WE 507 & 509

507:  Single line long loop w/ 419A diode, 425C net and 151C or 151F fixed gain amp

509:  Party line long loop w/ 426A gas tube, 419A diode, 425C net and 151C or 151F fixed gain amp

©2006 paulf.  All rights reserved.
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WE507B   WE509
 

  WE507 inside   WE509 inside

Steph provided the following from BSP C38.595.05:
The 425C network "contains a sidetone balancing network especially designed to provide a satisfactory impedance to open wire on loaded cable, see Fig. 2."

(No doubt they mean open wire *or* loaded cable.)
 

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©2006 paulf.  All rights reserved.