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
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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