Power Engineering. Соколова Е.В. - 43 стр.

      15.4 Dual-rurpose tubes

       For reasons of economy or convenience several functions that would otherwise
be accomplished by two or more tubes may be handled by a single multipurpose tube.
Such a tube consists of the elements of two or more tubes all mounted within a single
envelope, each unit acting independently of the others, or it may be a combination
that depends for its operation of some sort between the several elements.
       An example of the first class in the twin triode, such as the 608-G, which con-
tains all the elements of two entirely distinct triodes, except that a single heater is
used for both cathodes. It may be used in any circuit applications calling for two
similar triodes. Another example is the 3 A8-GT, which contains a diode, a triode and
a pentode but in this case the cathode, which is of the filamentary type, is common to
all three. An added feature making for flexibility is that a filament tap is brought out,
so that the tube may be operated at either 1,4 or 2,8 Vfor filament heating, by using
the two halves in parallel or series respectively. The second class of multipurpose
tubes can be illustrated by the 6A8, called a pentagrid converter. It contains five
grids, which are referred to by number counting from cathode towards plate. In a
typical application. No. 1 grid and No. 2 grid serve as grid and anode respectively of
a triode oscillator. Grids No. 3 and No. 5 are tied together and serve as screen, shield-
ing No. 2 and No. 4 from each other and from the plate. Grid No. 4 receives the in-
coming signal, and its A.C. voltage modulates the electron stream passing from the
oscillator section to the plate. The plate current is then a combination of the effects of
oscillator voltage and the signal voltage, but the only type of coupling between the
two sources is by the electron flow, electrically the two circuits are isolated by the
shielding action of the two screen grids. Among other tubes depending on electron
coupling are the 6L7, the 6J8, and the 6K8, which should be studied in the tube
manuals

      15.5 Filter circuits

        The output wave forms are entirely satisfactory for many applications, such as
the operation of relays, battery charging, and so forth, but they are not smooth and
continuous enough to be useful for B-voltage supply of amplifiers and radio receiv-
ers. Service of this sort requires that the supply voltage be practically pure D.C., with
very little ripple superimposed upon it. Smoothing of the rectified A.C. voltage is ac-
complished by the use of filter circuits composed of inductance and capacitance or
resistance and capacitance.
        Another form of filter circuit is known as the condenser-input filter, since the
condenser Cl is supplied directly by the rectifier. In operation, the condenser Cl is
charged to the peak voltage available from the rectifier and this charge is, withdrawn
gradually by the load current. Fluctuations in current and voltage are smoothed out by
L and С 1 as in the choke-input filter. No further current is supplied by the rectifier
until its voltage is again higher than that remaining on Cl.
        In comparing the two types of filters, it is seen that rectified current flows con-
                                                                                         45