Dimensions: Dimensions, outline diagrams of the tubes and bases, and the arrangement of electrode connections to the base terminals are shown in Figures 1 and 2.
Base and Mounting: These vacuum tubes employ medium, four-pin thrust type bases suitable for use in Western Electric 143B or similar sockets. The 300B tube has the bayonet pin so located that it may also be mounted in a Western Electric 1OOM, 115B or similar socket. The tubes may be mounted in either a vertical or horizontal position. If mounted in a horizontal position, the plane of the filament, which is indicated in Figure 2, should be vertical.
Average Direct Interelectrode Capacitances
- Grid to plate 15 µµf.
- Grid to filament 9 µµf.
- Plate to filament 4.3 µµf.
The filaments of these tubes are designed to operate on a voltage basis and should be operated at as near the rated voltage as possible. When alternating current is used for heating the filament, the grid and plate returns should be connected to a center tap on the secondary of the filament transformer.
- Filament voltage 5.0 volts, a.c. or d.c.
- Nominal filament current 1.2 amperes
Characteristics of the 300B Vacuum Tube
- Average characteristics -
(Ef = 5.0 volts, a.c., Eb = 300 volts and Ec = -61 volts).
- Plate current 60 milliamperes
- Amplification factor 3.85
- Plate resistance 700 ohms
- Grid to plate transconductance 5500 micromhos
- Limiting Operating Conditions for Safe Operation
(Not simultaneous ratings)
- Maximum plate voltage 450 volts Maximum plate dissipation: 40 watts
- Maximum plate current of average tube for fixed grid bias: 70 milliamperes
- Maximum plate current for manually adjusted grid bias or self-biasing circuit: 100 milliamperes
Recommended Operating Conditions
Recommended and maximum conditions for alternating-current filament supply are given in the table. Recommended conditions or others of no greater severity should be selected in preference to maximum conditions wherever possible. The life of the tube at maximum operating conditions will be shorter than at the recommended conditions.
Where it is necessary to operate the tube at or near the maximum plate current of 100 milliamperes, provision should be made for adjusting the grid bias of each tube independently, so that the maximum safe plate current will not be exceeded in any tube. Alternatively, a self-biasing circuit may be used, in which the grid bias for the tube is obtained from the voltage drop produced by the plate current of that tube flowing through a resistance.
Where it is necessary to use a fixed grid bias, the plate current of the average tube should be limited to a maximum value of 70 milliamperes, so that tubes having plate currents higher than the average will not exceed the maximum safe plate current.