two windings, a primary and a
secondary. Some output transformers have multiple impedance taps on the secondary side, to allow
matching to different speaker cabinets, typically 4, 8, and 16 ohms.
Phase inverter - a circuit that generates two output signals, each 180 degrees, out of phase with the other.
This is a bit of a misnomer, since it does more than just invert the phase of a signal, it actually generates
two out of phase signals.
Phase splitter - another name for a phase inverter.
23
Plate - the "current collecting" element in a vacuum tube. Also called the "anode". This is also the term
used for each of the two terminals of a capacitor, which are on either side of the dielectric.
Pot - short for "potentiometer".
Potentiometer - a variable resistor. It usually has three terminals: the two end terminals, across which the
entire resistance appears, and a third terminal, the "wiper", which moves to a different spot on the resistor
as the shaft is turned. In this manner, the resistance between the wiper and one end terminal gets smaller
while, at the same time, the resistance between the wiper and the other end gets larger. This allows the
potentiometer to be used as a variable voltage divider, for use in attenuators, such as volume controls or
tone controls.
Power amp - the high-level amplifying stage in a guitar amplifier. This is where the smaller preamp signal
is converted into a high power signal necessary to drive the speakers to the desired output level.
Power transformer - a transformer used to convert the incoming line (or mains) voltage to a higher or
lower value for use in the guitar amplifier. Typically, the power transformer will have at least one primary,
but sometimes two or more, to allow use at 120V/240V/etc. mains voltages. There is also usually a 6.3V
filament winding, sometimes center-tapped to allow balancing the filament string symmetrically around
ground for hum reduction. There is sometimes a 5V winding for use with a tube rectifier. This winding is
eliminated when using a solid-state rectifier. There is also a third winding for generating the high voltage,
or "B+", as it is commonly called. This winding may be center-tapped, unless a bridge rectifier is used.
Preamp - the low-level amplifying stages in a guitar amplifier. This is where the tiny signal from the guitar
pickup is amplified and shaped for the desired tonality before being sent to the power amplifier, which
generates the high power signal needed to drive the speakers.
Push-pull - In a push-pull amplifier, the power supply is connected to the center-tap of the transformer and
a tube is connected to both the upper and lower end of the center-tapped primary. This allows the tubes to
conduct on alternate cycles of the input waveform. A push-pull stage can be biased class A, where current
flows in both tubes for the entire input cycle (but in opposite directions), or class AB, where current flows
alternately in both halves, but less than a full cycle in each, or class B, where current flows only half the
time in each tube. Most designs are biased class AB for best efficiency and power output with minimal
crossover distortion (but not necessarily best "tone", although this is subjective). A push-pull stage requires
at least two tubes to operate, but can have more connected in parallel with each side, resulting in an amp
with four, six, or even eight output tubes for higher-power amps. This is called "parallel push-pull"
operation, or PPP.
R - the symbol for resistance.
Rectifier - this is the same thing as a diode, but the term is usually reserved for diodes used in the power
supply section of an amplifier.
Resistance the value or "size" of a resistor. The unit of resistance is the ohm. Resistors vary in size from fractions of an ohm to several million ohms. The prefix "k" is used for kilo-ohms, or thousands of ohms,
and the prefix "M" is used for meg-ohms, or millions of ohms.
Resistive load - a load that contains no inductance or capacitance, just pure resistance an example of a
resistive load is a dummy test load consisting of a single resistance equal to the output impedance of the
amplifier under test. The resistive load has impedance that is flat for all frequencies in the range of a guitar
amplifier.
Resistor - a circuit element, that presents a resistance to the flow of electric current. A current flowing
through a resistance will create a voltage drop across that resistance in accordance with Ohm's law.
Speaker - a transducer designed to reproduce audio frequencies. There are many different models of guitar
speakers, each with its own particular power handling capability and tone.
24
Star ground - a preferred amplifier circuit grounding system, where all the local grounds for each stage are
connected together, and a wire is run from that point to a single ground point on the chassis, back at the
power supply ground. Sometimes multiple star points are used for lower hum and noise levels in the
amplifier. Star ground nuts need to be firmly tightened.
Switch - a device that opens and closes an electric circuit.
Tone control - a potentiometer used for controlling the tone of an amplifier. This may be a single control
or there may be multiple tone controls, commonly called a "tone stack".
