Dual-purpose tubes

Today's radio electronics and engineering are facing some problems those are an obstacle in the way of its further development. They are simplification of the radio tools and devices operation and designing the most reliable and efficient ones.

For reasons of economy and 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 on its operation of some sort between the several elements.

An example of the first class is the twin triode that contains 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 tube that 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 V for 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.

Filter circuits

The output waveforms are entirely satisfactory for many applications such as the operation of relays and battery charging. But they are not smooth and continuous enough to be useful for B-voltage supply of amplifiers and radio-receivers. 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 accomplished 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 is supplied directly by the rectifier. In operation the condenser is charged to the peak voltage available from the rectifier and this charge is withdrawn gradually by the load current.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 L (inductance) and condenser as in the choke-input filter. No further current is supplied by the rectifier until its voltage is again higher than that remaining on the condenser.

Oscillators

In a typical commercial broadcasting station the carrier frequency is generated by a carefully controlled quartz-crystal oscillator. The fundamental method of controlling frequencies in most radio work is by means of tank circuits, or tuned circuits. They have specific values of inductance and capacitance, and they are suitable for the production of alternating currents of a particular frequency and discourage the flow of currents of other frequencies.

In cases where the frequency must be extremely stable, however, a quartz crystal with a definite natural frequency of electrical oscillation is used to stabilize the oscillations. The oscillations are actually generated at low power by an electron tube and are amplified in a series of power amplifiers that act as buffers to prevent interaction of the oscillator with the other components of the transmitter, because such interaction would alter the frequency. The crystal is shaped accurately to the dimensions required to give the desired frequency, which may then be modified slightly by adding a condenser to the circuit to give the exact frequency desired. In a well-designed circuit, such an oscillator does not vary by more than one-hundredth of 1 percent in frequency. Mounting the crystal in a vacuum at constant temperature and stabilizing the supply voltages may produce a frequency stability approaching one-millionth of 1 percent. Crystal oscillators are most useful in the ranges termed very low frequency, low frequency, and medium frequency (VLF, LF, and MF). When frequencies higher than about 10 MHz must be generated, the master oscillator is designed to generate a medium frequency, which is then doubled as often as necessary in special electronic circuits. In cases where rigid frequency control is not required, tuned circuits may be used with conventional electron tubes to generate oscillations up to about 1000 MHz. And reflex klystrons are used to generate the higher frequencies up to 30,000 MHz. Magnetrons are substituted for klystrons when even larger amounts of power must be generated.