BluesBox: First Tests

Plugging in the guitar and turning on the BluesBox for the first time was a pretty cool experience. It almost worked as expected! Testing it for about an hour with various settings and levels revealed following problems:

• Turned all the way up it produced a lot of gain but was too noisy. The source of the noise turned out to be the DC/DC converter. That's something I expected - I also had an idea what I could do about it.
• The tone stack driver stage produced a lot of noise on its own. The source of that wasn't clear to me. By using the tone stack bypass switch, this noise could be eliminated (loosing the tone controls doing so).
• There was a low but audible high frequency chirping/beeping sound. The source of it I couldn't easily identify at first.
• It looked like I needed to modify the pots for the tone stack. In the 12pm position of both the bass and treeble controls the sound didn't seem to have a flat frequency response.

Regardless of the above issues, it sounded really cool. With the separate bass, treeble and the two gain controls I was able to mimic the sounds of a DigiTech Bad Monkey in an A/B switch setup very well. My box was noisier, but the sound hat more dynamics and overall sounded very promising.

The following frequency diagrams show some of the characteristics of the BluesBox. The diagrams were created with Audacity on a laptop. I generated a sine sweep from 20Hz to 20000Hz, fed that into the BluesBox and then recorded the BluesBox output.

Frequency response of the 12at7 tube. The two cascaded triodes of the tube produce a maximum gain of about 25dB in my circuit. (The output level with the tube stage on bypass lies at -60dB. With the tube stage on, the level is at about -35dB.)

Frequency repsonse of the tone stack at positions 12pm and 12pm. It should be flat!

Frequency response of the tone stack at positions 8:30am and 8:30am. Expected would be a mid-boost around 700Hz.

A recording of the noise and chirping sounds.