RealTube-One - A 12at7 Tube Overdrive Pedal

In an earlier post I presented the BluesBox - my own design of a valve-based overdrive pedal. It was a great learning experience but didn't quite hit the sweet spot I was looking for with respect to sound and usability.

I'm presenting now the RT-1 pedal. A complete overhaul of the original BluesBox design. This time... yeah! Exactly what I was looking for. A fantastic sounding pedal. My specs to start with:

• very transparent and open
• very dynamic
• a large amount of sweet sounding even order harmonics
• full, rolling tube clipping and distortion
• tone controls which, at their 12 o'clock position, provide a linear (or near linear) frequency response

The Result - RealTube-One

Technical Specs

• two valve (triode) gain stages (a 12AT7 twin triode)
• three discrete transistor gain stages
• maximum gain of 30dB (mileage varies based on component selection)
• controls for level (volume), "crunch" (gain/distortion), treble, bass
• 9 to 12V DC power input at about 170mA
• 33V DC from an internal DC/DC converter (top-right in the picture above) to run the triodes and two of the transistor stages
• 1M Ohm input impedance
• 3.3K Ohm output impedance
• high quality foil capacitors in the signal path

Below I'm providing the schematic, strip-board layout and other helpful resources. 

The Schematic

Download LTSpice (.asc) file here (Note: I used LTSpice just for drawing. The schematic cannot be run as a simulation). LTSpice is from Linear Technology.


The Strip-Board Layout

Download the layout file as PDF, or as a complete DIY project file. The DIY file opens with the DIY Layout Creator.


The Jumpers

Download the jumpers file as PDF.


Trace Cuts and Solder Bridges

Download the file as PDF. This file is a mirror image, looking at the board's trace side. The blue jumpers are provided as a reference. They help locate the correct positions of the cuts.

Notes on the DC/DC converter and a PCB layout for it will follow in a separate post.

I used the Type BB enclosure from Musikding. It's easy to work with (like drilling the holes) and has pleasing dimensions. Fitting everything into the box, however, was a bit of a challenge.

One of the coming weekends I'm planning to spray-paint the box and put some nice white lettering onto it (using the DecalPro system).


Possible Modifications

• The two bias trim pots are not too important. They can be replaced with 100 Ohm resistors.
• The first transistor stage in the design is a JFET based input buffer / booster. With a 1.2K drain resistor (or 1.3K as shown in the schematic), it provides very little gain. By incrementing the drain resistor you can get a little more gain out of the input buffer stage. At the same time, you will also start to get distortion from the transistor. The level of this distortion depends a lot on the output level of your pickups. By tickling the MPF102 just a little, you get very nice first and second order harmonic distortion. By pushing it too far, you get some pretty bad sounding distortion. However, this can be used in an interesting way: driving the signal further, through the next transistor stages and finally through the tube stages, you will get some pretty cool fuzz-like sounds out of the box. The modification I have in mind is a three way switch allowing to switch between three sizes of the drain resistor: 1.2K, 1.6K and 2K (clean, harmonic and fuzz).
• The tone controls allow to adjust for bass boost, bass cut, treble boost and treble cut quite nicely. You can also get a nice mid scooped kind of frequency response. Mid boost is more difficult to get. I'm considering modifying the tone stack to also be able to shape a mid-boost kind of frequency shape.

Disclaimer: The schematic and all other resources of this post are provided "as is". I put a lot of effort into reviewing the schematic and layout for errors. However, I do not provide any guarantee as to their correctness. Do not use the pictures as a reference (for example, the transistor orientation is wrong on some of the pictures). Always refer the provided PDFs and DIY file.

Should you decide to build this for yourself: it is not an easy project! If you can't read the schematic at all you will not be able to turn it into a functioning device. I will not be able to provide any support. General questions and comments, however, are welcome.

Copyright notice: All resources provided are my copyright. I'm granting you the right to use these resources for your own use with no limitations, except when used as a basis for commercial production of sound effect products.