Richard Prosser <rhprosser <at> gmail.com> writes:

> 
> Once the cathode heats up, won't the grids etc develop a negative voltage
> due to electron emission (space charge?).
> 
> Also, if the multimeter is set to the high megohms range, there may be some
> resistance measurement cathode - anode which should be polarity dependent -
> and should vary depending on voltages placed on the control grids. I
> wouldn't have a clue as to likely values, but it _may_ be useful for
> comparitive purposes (with a known good valve as a reference).

Once the filament heats up a negative voltage appears between K and G1 (or the
'first' anode if there is no grid). This, when measured with a 10MOhm DVM will
be somewhere between -0.2 and -1.5Vdc. No voltage will appear on the other
anodes or grids usually. Keep your hands off the probes when doing this to avoid
false readings. The voltage depends a lot on the type of cathode used and little
on its emission although conclusions can be drawn by referencing the voltage
generated by the emission current with that produced by a known new tube of the
same type.

Conduction tests can be made using the same DVM on volts scale (!) and as little
as 24Vdc of anode supply for all known tubes. This implies putting the DVM in
series with the anode and the 24V supply, applying heater, and then polarising
the grids as appropriate to 'open' the tube. The tube is expected to pass as
little as 10uA in this mode when good (which reads as about 10V on the DVM used
as a current meter).

Peter P.


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