Lets take the anode voltage first, suppose the anode voltage is selected to be 250 volts, it's clearly 250 volts AC from one of the secondary windings of the transformer, so between anode and cathode we have 250 volts AC because the other end of that winding is connected to the cathode, grid bias aside, the valve can only conduct when the anode is +ve with respect to the cathode ie: the valve only conducts during the +ve half of the AC cycle.
The set zero/mA/V switch is arranged to supply the grid voltages, when the anode voltage is supplied with the +ve half of the AC cycle one end of the grid winding will be -ve with respect to the center tap and the other end will be +ve with respect to the center tap, the center tap is also connected to the cathode, therefore a change of grid voltage of 1 volt can be obtained by switching the grid between one end of the winding and the other.
So far as I can make out...
EDIT: Was writing this while Terry posted.
Lawrence.
The set zero/mA/V switch is arranged to supply the grid voltages, when the anode voltage is supplied with the +ve half of the AC cycle one end of the grid winding will be -ve with respect to the center tap and the other end will be +ve with respect to the center tap, the center tap is also connected to the cathode, therefore a change of grid voltage of 1 volt can be obtained by switching the grid between one end of the winding and the other.
So far as I can make out...
EDIT: Was writing this while Terry posted.
Lawrence.
