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Gil's Notes:
The collection of schematics and mods you'll find below contain the most popular and best mods for improving the tone and extending the tube life of a Valve Jr amp. The ideas and theory at work here can easily be tweaked, modded, and applied to the modding of most any Single Ended tube amp similar to the VJr, such as the venerable Fender Champ and many many others.

Schematics shown with dates above links (and a few that aren't, such as the PA-In mod and the combined Stock VJr schematic immediately below) have been thoroughly scrutinized and are provided courtesy of Nomad100; to whom we shall forever owe our undying gratitude for going back in to fix stuff when we've begged him to.

All schematics or links to other resources are provided as is, with no warranty of accuracy, and no guarantee that they'll do anything other than destroy your amp; cuz the natural law of amps sez that's exactly what happens when things go wrong. Yep. Even for the experts. As always, your life and not just your amp's life is in your own hands; so proceed at your own risk, and always observe proper safety precautions whenever working with lethally high voltage circuits. Have fun, and be safe!
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Stock Valve Junior: Versions 1, 2, and 3.
http://www.sewatt.com/files/sewatt/StockVJ_1-3.pdf
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Epiphone Schematics for Valve Special/Valve Standard TMB Conversion(archived from 18watt) and Epi Valve Std Mods from 18watt (active) Threads and Schematics,
and Blues Custom 30 schematic
Valve Special
Valve Standard
Blues Custom 30 schematic
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BlackHeart Schematics
Little Giant
Killer Ant
Rowbi's Blackheart Mod page
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Harley-Benton GA-5 Schematics
http://www.sewatt.com/files/sewatt/HB-GA5.pdf
HB-GA5-Stock_0.pdf English Version
http://www.sewatt.com/files/sewatt/HB-GA5-Mod1-Rev.2.pdf
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Marshall Class 5 Schematic
http://www.sewatt.com/files/sewatt/C5_Circuit_Diagram_complete.pdf
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Orange AD-5 Schematic
http://www.sewatt.com/files/sewatt/Orange%20AD5%20Schematic.pdf
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VHT Special 6 Schematic
http://www.vhtamp.com/pdf/VHT_Special_6_Schematic_5-17-10.pdf
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Starter Mods: These VJr Mods will help improve tube life, and improve your tone without dramatically changing the amp.
02-14-08
http://www.sewatt.com/files/sewatt/Basic.pdf
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These are the original mods from 18watt.com:
02-14-08
http://www.sewatt.com/files/sewatt/Fender.pdf
http://www.sewatt.com/files/sewatt/Baby_Voxy.pdf
http://www.sewatt.com/files/sewatt/Marshall.pdf
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General SE Amp Mods
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BIAS, biasing a VJr power tube
http://www.diycustomamps.com/valvejunior.htm#vjbias
- note - Steve's diycustomamps Link Down - try this one -
http://web.archive.org/web/20071213061717/http://www.diycustomamps.com/v...

Blackheart Variable Boost Footswitch Mod
http://www.sewatt.com/files/sewatt/BH5H_boost_FS_MOD.pdf

Hammond OT wiring
http://www.hammondmfg.com/125SE.htm
Hammond OT - VJr installation w/pics
How Do I Hook Up The OT (VJr Thread)

Gabi's Jumpy Mod, aka The Jp1-clip/C3-lift Mod
Jp1-Clip Mod

Input Impedance, Dual Jacks or Switch (think Fender & Marshall's Hi/Lo inputs)
http://www.18watt.com/files/high_and_low_input_theory_of_operation_2.gif
http://www.sewatt.com/files/sewatt/Dual%20inputs.jpg
http://www.sewatt.com/files/sewatt/Input%20jack%20wiring.pdf
http://www.sewatt.com/files/sewatt/impedance%20switching.GIF
http://www.sewatt.com/files/sewatt/impedance%20switching%20-%20SPDT.GIF

Line-Outs and FX Loops
http://www.sewatt.com/files/sewatt/DI%20BOX_0.pdf
http://www.sewatt.com/files/sewatt/Firezog.gif
http://www.sewatt.com/files/sewatt/FenderFixedDI_Out.JPG
http://www.sewatt.com/files/sewatt/NFB_line-out.pdf
http://www.sewatt.com/files/sewatt/PA_IN.pdf
http://www.sewatt.com/files/sewatt/VJr_FX-Loops.JPG
Dumble's Tube Buffered FX Loop

