732 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 | 



i 



netic flux, it is capable of responding faithfully to frequencies approach- 

 ing ten megacycles per second. j 



The Timing Wheels and Associated Heads | 



The synchronizing pulses derived from the drum originate from aj 

 ol2-tooth soft-steel gear mounted at the top end of the drum. In com-^'i 

 bination with a polarized reproducing head, the gear generates a timing i 

 signal which proA'ides means for permanently locating the various cells ' 

 used to store information on the drum surface. The polarized head differs 

 from those used on the drum proper, being of a form which is conven- 

 tional in tone-generators where, as in this instance, a sinusoidal output 

 is desired. | 



A second gear is mounted at the bottom of the drum, carrying a single \ 

 tooth of the same proportions as the teeth on the upper gear. In combina- ! 

 tion with a polarized reproducing head, otherwise quite similar to those 

 used on the drum proper, this single tooth provides a signal once per rev- 

 olution of the drum which (as will be shown later) is necessary for the 

 operation of the administration unit. 



The Reading Am-plifier 



One of the 80 plug-in reading amplifiers is pictured at the far left in 

 Fig. 7. It employs two twin-triode vacuum tubes, and consists of a three- :^ 

 stage ac-coupled linear broad-band feedback amplifier, followed by aj 

 threshold output stage. 



As shown in the circuit schematic of Fig. 10, the two halves of vi and] 

 the left-hand half of V2 constitute the linear broad-band amplifier. A 

 suitable choice of coupling elements insures that the amplification ^^ill| 

 diminish, with decreasing frequency, at a controlled rate for frequenciesj 

 below a few hundred cycles per second. It is unnecessary to provide am- 

 plification at low frequencies, since the signals to be handled have noJ 

 low-frequency components, and it is undesirable to do so from the stand- 

 point of hum pickup. There is about 20db of feedback in the important 

 part of the frequency range and the amplifier is thus substantially sta- 

 bilized against variations of gain due to change in operating voltages and 

 aging of tubes. The over-all operating voltage gain of the linear stages, 

 with feedback, is about 56 db; the 3 db points are approximately 300 

 c/sec and 700 kc/sec. 



The grid of the fourth stage of the reading amplifier is coupled to the 

 output of the linear amplifier and is biased to about twice the plate-cur- 

 rent cut-off value. The output signal from the plate of this stage, occa- 



