Building the BluesBox

The schematic for the first complete build is depicted in the following illustration:

Prototyping the pre-gain and tube (distortion) stages:

Building it:

Top-right is the 9V to 34V DC/DC converter that generates the plate (anode) voltage for the tube. It is based on a Linear Technology  LT1615-1 DC/DC step-up converter. Top left is the tone stack driver, bottom left is the tube driver (pre-gain) stage.

The following picture shows a gut-shot of the completed pedal. There was no space left for a proper tube socket. I therefore decided to solder the wires straight to the tube's pins and hold it in place with a piece of folded sheet metal.

Links and Resources

In my last post I wrote about the conceptual design of the "BluesBox". The main components (or stages) are a simple EQ (tone stack), a booster stage and a clipping (distortion) stage. Various example circuits for these stages can easily be found on the web. The purpose of this post is to list the most valuable resources I came across during my research.

Helpful links about tube amplifier design:

• FUN WITH TUBES
• The Basic Triode Gain Stage
• Valve Technology - A Practical Guide
• Designing Common-Cathode Triode Amplifiers
• The Cool Sound of Tubes
• Building Valve Pedals

For my design I decided to use a 12AT7 tube (a dual triode). Both triodes operate in a Common Cathode circuit, the first feeding the second one. The output volume pot of the preceding booster (a bipolar transistor stage) adjusts the clipping of the first triode and a second volume pot after the first triode stage adjusts the clipping of the second triode. This results in two gain pots allowing to independently control the clipping of the positive and negative semi-cycles of the guitar signal.

Input-buffer and booster circuits

• A Discrete FET Guitar Preamp
• FET Preamplifier
• Design Guidelines for JFET Audio Preamplifier Circuits
• Design Guidelines for Bipolar Transistor Audio Preamplifier Circuits
• Bipolar Transistor Amplifiers
• A Comparison of Various Bipolar Transistor Biasing Circuits
• An Introduction to the Amplifier Tutorial

For the BluesBox I picked a JFet pre-amplifier / input buffer circuit (the second link in the list above) followed by a bipolar (NPN) transistor "booster" circuit.

Tone stacks

• The Tone Stack Explained in English for Humans
• Adam's Amplifiers: Tone Stacks
• Practical Tone Control Circuits
• Passive Baxandall

I decided to incorporate a Baxandall tone stack. It's a two-knob tone control allowing to independently adjust the lows and highs of the signal. Combining the Baxandall tone stack with a booster stage (and booster output volume pot) you get three pots in total allowing you to quite nicely adjust the lows, highs and, with the volume pot, mid-boost or mid-scoop of the signal.

Buffer circuits

• Basic Buffers
• Guitar Buffer Circuits

The output signal of the tube stage I decided to run through an output buffer circuit yielding a low output impedance of the pedal.

General info and calculators

• All About Capacitors
• How to choose an emitter bypass cap value
• Coupling Capacitor Calculator
• Hochpass- und Tiefpass- Berechnung (hi-pass & low-pass calculator; in German)
• Sengpielaudio dB Calculator (in German)

...and some random pages I found particularly cool or enlightening

• GM Arts
• Selected Stompbox Schematics
• Hilltree Productions Projects Page
• The Beast with a Thousand JFETs

DIY Tube Overdrive

One post that really intrigued me and got me thinking about designing my own overdrive effects pedal was Dave Mac's version of the Matsumin Valve Caster. The Valve Caster is a simple circuit built around a 12AU7 tube running at 9 volts. The 9 volts feed the tube's heater and also provide the tube's plate (anode) voltage.

At 9 volts, the tube operates way below its intended operating voltage. Most designs use plate voltages of between 100 and 300 volts. I was skeptical that at 9 volts, the tube would sound and perform the way I had in mind. I decided to use a higher plate voltage. I also decided to use a higher mu (amplification factor) tube.

I finally picked the 12AT7 tube for my design. In terms of mu it lies between the 12AU7 and the 12AX7. The 12AX7 is a very popular tube used in the pre-amp stages of many great guitar amps. The main reason I chose the 12AT7 over the 12AX7 was its higher idle current - something I considered an advantage in a low voltage design.

So the heart of my overdrive circuit would be a 12AT7 tube running at more than 9 volts. However, I wanted my pedal to be able to run off the standard 9 volts of power supply most pedals use. As a result, I needed an internal DC/DC voltage converter in my design. I also decided to run a pre-amp stage in front of the tube with adjustable output in order to vary the level of overdrive the tube would generate. A simple EQ would be nice too I thought, to independently shape the lows and highs going into the tube stage.

The following diagram shows the main stages of what I had in mind for my circuit.

Overall block diagram - the guitar tone going into a simple EQ, then into a booster (with adjustable output) and finally into a tube amplification stage producing (hopefully) a nice over-driven guitar tone.

The challenge now was to turn this conceptual design into a working electronic circuit! Quite a challenge considering that my knowledge of electronics and amplifier design circuits was (and still is) very limited. I needed to do some serious research!

Tom's Guitar Projects

Picking up a guitar for the first time a few years ago opened up a completely new and wonderful world to me. I was forty years old at the time, driving to a skiing resort and listening to Dire Straits' Brothers in Arms in the car, I suddenly realized that I had to get a guitar and learn how to play it.

Back in town I went to the next guitar shop and bought an entry-level Ibanez E-guitar with an HSS pickup configuration (humbucker/single-coil/single-coil), a 5-way switch and a whammy bar. The amp I got was an inexpensive Vox modelling combo (combo = pre-amp, power amp and speaker(s) all in one box). I also booked a set of guitar lessons with the guy who sold me the gear.

I always liked the feel and looks of the guitar, however, over time, I grew less and less impressed by the way it sounded. So one day, after some internet research, I decided to completely replace the electronics and pickups with new parts. The original single-coil pickups I replaced with GFS Pro-Tube lipstick pickups. The humbucker I replaced with an Artec Classic (CLS) pickup. I also got a new amp: a Fender Blues Junior III.

Other modifications to the guitar included the complete blocking of the tremolo mechanics resulting in a fixed (aka hard-tail) bridge and an additional toggle switch, allowing me to play the two single-coil pickups electrically in series. All of the mentioned modifications noticeably improved the tone. The most drastic improvements being the the upgrade to the new single-coil pickups and the new in-series wiring.

Working on my old guitar (with a soldering iron rather than my finger tips) triggered a big general interest in guitar related DIY projects. I realized that there was a huge amount of information readily available on the internet on the subject. A a consequence, when I decided to replace the overdrive pedal I used a the time, the first thing I did was to look for suitable DIY effect projects.

That was last summer. Today, 9 months later, I'm the proud owner of my own guitar overdrive effects pedal: The Blues-Box - designed and built from scratch with the aim to get a warm tube-like overdrive tone with a lot of dynamics and transparency. In the next few posts I will show how I got there.

BluesBox - a tube overdrive guitar effects pedal