Radial SB-15 Tailbone

The Radial SB-15 Tailbone is a combination hi-Z signal buffer and DC power adaptor for pedalboards, made by Radial Engineering. Announced in April 2014 and shipping at $90 USD, it is built around a 100% discrete class-A FET buffer that Radial says delivers optimal signal transfer, plus a Dragster load correction circuit that lets you tune how the guitar feels through the chain¹²³. Radial has since discontinued it, so it is now a second-hand find rather than a new purchase⁴.

A class-A buffer for the start of the chain

The Tailbone is what Radial calls a high performance signal buffer designed to sit at the beginning of the signal chain and drive multiple pedals without the added noise that often plagues high impedance circuits³. The signal path starts with a standard hi-Z instrument input feeding the class-A buffer circuit³⁵. Once buffered, the SB-15 lowers the impedance and the susceptibility to hum and buzz caused by radio interference and electromagnetic fields³.

The buffer is set to unity gain, with no level adjustment available. Radial's own FAQ puts it plainly²:

That makes it a set-and-forget box: unlike a boost pedal or a buffer with a level knob, the Tailbone does not color the volume, it only conditions the signal for the pedals that follow².

Discrete class-A, not ICs

Where many buffers employ ICs or integrated circuits, the Tailbone is a 100% discrete, class-A unity gain amplifier. Radial says the discrete design reduces the need for phase-canceling negative feedback³. The buffer circuit itself is the same award-winning class-A design used in the Radial JD7, a rackmount guitar distribution center that can drive up to seven amps or pedal chains simultaneously³⁶. Radial also traces the basic class-A circuit design in its power boosters back to the JD7⁷.

The Dragster load correction circuit

After the buffer, the signal is, in Radial's words, "tamed" using the Dragster load correction circuit, which replicates the tone and feel as if connected directly to the amplifier³. In practice this is the one control on the box: a Drag Control knob that adjusts the load the pickup sees, letting you dial back the slight brightness or stiffness a buffer can introduce and recover the feel of a straight guitar-to-amp connection¹³.

The enclosure is 14-gauge steel with a powder coat finish, built to survive pedalboard duty¹⁵.

Power for Tonebone pedals: 15VDC, 400mA

The second half of the Tailbone is a DC to DC converter. It takes two 9V supply lines from a typical power brick and converts them to 15 volts, delivering a special 15VDC 400mA output designed to supply Tonebone pedals¹⁵. Radial positions this as an easy way to power a 15V Tonebone from the 9V outputs most players already have on their board⁴.

The power connections work as follows⁵:

The output connector is a standard 2.1mm barrel with center positive polarity¹. Radial's spec sheet lists overall power as +15V DC 400mA¹; the buffer portion itself runs from the 9V supplies, with the two-supply arrangement needed only when powering a 15VDC Tonebone pedal⁵.

Where it came from and what it cost

Radial Engineering announced the SB-15 Tailbone on April 7, 2014, in a press release covered by Premier Guitar, with the product shipping immediately at a list price of $90 USD³. It belongs to Radial's StageBug line of compact problem-solvers, and its buffer lineage runs through the JD7 Injector and the same Drag control load correction used elsewhere in Radial's range⁶⁷. Radial has since discontinued the product⁴.

Players comparing buffers today can look at alternatives such as the TC Electronic Bonafide Buffer, the Empress Effects Buffer, or Radial's own Radial Elevator, which pairs a buffer with a boost section.

Questions