151. Description of Monitor Loudspeakers.-Several types of dynamic monitor loudspeakers are in use.
PG-1 to 8 equipments will be found to have one of two types. That illustrated in Figure 100 uses a
Rectox unit mounted directly on the housing of the field coil. This coil is somewhat smaller than those
used on other speakers but it is a 100 volt winding. This monitor may be used where a 1.10 volt
AC supply is available. The AC is connected direct to the Rectox unit. The voice coil is connected to
the amplifier through the monitor jack.
The other type of monitor (Type PL-7) used on these equipments is shown in Figure 101. This has a 100
volt field, which is supplied with direct current by means of an SPU (Type PK-6) using a UX-280 rectifier
tube. Both of these monitors have a volume control consisting of a variable resistance in series with the
voice coil. The monitor speakers (Type PL-10) for both the PG-10 and 13 equipments use a 12 volt field
supplied in the PG-10 by the storage battery and in the PG-13 from the 12 volt generator. These units are
slightly different in construction and appearance but in general are very similar to the type illustrated in
Figure 101, except that no rectifier tube is used.
All monitors use a flat baffle board mounted as illustrated to separate the sound waves from the front
and rear of the cone, and is essential for best reproduction. The baffle is made of an asbestos board
composition in all types except the one illustrated in Figure 100, in which case the metal casing serves
as a baffle.