Why the Term B+? . . . What does it mean? . . . Where does it come from?
I refer to the battery positive post as +BATT and the positive voltage measured there as "B+." The term “B+” is used to describe the highest positive voltage source in a circuit. You might wonder where the term “B+” came from. It goes back to the time of vacuum tubes in electronic equipment which was long before transistors and other solid-state components came on the scene.
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The first vacuum tube was invented in 1904 by John A. Fleming and opened up the technology of radio electronics communications in the early 1900s. A vacuum tube requires two voltage power supplies to operate. They are labeled the “A” supply and the “B” supply voltage. Here is how these are used in a vacuum tube circuit.
Figure 2 shows the schematic of a simple vacuum tube circuit. The vacuum tube is drawn as a circle on schematic diagrams and the numbers around the circle are the pin numbers of the vacuum tube pins that connect to the elements inside the vacuum tube. In this vacuum tube Pins 1 and 2 connect to the Filament that heats up the tube.
Pin 4 is connected to the Cathode which is grounded to receive electrons. Pin 8 is connected to the Plate. The load in the circuit is wired in series with the Plate circuit. When an electron current flows through the vacuum tube it also flows through the load which becomes excited and goes into operation. Pin 5 is the Control Grid that helps control or vary the amount of electron current through the vacuum tube. The control grid is another subject for another time and is not relevant to our discussion explaining B+.
A+ and A-
The “A” voltage power supply is a low-voltage power source which supplies an electron current to heat up the filament which is made of similar material as the filament in an incandescent lamp. The “A” supply’s A+ and A- polarity connect to the vacuum tube’s filament circuit, Pins 1 and 2 as shown in Figure 3. Filament voltage can be in the range of 1.5 -12 volts depending on the vacuum tube specifications. Sufficient electron current through the filament heats up the vacuum tube. This process is called “thermionic emission” which means a “thermally-excited charge emission” that creates electrons.
A small electron current flows through the filament causing it to glow red hot. This action heats up the Cathode which develops a “negatively charged electron cloud,” as shown in Figure 3. The heat literally boils electrons off the Cathode surface which hover around the Cathode. To move these electrons through the vacuum tube and through the load circuit to operate the load, a high positive voltage (well over 100 V) is applied to the vacuum tube’s Plate to attract the negatively charged electrons through the vacuum tube and the load.