[Mondial]

The shadow meter is actually a DC milliameter. It is connected in series with the plate B+ supply to the AVC controlled RF and converter tubes. If the meter is unplugged, these tubes will not receive plate voltage.

The meter indicates tuning and signal strength because the plate current of these tubes is dependent on how much AVC voltage is generated. On a weak or no signal, there is no negative AVC bias and the tubes operate at full plate current, which makes the shadow meter deflect and cast a wide shadow. With a strong signal, there is a high negative AVC voltage applied to the RF and converter tubes, which reduces their gain and plate current to a low level. Therefore the meter deflects very little and the shadow is narrow.

So the shadow meter really displays tuning by the inverse relationship of the plate current of the tubes controlled by AVC and the level of the received signal.