Note that the polar pattern is switchable by means of selecting the signal from one or both the static mesh "plates" either side of the diapham - that forming the other, common, energised electrode. The diaphram is held at a constant tension of about 60V and the charge is held steady by feeding this through a very large resistance (>10Mohm). The amplifier is the pentode valve, strapped as a triode, whose anode load is formed by R5 and the primary of U1. C2 provides AC coupling. The transformer translates the impedance to that suitable for a microphone input on a console (50 or 600 ohms) at the expense of a loss of voltage gain. Note that there is only one supply (105V at 40mA) which is used for the HT and for the heater supply for the valve, suitably dropped by R4. Those who have a nostalgia for valve equipment might note that the microphone consumes 4 Watts. Only 1mA of the 40mA the microphone requires is used in the amplifier stage: 39mA passes through the heater circuit and the larger part of the internal dissipation (2.75 Watts) is in R4!
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