AT 2 Amplifier (Subsonic). The cirucit of the AT 2 subsonic amplifier appears in fig-ure 213. The amplifier consists of three vacuum-tubes. All are glass pentodes type RV 2.4 P 700. The magentophone output is fed to a sharply-tuned transformer circuit. This circuit is tuned to 20-25 cps. The first amplifier tube may be tapped at three different points to determine the sensitivity of the unit. The output is condenser-coupled to the second stage. The output of the second stage through a broadly-tuned transformer cir-cuit. When the subsonic amplifier is switched on, contact (r1) breaks the connection between the grid of the third pentode with the ground through the 10 Ohm resistor. Thus, the grind potential of the third stage pentode will depend upon the output of the second stage. The output of the second stage is rectified and fed to a time-delay circuit with a total delay of approximately two seconds. At the end of this delay the grid of the third stage pentode becomes more positive and draws plate and screen grid current through the operation coil of relay R2. When contact (r2) closes, the detonator is fired.

Figure 213 – AT 2 Unit Circuit

Figure 221 – AT 2 Unit