The pickup system on Wurlitzer pianos is formed of two metallic bars, one for the bass and treble section of the piano, that is housed on the rear portion of the reed bar. It is a comb shaped structure that surrounds the reed of each note. It is located under the dampers and damper arms.
When the piano is turned on, high voltage low current electricity is sent to the reed bar. This high voltage creates an electrostatic charge that is changed when the reeds vibrate. This signal is then sent to the amplifier which increases the volume of the signal.
Function:
As with all electric pianos the purpose of thro pickup is to boost the signal and contribute of the tone. However unlike a Rhodes, Clavinet or Pianet that use electromagnetic pickups, the Wurlitzer is unique in the electric piano world that it uses electrostatic pickups. This makes the instrument active, rather than passive in its signal generation.
In more detail of how the pickup works, firstly the transformer that converts AC 120V or 240V electricity splits into three different voltages. Of these the high voltage line (blue wire on 200 series pianos), is intended for powering the pickups on the reed bar. This is sent through three capacitors which significantly lower the current, making it usable as an electrostatic pickup. The voltage of this circuit is approximately 180V DC.
From here when one or more reeds is played the vibration affects the capacitance between the reed and the pickup, causing a small fluctuation of the voltage. It is this change in voltage that captures the audio signal that is sent to the amplifier. The reason the current is low is that it is the voltage change that is required for capture of the audio, not current. This is also known as a high impedance circuit. Having a lower current is necessary to create the conditions for a high impedance circuit, as the circuit needs to only capture signals, not transfer large amounts of power. The low current also improves safety by reducing heat and making the high voltage electricity less dangerous*
Safety:
*Even though the current in this portion of the Wurlitzer is low, the voltage is still very high (180VDC), meaning electric shocks are still dangerous. The shock will be painful! The Always turn the power off and unplug the cable before working on the reed bar and pickups.
