The Colourbox

The Colourbox is a collaborative project between The Peasant and John Greczula of Xenolab Design. John had an old vacuum tube amplifier chassis on hand and was interested in developing it into a signal processing effects unit. He was planning on using it to add some vacuum tube "warmth" and distortion to his sounds.

The plan was for me to design the electronics using the original amplifer, converting it into a line-level device and adding extra features and functions. John would design the panel layout and graphics for the project and choose which features he wanted us to include.

Below is the original amplifier chassis. This particular amplifier was designed to drive a speaker as well as a reverb tank, and has an interesting output current limiting circuit for the reverb using incandescent lamps. In this picture the actual lamps for this are shown mounted on the top of the chassis next to the large filter capacitor. They were originally located inside the chassis, but were moved to the top to allow easier access in the final assembly. There was also a damaged screw type terminal strip on the front of the chassis that was removed to make room for the new wiring, some of which is visible here. The hole left over after removing the terminal strip was lined with plastic grommet strip to protect the wires.

Below are a couple of pictures of the inside of the chassis. The first one shows the original components as well as a few new connector strips and parts, including the big blue filter capacitor. The second one has most of the additional chassis wiring and circuitry installed.

Next is the final schematic for the device. The black parts represent the original amplifier circuits and the red parts show the new circuitry that has been added. Panel mounted components are shown shaded.

John had an old analogue meter that he wanted to include, and he obtained some 6E1P/EM80 "magic eye" tubes to use as level indicators. Some of the other features that we added include a mute switch, bypass/effect switch, line/instrument input switch, front and rear switched input and output jacks, as well as input and output level controls. A potentiomenter was added to allow reduction of the amplifier negative feedback, the driver stage plate voltage was made adjustable using the analogue meter as a voltage monitor, and another potentiometer was added to unbalance the output tubes. The incandescent current limiter circuit was given an enable switch, extra filtering was added to the power supply, and some extra capacitors were added to the AC coupling capacitors. Finally, a gain control for the transistor input stage on the original chassis was relocated on the front panel.

Here is a picture of the finished faceplate that John designed, very nice with an attractive powder-coated finish. The powder coating is "Prismatic Black TGIC" sourced from Columbia Coatings. John sent the front panel and powder coating powder out to pinball artist extraordinaire Wade Krause in California. Wade cut additional front panels with all the required holes from aluminum sheet using a CNC router. He took care of having two panels powder coated, then expertly silkscreened the white graphics on them.

The completed project will be housed in a standard 3U rack mount enclosure, specifically a "10 Series" that was purchased from Par Metal (

The next two pictures show the faceplate with all of the parts mounted and then with some of the panel wiring completed. The tube socket brackets were designed by John and made from laser cut acrylic plastic via

This is a picture of the panel being wired to the rest of the unit. The metal amplifier chassis is isolated from the rack enclosure using ridged plastic washers, to prevent shorting the signal ground to AC chassis ground.

And here you can see that the internal wiring has been completed and all of the tubes have been installed.

Next are a couple of views of the completed unit.

Finally, the finished project up and running. You can see the magic eye tubes in operation through their windows. The sparkly rainbow powdercoat finish sure looks sharp!

Below are a few oscilloscope shots showing what some of the different settings of the unit can do to the output. The top left 1 kHz square wave shows the input signal that was used.

And here is a short sound file of a sequencer feeding the Colourbox. It begins with the original dry signal, then the effect switch is turned on and the various controls are adjusted, and finally the effect switch is turned back off just before the end.


To see other work that The Peasant has done with vacuum tubes, go to the Vacuum Tube Synthesizer page and the Thermiomniverb page.