Monday, January 12, 2015

LeSpork Bass Prototype - Under My Thumb

It's down to me
The way she talks when she's spoken to
Down to me, the change has come,
She's under my thumb

As pop music bands of the 1960s started experimenting with tone and range, the fuzz bass became a familiar sound.  An early popular example is Bill Wyman of the Rolling Stones using it in the song "Under My Thumb" in 1965.

With the basic Electra Distortion circuit built and working, it was down to me to change a few parts to get the synthetic-inspired sound that was in my head, rich with rumbling odd-numbered harmonics.  The initial circuit already had some distortion and gain.  What I wanted to do was to increase the sustain and edginess.  The initial circuit also had less definition with lower notes of the bass.  I also wanted to achieve the distortion effect more evenly throughout the frequency range.

I started tweaking the gain by bypassing the 680-ohm resistor between the emitter and ground.  This gave too much gain, so I put it back in.  I moved on to tweaking the distortion by changing the diodes.  Of all the combinations of IN4818, IN4001 and IN914 I tried, I settled on a mixed pair of an IN914 with an IN4001.  This pair gave a consistent amount of distortion over the frequency range of this bass as well as the middle-of-the-road fuzziness I had in mind.  Moving back to the gain, I tried a more-powerful 2N5089 transistor.  It also had too much gain so I switched back to the 2N3904 plus the 680-ohm resistor.  Swapping out the input cap with other values I had on hand made little difference in tone, so I'm sticking with the original one, for now.  So, other than using a different pair of diodes, what I have at the moment is the circuit as originally designed.

Next time, I'll add in the volume and passive tone controls, and check for any interactions between them with the effects part of the circuit going in to This Old Guitar.

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