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