A transformer is a device in an electronic circuit that changes the voltage of AC current. It consists of two coils of wire wrapped around a magnetic core. The primary winding is connected to the input voltage, usually mains power. The secondary winding is connected to the output for the electronic device.
In an Wurlitzer or Rhodes suitcase amplifier, mains electricity is converted into different voltages for practical applications in the amp. Amplifiers usually require much less voltage than mains electricity, so a transformer is essential for stepping down the high voltage of mains electricity.
In a Wurlitzer piano, the transformer converts mains electricity into two voltages. One is high voltage for powering the reed bar, and the other is low voltage for powering the amplifier. The are four cloth/fabric wires that connect the transformer to the amp:
- 1x blue wire – high voltage 150V
- 2x red wires – low voltage 25V
- 1x red/yellow wire – ground
These are connected to the right hand side of the PCB. Immediately afterwards all the transformer wires are connected to diodes to convert them from AC to DC current. The 150V high voltage section provides power to the electrostatic reed bar. The high voltage allows the vibrations of the reeds when struck by the hammer to be picked up. The 25V low voltage section powers the capacitors, resistors, transistors and integrated circuits that make up the amp.
The individual wires are usually detachable from the circuit board if removing the amp is needed, though when servicing the underside of the amp this is usually not necessary.
Depending on the voltage of the region, the transformer in a Wurlitzer piano will either step-up or step-down the voltage. In North America the voltage is 120 AC, so the transformer needs to step-up the voltage in order to power the reed bar to 150VDC. In Europe and Asia, the 240V voltage needs to be stepped down to achieve the intended voltage. This is achieved in the transformer by the method of wiring:
- 120VAC – wiring is in parallel.
- 240VAC – wiring is in series.
The transformer is connected to the mains via terminal strip where the wiring is series or parallel depending on region. The live mains wire connects to the power switch, then a fuse, which then connects to the black/red wire that enters the transformer.
Another reason why transformers are essential in amps is for electrical safety. A transformer acts as a barrier between the mains current and the components of the amp. In some older vintage tube amplifiers, mains current was wired directly into the amp. A short circuit or malfunction can cause live electricity to transfer into places where it absolutely should not. An example of this would be through the jack cable. This is clearly extremely dangerous and thankfully was phased out many decades ago. A transformer acts as a barrier to prevent this, which gives another level of safety into the system.
Transformers also act as a barrier that can protect against electrical surges from mains voltage, and can help to reduce ground loops and noise reduction. Ground looping is however an issue in some Wurlitzer pianos that is inherent in the design.
