12-10-2016, 08:45 AM
(12-10-2016, 07:50 AM), whenSparks Wrote: I will read it, but afraid I never was much good at theory, and it is unlikely that I will understand it, on account of the fact that there is so much to read, but I hope to figure it out by use, and comparing known good valves to known poor ones!
Bob
There's not a lot of theory in it, some folks can make the mistake of trying to hard and missing the point in the process, sometimes me included.
When trying to figure out a particular part of a circuit in a schematic it can help to print a copy and high light with a marker pen or pencil the relevant bit's when the relevant switches are in their selected position.
Basically mutual conductance (mA/V) is given by a change of anode current for a change of grid voltage whilst the anode voltage is held at a fixed voltage, the anode is fed from a low impedance source compared to the anodes resistance so in the grand scheme of things the anode voltage will be relatively constant for a change in anode current, all that's required now is to change the the grid voltage by 1 volt and note the change in anode current, that's indicated by the meter, if the anode current changes say by 5 mA for a change of grid voltage of 1 volt then the mutual conductance would be 5mA per volt (5mA/V)
With your tester when you set the meter to zero, anode current still flows (standing current) but that current isn't now indicated on the meter as it's been balanced out "backed off" so far as the meter is concerned, that's what the zero control, rectifier and it's feed winding etc do, when the mA/V switch is engaged the grid goes more +ve therefore more anode current will flow, the current through the meter is now not in balance with the increased anode current and thus will give a deflection that indicates the change of anode current, it might sound complicated but it's all Ohms law kind of stuff
Lawrence.