Transformer - a device for changing levels of AC signals, or for changing impedances of circuits. It
consists of a minimum of two coils, the primary and the secondary, wound on the same core. The core
material can be ferrous (magnetic, such as iron), or non-ferrous (non-magnetic, such as an air core).
Transformers used in guitar amplifiers are invariably wound on iron cores. An ideal transformer has no
losses, it merely steps a voltage up or down in proportion to the turns ratio between the primary and the
secondary. This is useful in converting the voltage from a wall outlet, typically 120 or 240 volts, into a
higher voltage for the tube plate supply, typically 400V or more, and a lower voltage for the tube filament,
typically 6.3 or 12.6V. The transformer will also "reflect back" to the primary the impedance which is
connected to the secondary, in proportion to the square of the turns ratio. That is, if you have a 20:1
transformer with a 16 ohm impedance connected to the secondary, it will "look like" 6.4K ohm impedance
on the primary side. This is useful in matching the plate of a tube, which is very high impedance, typically
on the order of several thousand ohms, to a speaker, which is very low impedance, typically on the order of
4, 8, or 16 ohms.
Triode - a three-element electron tube, containing a grid, cathode, and plate as active elements, in addition
to the filament.
Tube - short for "electron tube".
Voltage - the term for electric force. Voltage is the energy per unit charge created when positive and
negative charges are separated.
Volume control - a potentiometer used for controlling the volume of an amplifier. Best setting is usually
on "10" or higher.
Watt a unit of power. Contrary to popular belief, more is not always better.
Glossary of terms from: http://www.aikenamps.com/AmpTerms.html
25
9. References
[1] Personal communication, by interview, of the amplifiers designer: Gary Gerhart.
[2] The Safety Precautions section 1.6 is a paraphrased compilation of concepts from these sources:
URL: http://www.drifteramps.com/safety.html
Author: Gino Iorfida [email protected] Found: 12/04/02
URL: http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#safety
Author: R.G. Keen Found: 12/04/02
[3] The Personal Injury/property liability Disclaimer section 1.6 is a paraphrased compilation of concepts from these sources:
Title: How to Service Your Own Tube Amp (no page number)
Author: Tom Mitchell
ISBN 0-962817-0-7
URL: http://aga.rru.com/legalstuff.html
Author: Miles ONeal
Found 12/04/02
[4] Symbols (GIF images) in the glossary of symbols (section 7) are from:
URL: http://www.thinkquest.org/library/IC_index.html
Found: 12/04/02
[5] Definitions in the Glossary of terms (section 8) are selected excerpts from:
URL: http://www.aikenamps.com/AmpTerms.html
Author: Randall Aiken used by permission
26
10. Appendixes
Appendix A
Suggested reading (Helpful books):
Title: How to Service Your Own Tube Amp
Description: A complete guide to servicing most common tube amp problems. It includes
basic electronic theory, detailed descriptions of parts, and troubleshooting flow
charts. This is arguably the best beginner, tube amp service and building book!
Author: Tom Mitchell
ISBN 0-962817-0-7
Title: A Desktop Reference of Hip Vintage Guitar Amps
Description: Discussion of basic tube amp theory, in layman terms, and overview of
traditional vintage amp construction and modifications. Includes many
Schematics.
Author: Gerald Weber
ISBN 0-9641060-0-0
Title: The Tube Amp Book
Description: Discussion of basic tube amp theory, troubleshooting, tube descriptions and
tube chart by manufacturer. Includes schematics.
Author: Aspen Pittman
Internet links: (Webliography)
GEOFEX
Description: Covers a broad range of topics on the service and building of tube amps.
Click the links for- the Tube Amp FAQ, The Tube Amp Debugging Page,
and other helpful resources.
Author: R.G. Keen
URL: http://www.geofex.com/
Duncans Amp Pages
Description: This site contains a tube data sheet locator, an index of tube amp schematics
FREE amplifier design software and more.
Author: Duncan Munro
URL: http://www.duncanamps.com/
27
Appendix B (figure 5)
Parts List for Basic Gilmore Jr. Kit
Figure 5: Parts List for Basic Gilmore Jr. Kit
ID Description Function
C1 22uF 50v Atom V1a cathode bypass. Affects gain, low freq response.
