[ARC5] BC-456 and the microphone circuit. (Was reverse engineering the BC-456)

[Leslie Smith]

Fellow list-readers.

Looking at the circuit diagram Fig 27-1 and the parts list I see:

The carbon mic is in series with two resistors, R-54 and R-66.  Para 9-f
states the R-66 is normally "out" of the circuit, and gives instructions
to check this is so.

R-66 has a value of 360 ohms.  
The transformer has the following characteristics:  
Primary resistance: 280 ohms.  Turns ratio 1:10 (step-up)
Not stated:  impedance step up: 1:100, but this is easily calculated.

The circuit is fed with the heater supply:  24V,  

This suggests the mike current is in the order of 20 to 50mA
I understand that the output to the 1625 (grid) is in the order of 3 to
10 volts.
This suggests (by inspection and commonsense) that the gain of the PA
stage is in the order of x40 to x100.
(Perhaps less if the modulation transformer has a significant turns
ratio).

The thing is this:  The mic jack is mounted on the FT-451-A control box.
What other arrangements do people here use when the BC-456 modulator or
FT-451-A aren't available?

Les Smith.