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Brian, the diode & rectifier tests were added later even though a lot of testers do not cater for these tests as it is often considered as being not reliable enough. I put the circuitry in so that I could at least get some idea of the workings of a diode/rectifier rather than none at all. For small signal diodes like the EB91, the pin switch positions would be 192310800 with 6.3V heaters selected and the Anode Voltage set to 50V. This 50V setting was added to the original Anode voltage selector switch specifically for testing small diodes. You must use D1 and D2 on the Electrode Selector switch as this brings in the lower mA current limit circuitry, the EB91 is rated for 9mA. The Sussex circuitry will be limiting this current to 5mA which should be sufficient to prove the diode is actually working. When testing a Rectifier, say the UU9 with a switch setting of 290008130, You could set the Anode voltage to the same 50V and of course use D1 and/or D2 to provide current limiting for the tests. However the UU9 has a maximum anode current of 30mA so whilst testing this valve, briefly press the 50mA button and check that 30mA is reached. If not then advance the Anode Voltage a step at a time to see if 30mA can be achieved. These measurements should be taken briefly. Diodes do not have a grid to control Anode current which is why it is important to use D1 and/or D2 which will bring in the required current limiting. Only Anode voltages are required as this circuit is used for D1 and D2, the Screen voltages can be left at zero.