Modifying the

Epiphone Valve Junior Guitar Amplifier

"From least to beast"

by Gearscore.com

Introduction

Gearscore.com's Modifying the Epiphone Valve Junior Guitar Amplifier
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1.1 -- What and Who?

This article will describe the procedures for installing six modifications to the Epiphone Valve Junior amplifier, along the way there will be video, audio samples, pictures and more text. This project is NOT for everyone, anyone attempting the mods listed here (except Mod-1), should have some basic knowledge of electronics. That includes being able to read a schematic, able to use some basic hand tools and use a soldering iron fairly well.

What this article won't do is… make you into an amplifier technician, but it should educate you on the some of the basic principles of amplifier operation.

The idea here, is to take a very economical platform, like the Epiphone Valve Junior  (known here on as the EVJ), and apply a series of modifications to it. The mods range in price, and of course, not all mods need to be applied by anyone taking on this project. Each mod is standalone.

1.2 -- Basic Safety

As with any electronic component, there are voltages present when the device is powered on. In the case of a tube amplifier, such as the EVJ, household electrical supply in the form of AC (alternating current) is always present when the device is plugged into an outlet, unplug from wall outlet before working on any such device -- always!

In the case of many electronic devices using an AC power source, there is a transformer present to step the power down to electronic component levels. This typically takes place in an area of a circuit known as the power supply section -- more on this in a bit.

Often present when "clean" power is required, or voltage needs to be bumped up, capacitors (caps) are used. These caps are like batteries in the sense that they can store an electrical charge. In the case or power capacitors, often large "can" like looking components -- quite a bit of electrical charge can be stored, and stored for a considerable amount of time, even with the AC power source unplugged from the wall.

While it's not really necessary to drain the power caps in the EVJ, it's a good idea to know how to do it, in the event you pursue other more complex/dangerous projects after this. The reason the EVJ is relatively safe is because of two factors:

a) It's a very simple circuit with few parts and

b) The circuit doesn't have many physical breaks in it, like switches, potentiometers (pots) or "gates."

The design of the EVJ, as such means that the power caps drain to circuit ground fairly quickly. Just wait a few minutes after unplugging the EVJ, with any pots installed, turned up -- and the power caps will drain to the chassis, making for a safe platform to work on.

So, with that said -- I'll show you how to make a jumper/shunt and how it is used to drain a power cap -- although you may never need to use it on this particular project.

1.3 -- Tools

Table 1.1 provides a list of recommended tools for this project.

Table 1.1 Tool List

Note: Additional tools may be required specific to the mod being installed, such as a drill or hole cutter. Special tools will be noted in the mod chapter they apply to.

1.4 -- Audio Samples

Along the way, including at the beginning -- sound samples will be generated. The "stock" sound samples are done first for a baseline comparison for later mods Whenever mods are performed, the new sound samples will be analyzed and some commentary on them will be generated to explore the differences. Sound samples will be available to be played through a lightweight MP3 player -- to provide a reference player for all samples. It is recommended if your computer is connected to a stereo system, that you zero all tone controls out. This will give a fair audio description of the samples recorded.

All audio samples are recorded flat, as is, no post EQ or effect (reverb/echo) will be used. I’ve heard quite a few samples of mods on the internet that had reverb applied to them -- while it makes for nice sound, it brightens the tone somewhat -- giving a false audio reference.

The audio recording process is as stable as it can be, all samples will be recorded as played through a 2x12 Avatar speaker cabinet with Celestion G12H speakers. The microphone used is an AKG 770, chosen for less "coloring" than a Shure SM57. The speaker cabinet is mic'd at a distance of 4 feet, head pointed at the top edge of the speaker cabinet -- between the two speakers. The microphone signal is then taken to a non-powered mixer where input gain/clipping is controlled and passed on through to the computer recording application by way of a Presonus Firebox. Only level control is applied to compensate for the changing levels as the volume control on the amplifier is adjusted up/down.

The resulting sample is exported in to a 128 bit rate MP3 file for playback -- there IS some high end loss using this process, but all samples will be the same for comparison.

I'll be using a drum track for actual musical tone value, and the musical sample with be a simple A/E progression with a change to C/B -- hopefully it will work well enough to describe both rhythm and lead tones as they are played.

