Kingsley Constable - Valve Preamplifier




The Kingsley Constable is a tube guitar preamplifier modeled after the classic "plexi" design of the late 1960's. It is designed to push a power amplifier directly but may also be used into the effects return of a guitar amplifier effects loop. While the most authentic plexi tones are achieved by using one of the above methods, the Constable also features a mode of operation that attenuates its output to allow it to be run into the front end of a guitar amplifier or in front of other guitar effects pedals, for a clean boost or mild to medium drive pedal.

The Kingsley Constable uses two 12ax7 tubes running at high voltage (around 270VDC) to get the most authentic tube tone out of its hand wired circuit.


The full feature set of a plexi preamplifier is provided, including controls for Volume 1, Volume 2, Treble, Middle and Bass. There is also a defeatable Master Volume, as well as a 3-way "Master" switch and a 3-way "Mode" switch.

Position 1 on the Master switch attenuates the output for driving the front end of a guitar amplifier or for running into other effects pedals. In this mode the Constable becomes a clean boost or mild to medium drive pedal. Master switch position 2 allows full output for driving a power amplifier directly, while keeping the Master Volume control active. Master switch position 3 defeats the Master Volume control for the most authentic plexi control set.

The "Mode" switch determines the function of the left footswitch. In Mode 1 the left footswitch toggles between channel 1 and channel 2 (Volume 1 and Volume 2). In Mode 2 both channels are active simultaneously. This is like using a Y-cable to jumper the two channels on a 4-input plexi amplifier. In Mode 3 the left footswitch toggles between channel 2 and channel 1 plus channel 2.

Channel 2 provides the fullest, fattest sound, whereas channel 1 is considerably brighter, with cutting upper mids. By combining channel 1 and channel 2 (by using Mode 2 or Mode 3), a very wide variety of tones and responses are available.

It should be noted that a large part of the plexi tone comes from the preamplifier driving the power amplifier hard. Most of the overdrive or distortion in a plexi amp comes from the phase inverter and some from the power tubes. The preamplifier alone only produces moderate breakup capabilities. As such the Constable will produce the most authentic plexi type drive tones when driving a power amplifier directly.

Power requirements:

  • 12VDC @ 500mA, with a center positive 2.1mm plug.

  • The Constable ships with its own power supply, which works at any mains voltage from 120VAC up to 240VAC, allowing use in most countries around the world. However, a pin adapter plug may be required depending on the particular country.

  • If you wish to use an alternative power supply please make sure that it has the right specs and in particular note the center positive polarity requirement.


  • The Constable measures 5.25” wide x 5” deep. The front is 1.75” tall, sloping up to 2” at the back.

  • Weight: 1.8 lbs (815g)

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