AT 3 Amplifier (Subsonic). The cirucit of the AT 3 subsonic amplifier appears in fig-ure 214. The amplifier consists of two vacuum-tubes. Both are metal pentodes, type DF 11. The circuit is essentially similar to AT 2, except that the substitution of the metal tubes with lower heater drain currents increases the continuous drain life on the amplifier batteries from 50 hours (in AT 2) to 14 days. The AT 3 has no provision for setting sen-sitivity similar to that used in AT 2. As well, the third stage is omitted, and the output of the second stage is transformer-coupled to a full-wave rectifier and time-delay circuit which operates a relay R2. Closure of contact (r) puts the 96-volt battery across relay R4. Closure of (r2) fires the mine.

Figure 215 shows a German representation of various phases of AT mine characteris-tics. A sketch and a schematic representation of the magnetophone are shown. The re-lative response of the sonic microphone and the subsonic microphone is shown in the first chart. The second chart shows the subsonic effect at a point due to explosions at distance.

Figure 214 – AT 3 Unit Circuit

Figure 215 – Principles of Subsonic Firing