1.5 -- Video Clips

I'll be making video clips along the way to demonstrate certain processes, or to make a point regarding a particular mod. Please note that the parts list for each mod will not be included -- as that belongs to the maker of the mod kit itself, only the part number or name for the kit itself will be published. I bring this up because during video clips, you may be able to see/identify component values -- not all will be shown to protect the vendors kit contents.

1.6 -- Pictures

Lots and lots of pictures!

I'll be using many pictures to supplement the text where video would be a waste of time, and a simple static picture can do the job.

1.7 -- Very Basic Electronics!

I thought this would be a good idea, since the "language" of electronics and its symbols will be used extensively through the modification processes, so here are a few terms and what they mean.

Electronic parts are referred to as "components" -- components have a symbol and a name, the name is called a reference designator. For instance, a resistor is symbolized as a squiggly line and its name begins with an "R." The first resistor is R1, the second is R2 and so on. Printed Circuit Boards (PCBs) are drilled for components and have the RefDes printed near the holes where a particular component is to be mounted.

Figure 1.1 illustrates a PCB and a resistor with its name printed on it.

Figure 1.1 Resistor R3

Table 1.2 provides a list of commonly used RefDes, symbols and the values they are measured in.

Table 1.2 Reference Designators and Symbols

More info: Resistors and potentiometers are measured in ohms, a potentiometer is nothing more than a variable value resistor. Capacitors come in two flavors, regular and polarized -- the latter of which is very important to know. A polarized capacitor has a "+" symbol marked on one end, is often the shape of a small can and must be positioned according to its polarity in a circuit. Failure to properly align a polarized power capacitor may result in burning it up or even exploding on -- very important to pay attention to polarity direction!

For a lesson on how to read the bands on a resistor, to get their value, follow this link. www.breakup.de/resources/resistor.html

It has a calculator and shows various types of resistors -- most of the resistors used in this project will be the 5 band type.

This link also has resistor color codes as well as capacitor codes and other components. www.arar93.dsl.pipex.com/mds975/Content/components01.html

For a good reference to soldering, follow this link. www.kpsec.freeuk.com/docs/solder.pdf

1.8 -- Basic Amplifier Theory

This is just a simple primer for those that want to understand how a signal from your guitar gets amplified and what is basically going on inside of that "box." All amplifiers follow the same principle.

• Take an audio signal

• Change tonal properties (optional)

• Apply gain (preamp)

• Feed preamp signal to power section

• Output amplified signal

Figure 1.2 illustrates a typical amplifier circuit.

Figure 1.2 Amplifier Block Diagram

Typically, a weak signal, such as that from a guitar -- is fed into the input jack at the front end of the amplifier preamp section. Preamp is a much used term that simply means, pre-amplify -- that is, condition the voltage and shape of the audio signal to line level quality, for used in a power amplifier. The preamp section provides tone, inter-stage gain, and line level output to the main power section of the amplifier.

The power section of an amplifier receives line level quality signal from a preamp, and accelerates voltage to amplify the signal, for use in an output transformer.

The output transformer is really the heart and soul of actual audio output. A preamp shapes the signal, but the quality of that signal output is determined by the quality of the output transformer used. Think in terms of a magnifying glass -- the object (signal), can be seen from various distances through the magnifying glass -- but is out of focus in all but one place.

Generic transformers in many amplifiers tend to be "ballpark" in focus, so to speak. Boutique amplifiers have much better transformers and are used based on many characteristics to get a particular sound from an amplifier, or "tuned." This is one of the main components that often drives the cost of a "boutique" amplifier up.

A transformer that is better suited to the application/design of an amplifier circuit results in better articulation of notes and clarity of signal, very evident in cleans and the difference between muddy and crisp overdrive.

The last section that many overlook, is the power supply section, remember, this is all an electrical circuit and it must be powered at all stages. The quality of power supplied to the preamp and power section circuits of an amplifier are directly relative to the performance of those circuits. Think of this analogy -- washing your clothes, if you wash your clothes in dirty water, they will come out dull and stained. If you use good clear water and a good soap, it will scrub the clothes much better -- resulting in cleaner or even brilliant colored clothes. Same with the power supply, removing AC line noise and RF interference is crucial, as is providing a consistent, ripple free current supply for the operation of the circuits.

In the mods that follow, we will be making mods to the tone circuit in the preamp section as well as modifying the output section and making additions to the power supply section -- with new transformers and components.

Gearscore.com's Modifying the Epiphone Valve Junior Guitar Amplifier
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