Of course, if a tube rectifier is used, the 300B will require its own separate heater transformer.
    The last advantage to the 300B as a regulator tube is its low price. What? Aren't NOS WE 300Bs going for up to $700 each? Yes they are, but Chinese 300Bs can be had for under $60. So from a dissipation watt per dollar basis, these tube can be seen as cost effective.
Series Regulators
    The difference between the series and the shunt regulator has been covered in past issues. So only the briefest recap is given on how each regulator type works. The series regulator is best suited to applications that have a varying current draw, such as a Class-AB, B amplifier. The pass device, the 300B in this case, sees all of the current flowing into the load, which sets a current limit of 100 mA per 300B. A feedback mechanism and a voltage reference are all that is needed to complete the regulator.
    The circuit below uses a 5687 as the main feedback mechanism and a zener for the voltage reference.

The 300B as Regulator

     The 300B has justly won the reputation for being a superb output tube in single-ended amplifiers. Of course, it actually has a much wider application, such as a push-pull amplifier's output tube and possibly as a headphone amplifier tube or super-buffed line stage tube. But it will take some rethinking to imagine this great tube in a non-audio role. It is sort of like trying to imagine Arnold Schwarzenegger playing Goethe: sure he has the accent, but are the biceps really necessary or even desirable?
    The pluses the 300B brings to a voltage regulator is its ability to pass a fair amount of current at low cathode-to-plate voltages, the result of having a low rp. And its 40 watt plate dissipation limit does not hurt either. An advantage few would highlight, however, is its 5 volt heater specification. Why would this count as an advantage? Most high voltage power transformers retain a 5 vac winding for heating a tube rectifier. Rectifiers with rare exception require a floating 5 volt power supply. Had rectifiers used the same nominal 6.3 volts that most tubes use, many would be tempted to attach the rectifier's heater to the common 6.3 vac winding that is referenced to ground. This would spell catastrophe, as the isolation material the separates the heater from the cathode cannot withstand more than 100 volts in most tubes; in fact, most indirectly heated rectifiers have the one leg of the heater element connected to the cathode and thus have a 0 volt heater-to-cathode voltage limit. And those rectifiers that use directly heated cathodes would obviously also short the high-voltage to ground! Choosing 5 volts for rectifiers was a wise decision. Coincidentally, the 300B uses a 5 volt heater. This heater voltage allows us to use the often free 5 vac winding, free because solid-state rectifiers were used instead. (Solid-state rectifiers result in a higher power supply voltage and a much low power supply impedance. High speed rectifiers do perform much better than the slower variety and are heartedly recommended.) 

    The cascode circuit provides a relatively high gain and only draws 4.7 mA of current. The value of resistor Ra is determined by the finding of the required negative grid voltage for the 300B at the given cathode-to-plate voltage and desired idle current and then taking this voltage


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