The White Spiral Boost is a dirty boost and the result of my exploration of various JFET circuits initially inspired by Jimmy Page’s use of the Barcus Berry Preamp. While Page is only known to have used it on acoustic, I found that the BB 1330-1, and the simpler BB 1330S, work great with electric guitars that have low-output pickups and as a general boost circuit. The White Spiral circuit has evolved over time to become a hybrid circuit with bits of the BB 1330S, National Semiconductor Mu-Amp, and the Yellow Spiral. The White Spiral uses a dual JFET for amplification, the Grit is a combination of JFET and Silicon clipping, the Girth control comes from the Yellow Spiral but with a twist, the White also has 18v internal operation to provide increased headroom.
The location of the Girth Control in the circuit is user-definable by the Alpha/Omega two-way toggle. In the Alpha (A - Left) position the Girth control is placed before the low-gain Grit clipping section of the White Spiral. This position allows more, or less, bass to be sent to the Grit, and changes the character and compression of the Grit. The Omega (Ω - Right) position places the Girth Control after the Grit Control and this position changes how much bass content is passed through the circuit. This position is still dirty but it is more open, glassy, and Hi-Fi sounding than the Alpha setting. Bassists will LOVE the Omega selection!
The Grit Control does double duty, it both changes the performance of the JFET’s to create distortion, and also brings in Silicon clipping. These two types of clipping together are slightly asymmetrical and the result is a natural-sounding low-gain grit/grind that mates well with already dirty amp channels.