The fundamental differences between tubes and transistors

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In yesterday's post we covered how the mysterious crystals used to make radio receivers first were spun off to the vacuum tube and later rediscovered to become transistors. Today let's cover some of the fundamental differences between tubes and transistors. Let's start with what the two have in common: they both are linear amplification devices and they both have three nodes to control them. Linear amplification means that when you place a small signal (voltage and current - like that of a phono cartridge) on their input, they will make a larger and more powerful replica of the input. The three nodes we have to remember are that each device type has one node that goes to the "-" of a battery, another goes to the "+" of a battery and the third - the middle one - is used as an input for the small signal you want to convert to a bigger signal across the other two nodes. Actually there are numerous other schemes using these 3 nodes to amplify but those are beyond the scope of this short primer. Next let's understand there are two types of transistors: junction and field effect. Bipolar junction transistors (BJT) are what most of us would think of when we picture a transistor. This transistor fits all the classic models and is used in the majority of today's amplification devices and early computers. The field effect transistor (FET), on the other hand, acts very much like a tube. This is the device the very first researchers described but could never figure out how to make until much later than the junction transistor came into use. Some high-end amplification devices and most computers use FETs today. PS Audio, for example, uses FETs in all of our audio amplification products to get a bit of the warmth associated with tubes - Nelson Pass is also a big advocate of the FET. Skipping some of the obvious physical differences between tiny solid state devices (transistors) and larger glowing glass envelopes sometimes called "fire bottles" (tubes), let's get to a couple of significant differences that will help us understand how these two classes of devices sound differently in high-end audio amplification applications. Perhaps the single biggest difference between transistors (both BJT and FET) and tubes is the voltage they operate at. Tubes operate at very high voltages, typically on the order of ten times higher, while transistors operate at relatively low voltages. There are, of course, exceptions to these broad views - there are both low voltage and high voltage devices in both classes - but I don't believe they are relevant to this post since 99% of high-end audio products use these choices in their classic architecture. The second biggest difference is two fold: the BJT amplifies current and is always off, the tube and FET amplifies voltage and are always on. How these two main differences in devices are managed and the results of using the devices in high-end audio is a topic for tomorrow's post: High voltage and linearity. See you then.
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Paul McGowan

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