360 Bass Preamp

The 360 Preamp is an adaptation of the Acoustic Control Corporation Model 360 bass preamp, first released in 1967. It is best known as the amp of choice for both John Paul Jones and Jaco Pastorius.

The 360 is a solid-state design comprised of a switchable fuzz circuit, an amplifier stage, a passive tone stack, and an active tone control with selectable frequency called “Variamp”. It uses primarily silicon transistors, with one germanium transistor in the fuzz section.

The fuzz section carries a very strong resemblance to the Mosrite Fuzzrite, which was released a year earlier in 1966. It has been converted to negative ground and utilizes NPN transistors, but the topology is nearly identical and it is clear that the designers looked to the Fuzzrite as the primary inspiration for the design.

Acoustic also released a guitar version of the amplifier called the 260. The circuit is nearly identical, with only the Variamp frequencies changed and three other very minor component changes to raise the bass cutoff frequency.

The 360 Preamp is a direct adaptation of the Acoustic 360, running at the same voltage as the original (24V for the main circuit and 7.5V for the fuzz section). We added two small modifications: a “Mode” toggle switch to improve the bass frequencies of the fuzz section (adapted from our Orpheus project), and a sixth frequency mode to the Variamp control to make full use of the more standard 6-pole rotary switch. Otherwise, it is faithful to the original circuit.

If you just want the fuzz and not the full preamp, the 360 Fuzz is available as a separate project.