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396
Radio Broadcast
The condenser used to tune the loop may be mounted on the base of the loop itself or in the usual position on the panel, where it is used to tune the secondary when the outside antenna is employed.
You will notice that the grid of the amplifier tube is connected through the secondary or loop as the case may be, to the centre terminal of the potentiometer, R3. It is possible, by this arrangement, to impose a suitable voltage on the grid under the control of the potentiometer knob. A potentiometer used in this manner is frequently called a "stabilizer." Where more than one stage of radio frequency is employed, it is a good practice to have all the grids connected to this terminal as shown in Fig. 5.
The plate of the amplifier tube is supplied with 90 volts and none but a hard tube should be employed. The plate voltage of the detector tube is twice variable because a "B" battery with taps is used and a potentiometer, R4, is also in circuit.
Tuning of the various arrangements is apparent to those who understand the tuning of other circuits, so there is no need of discussing it here. For those who are not familiar with this form of tuning, it would be well to read "Regenerative Radio Reception" page 58, RADIO BROADCAST for November, and "Paris
and Honolulu Are Calling You," page 132, RADIO BROADCAST for December.
TUNED AND TRANSFORMER-COUPLED R. F.
WHERE a standard regenerator or a variocoupler and twin variometer outfit is to be fitted with radio frequency, it is possible to take advantage of two stages with a single R. F. transformer. In these instances, which are illustrated in Figs. 5 and 6, the tuning of the antenna circuit is accomplished by means of a coil, which may be a multi-layer or a tapped single-layer coil in series with a variable condenser, the antenna and ground. For loop reception, it is but necessary to connect the loop and its shunt condenser between the grid and negative battery lead of the first amplifier tube.
By the arrangement shown in these two figures, the primary of the tuner is shunted by a variable condenser (23 or 43-plate) and may be tuned to any wavelength within the limit of its inductance and capacity. The secondary circuit in Fig. 5 may be tuned by the condenser and in Fig. 6 by the variometer. Here we have two tuned circuits forming the primary and secondary of the second radio-frequency transformer— in other words, we have one step of transformer-coupled and one step of tuned radio-frequency amplification.
FIG. 5
A single radio-frequency transformer and two amplifier tubes may be applied to a three-circuit regenerative receiver (shown within the dotted lines) to provide one stage of transformer-coupled and one stage of tuned radio-frequency amplification