C2 220pf 500v Silver Mica Tone control capacitor. Works with C3 & VR1 as high cut.
C3 .002 600V Orange Drop Tone control capacitor. Works with C2 & VR1 as high cut.
C4 .022 Mallory 150 Yellow Decoupling capacitor. Blocks DC, affects low freq response.
C5 500pf 500v Silver Mica Bypasses R6. Affects gain, high freq response.
C6 .022 Mallory 150 Yellow Decoupling capacitor. Blocks DC, affects low freq response.
C7 40uF 450v 1st filter capacitor. Supplies B+ to power amp.
C8 20uF 450v 2nd filter capacitor. Supplies B+ to preamp.
D1 1N4007 1A 1000V Diode. Forms full wave rectifier with D2
D2 1N4007 1A 1000V Diode. Forms full wave rectifier with D1
F1 Fuse 1A SloBlo (500ma for 220/240v line)
J1 1/4" Cliffs Jack Input jack
J2 1/4" Cliffs Jack Speaker jack
J3 IEC Receptacle Power cord jack
L1 Neon Light Power indicator
R1 1M 1/2w Metal Film V1a Grid resistor. Determines input impedance
R10 100K 1/2w Metal Film V2a grid resistor. Affects gain, references bias voltage.
R11 330R 2w Metal Oxide V2 bias resistor. Supplies bias voltage.
R12 10K 2w Metal Oxide PS decoupling resistor.
R2 68K 1/2w Metal Film V1a grid stopper. Blocks RF. Mount at socket.
R3 1.5K 1/2w Metal Film V1a bias resistor. Affects gain, operating point.
R4 100K 1/2w Metal Film V1a plate resistor. Affects gain, operating point.
R5 100K 1/2w Metal Film Tone slope resistor. Affects low end response, gain.
R6 470K 1/2w Metal Film V1b grid stopper. Affects gain, high freq response, oscillation.
R7 100K 1/2w Metal Film V1b plate resistor. Affects gain, operating point.
R8 1.5K 1/2w Metal Film V1b bias resistor. Affects gain, operating point.
R9 1.5K 1/2w Metal Film V2a grid stopper. Affects oscillation. Mount at socket.
S1 SPST Switch Power switch
S2 SPST Switch Standby switch
T1 GG50 Power Transformer, Custom, Mercury Magnetics
T2 GG088 Output Transformer, Custom, Mercury Magnetics
VR1 250K Log Tone control pot
VR2 1M Log Volume pot
TS1 9pin Socket w/shield Shielded socket is for V1 (preamp tube)
TS2 9pin Socket Tube socket for 6n1p (power amp tube)
TB1 Turret Board Used for mounting components
K1 &2Knob Chicken Head knobs
MAN1Manual Assembly Manual
Hok-UpWire Silver Conductor /Telfon Insulation
28
Appendix C (figure 6)
Additional Parts in Complete Gilmore Jr. Kit
Figure 6: Additional Parts in Complete Gilmore Jr. Kit
Description Quantity Chassis Top 1
Chassis Bottom 1
Oak End Cap 2
Feed-through Grommet 5
4-40 x 3/8 SS Pan Head Screw 4
4-40 x 1/4 SS Flat Head Screw 6
#4 SS Flat Washer 4
#4 SS Lock Washer 4
6-32 x 3/8 SS Pan Head Screw 4
#6 SS Flat Washer 4
#6 SS Lock Washer 4
6-32 SS Hex Nut 2
8-32 x 3/8 SS Pan Head Screw 4
#8 SS Flat Washer 4
#8 SS Lock Washer 4
10-32 x 1 Brass Binding Head Screw 6
#10 Brass Flat Washer 6
Tie Wraps 8
Tube Socket Plug 1
Hook-up Wire
Shrink Wrap
29
Appendix D (Figure 7)
Figure 7: Gilmore Jr. Schematic
A note about viewing and printing this schematic: The schematic is easiest viewed on a computer screen where you can magnify it. It might also be handy to copy the file and have a professional
print shop print a copy larger than a normal home printer is capable of.
List of symbols found on this schematic:
Resistor Potentiometer Capacitor Polarized Fuse 117V plug
Capacitor
Neon Lamp Tube, triode Switch, SPST Phone Jack, Transformer, Transformer,
2 conductor, tapped secondary tapped primary Interrupting
Chassis ground
