(See Power Transistor for this specific transistor type)
A transistor is an electronic component found inside most electronics. It is a type of semiconductor, and a key component in amplifying a signal in a circuit. Depending on the type, the way a transistor works is to act as an amplifier or switch – where signal is amplified or diverted through one input can vary the output of the two other two parts. Along with capacitors, resistors, and diodes they form a key part of all electronic circuits.
The type of transistors found in Rhodes amps and Wurlitzer pianos are bipolar junction transistors. These are small black plastic components with three metal prongs. The three sections of these transistors are called the base, collector, and emitter. Here’s a description of how they work:
- Base: Controls the flow of current between the collector and emitter. A low current or voltage to the base allows a higher current to flow between the collector and emitter.
- Collector: Collects the current flowing through the transistor. The terminal that the largest amount of current flows in or out of.
- Emitter: The emitter is the terminal from which the current flows. It emits the current into the other regions of the transistor.
There are two types of these transistors – NPN and PNP transistor. This refers to the polarity of the three material layers that are inside the transistor. The key difference is the flow of the current between the collector and emitter:
- NPN: refers to negative, positive, negative – when a low positive current flows into the base, it allows a larger current to flow from the collector to the emitter.
- PNP: refers to positive, negative, positive – when a low negative current flows out of the base, it allows a larger current to flow from the emitter to the collector.
The majority of transistors in the Wurlitzer circuitry are to amplify the signal that is carried from the reed bar. Depending on the model, the number and type of transistors varies; but the role inside is the same: to create a workable signal for the speakers and line out. The signal passes from the reed bar, to the preamp (on 200A models), bypassing through a vibrato circuit, a bias control, a pre driver, and then an output driver. Of these, the output driver, also known as a power transistor, is the most different. Power transistors are much larger, and their role within the amp is to provide higher power and heat dissipation, usually through a heat sink. This is because the current passing through the amplifier generates heat.
In older amplifiers and circuits, replacing transistors can be a way of reducing noise and hiss, correcting bias issues, and improvement overall of the amp. When replacing these parts make sure the base, collector, and emitter leads are correct when installing. Incorrect installation can damage the transistor and potentially the amplifier circuits.
