121. Three Element Amplifier Tubes.-The UX-250 Radiotron is a three element amplifier tube of the power amplifier type. The construction of this tube is shown in Figure 84, and with the exception that it has another element, the grid, its construction is similar to that of the UX-281 Radiotron. Most of the small three element Radiotrons are similar in construction to the UX250 Radiotron, the main difference being that of size. In this type of tube the electrons are emitted from the heated filament. Therefore the filament has two purposes, one is to generate heat and the other to emit electrons. This method is satisfactory when direct current is used to heat the filament, and in special cases, when alternating current is used for the same purpose. But in audio frequency amplifiers where the sound currents are amplified many times after leaving the tube, an objectionable amount of hum would be produced in the loud speakers if this type of tube were used with its filament heated by an alternating current. There are two reasons for this. One is that the temperature of the filament (and therefore the electron emission) is not constant, but varies slightly with the current, which changes from zero to a maximum one hundred and twenty times per second (on sixty cycle systems). The other reason is that the alternating voltage across the filament introduces variations in the grid to filament voltage. Both of the above effects can be largely taken care of in the design of the tube and circuit in which it is used. The temperature changes are reduced to a small value by making the filament large in cross-section, and most of the effect of the alternating filament voltage on the grid can be eliminated by connecting the grid circuit to the mid-tap of the filament transformer. However, these 'precautions are not sufficient in many cases and a tube of a different design is required. A type of tube, known as the heater type, fulfills the requirements adequately.

122. Heater Type Vacuum Tubes.-Figure 85 is an illustration of a UY-227 Radiotron, which is the most common of the heater type tubes. The cathode (emitting element) of these tubes is insulated from the filament or heating element. The cathode of this tube is maintained at a constant temperature and since it is insulated from the filament, it is not affected by the alternating voltage used to light the tube.

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