International projectionist (Jan 1943-Dec 1944)

primary of the output transformer is modulated by this signal (valve action of the grid again) and thus by transformer action a signal is obtained in the secondary of the output transformer. The principles of push-pull operation are so well known that discussion of them is not necessary here. We thus have two types of twin triodes used in the same amplifier, one as an amplifying and phase invertor tube and the other as a push-pull driver and push-pull power output tube. The secondary of the output transformer is tapped so that the impedance of different loads may be matched. When the output and load impedances are matched, maximum transfer of power from amplifier to load is obtained and distortion is reduced. The efficiency of the system is then at its maximum and tone quality at its best. In case it is not possible to select a tap on the transformer to match the load impedance, the artificial load shown connected between the output terminals is strapped so that its impedance, in parallel with the load impedance, matches that of one of the taps on the transformer. It is granted that there is some loss of power through the artificial load, but it still presents advantages over a mismatch. Some of you mathematicians may wish to make some calculations along these lines by delving into the formulae for power transfer and power or db loss in the artificial load. This amplifier also includes a tone control, shown top center, which is actually combined with the power switch shown at the lower left. The circuit consists of a potentiometer in series with .022 mf capacitor. A capacitor of a given size presents an impedance to the passage of a.c. current, this impedance decreasing as the frequency increases. Without the potentiometer in series, the capacitor would by-pass more and more of the signal as the frequency increases and absence of the higher frequencies would rapidly become noticable in the speakers or hearing aids connected to the output. The addition of the potentiometer increases the impedance of the by-pass circuit and reduces the loss of high frequencies. When the slider of the potentiometer is raised to the top, as shown in Figure 1, maximum high frequency response is obtained, as the impedance of the by-pass circuit is a maximum at all frequencies. Conversely, as the slider is moved downward, the impedance of this circuit decreases, the high frequencies fall off more rapidly until, when it is entirely out of the circuit, the sloping characteristic of the capacitor with frequency applies. By changing the values of the capacitor and the potentiometer a wide variation in the characteristics of this by-pass circuit may be obtained. N. T. S. MANAGER SAVES THEATRE FROM FIRE Scratch a National Theatre Supply Company manager and you are likely to find a fire fighter underneath. That's what they are saying about "unofficial fire chief" Bill Hutchins, National's branch manager at New Haven, Conn. Although N. T. S. men know how to get a theatre running again after a fire occursv Bill Hutchins had the good luck to be on the scene when a fire broke out at the Cheshire Theatre, on the outskirts of Cheshire, Conn. In addition to turning in the fire alarm and using a fire extinguisher on the blaze, Hutchins managed to keep the projection room and all its precious equipment intact. The Cheshire Volunteer Fire Company arrived shortly after the alarm was turned in and soon had the fire under control. The damage was limited to the ladies' room and a badly damaged switchboard. SLIDES MAKE BIG COME-BACK Slide manufacturers are reported to be experiencing an unprecedented boom while trailers become scarcer due to government restrictions in raw stock. Large circuits and big first-run houses are using slides to announce special events, and some houses are using them to announce coming attractions. A majority of theatres have slide projection equipment and those that do not have them are installing the machines as quickly as possible. According to Lou Semel, head of CinemaCraft, slide makers, the raw stock order has created almost a new industry within the film business. Regular scene trailers as supplied by National Screen Service are not affected. "input" "OUTPUT* SOO FIGURE 1 Diagram showing a portable type of amplifier which may be associated with sound systems for operation of various types of auxiliary speakers, or similar attachments. INTERNATIONAL, PROJECTIONIST*