Master Volume
http://www.sewatt.com/files/sewatt/Master_Volume.pdf

Negative FeedBack: NFB variations
01-18-08
http://www.sewatt.com/files/sewatt/NFB.pdf

Preliminary Acoustic/bass
01-23-08
http://www.sewatt.com/files/sewatt/Acoustic_Bass.pdf

Pentode/Triode switch
http://www.sewatt.com/files/sewatt/pentode-triode%20switch.jpg

Screen Grid Resistor Mod - VJr thread w/pictures
http://www.sewatt.com/node/19533

Tone Stacks
Princeton Tweed
Coopecb1's Blackface TB Stack
BitMo Kits
BitMo Tone Stack
BitMo Tone Stack - VJr Thread
Adam's Amplifiers:Tone Stacks
Duncan Amp's Tone Stack Calculator
Badcat_Minicat Tone Stack
Burchtone's "THAT" Tone Control
"THAT" Tone Control Thread
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Advanced SE Mods for VJr, Harley Benton, etc.
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High Octane Junior: schematic, bom, and build notes
http://www.sewatt.com/files/sewatt/The%20VJr%20HOJ%20mod.pdf

Octal
The Original VJr Octal Mod

Jones' Octal One - Champ 5C1 - Gibson GA-5 LP Jr - 6SJ7 preamp pentode mods
http://www.freewebs.com/jonesamps/JonesOctalOne.pdf
http://www.schematicheaven.com/gibsonamps/les_paul_jr.pdf
http://www.schematicheaven.com/fenderamps/champ_5c1_schem.pdf
Original Octal One Monster Thread

Plexi SE PCB Conversion for VJr
Latest Schematic, Parts List, & Build Guide
SE Plexi Drop in board - VJR Thread
http://rh-tech.org/public/z_stingray_pcb_files/

Pentode Preamp & Octal Mods
Stargeezer's OctNomad
David Jones' Octal One
David Jones' Octal Madness
Doug Hammond's Roctal Fatness
Kerry Fine & Zaphod_Phil's VF-10 preamp, with updates

Power Amp In
01-22-08
http://www.sewatt.com/files/sewatt/PA_IN.pdf

Savage Croaker - Tone Per Buck Club 6U8>6AQ5 Project
http://www.wattkins.com/files/wattkins/savagecroaker_final.pdf
Savage Croaker - Tone Per Buck Club Monster Thread

Tremolo
Tremolo Layout.doc
VibraValve Circuit Board with voltages.doc
Tube Tremolo in VJ - VJr thread
How would you add a trem to a Jvr? - VJr thread
Tremolo - VJr thread

Tube Reverb
http://hostbase.nfshost.com/tubereverb.pdf

VVR:
01-17-08
http://www.sewatt.com/files/sewatt/VVR.pdf
http://www.sewatt.com/files/sewatt/vvr_tagboard.jpg
New VVR Thread Section - Dana Hall's VVR support threads found here
VVR Circuit - Original Amp Building thread
Where can I get a VVR pcb? - Amp Building thread
VVR Mosfet Choices - Amp Building thread
VVR scaling only the PA - Amp Building thread
Need advice for problem with VVR - Amp Building thread
VVR - VJr thread
Just bought a VVR board from Dana/ur12....- VJr thread
VVR mosfet in OZ - VJr thread
Help! Dana's VVR Circuit - VJr thread
Speaking of VVR. - VJr thread
VVR set to lowest voltage - VJr thread
VVR maybe bad for filter caps??? - VJr thread
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18watt and 5E3 Conversion Mods
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JamesRR's Tweed 5E3 VJr Conversion Build Guide for Z_Stingray's 5E3 PCB
"Baby Will" 18watt Lite Mod: PCB's, Schematics, and Build Guides
StarGeezer's Absolute Minimum 18watt, with 6SJ7/5393, 12ax7 PI, 6V6's, and Edcor PT
14watt high gain, PPwatt Thread
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Schematics, general
How To Read A Schematic Diagram
http://www.18watt.com/
http://www.ax84.com/
http://blueguitar.org/
http://www.el34world.com/charts/Diagrams.htm
http://www.kbapps.com/schematics.html
http://www.schematicheaven.com/
http://www.schematicx.com/brands/

Schematic Symbols
http://www.hobbyprojects.com/general_theory/circuit_symbols.html