(5) Inter-communication amplifier

This is a two-stage resistance-capacity coupled amplifier comprising a pentode V1F (6K7G) followed by a beam tetrode V8B (6V6G). Negative feedback is employed.

The purpose of this amplifier is to enable members of a tank crew to communicate one with the other when the control units are switched to "I/C". In this position signals received by both the "A" and "B" sets can be heard as side-tone, and depression of the pressel switch does not operate the S/R relays.

The output from the moving-coil microphone is applied via the transformer T4B to the control grid of V1F. A.F. voltages produced across the anode load resistance R7K are passed through the coupling condenser C29C to the control grid of V8B. The output from this valve is taken to the phones via the transformer T6A. Grid bias is provided by means of resistors in the cathode leads of V1F and V8B, R9C and R39A respectively. The absence of decoupling condensers in the cathode circuits results in the application of series negative feedback. Further negative feedback is provided by connecting the secondary of T6A to the primary of T4B via R21B.

The resistors R2D and R2E across the windings of T4B and T6A modify the response curve. Further tone compensation is provided by the filter consisting of C14B and R23D.

WS No.19 contents    Low power wirelesses contents   Radio Pages     Home Page

Rod Beavon     17 Dean's Yard      London  SW1P 3PB

e-mail:  rod.beavon@clara.net