[AMRadio] Testing tubes

[John Coleman, ARS WA5BXO]

Actually Jim, 
	You could probably use a 100 ohm resistor in series with the
plate so as to have 1 volt measured change = 10 ma change.  The 10 volt
drop in plate voltage will not alter the plate current enough to be
perceptible so long as the screen is constant.  But it is easier to
measure and you wouldn't need quite the large filter on the power
supply.  It need only be large enough capacitance to bypass the testing
frequency.  And if the testing frequency was at about 1000 CPS then it
would be more accurate than at 60 CPS.  I have drawn up what I think
would be a good testing circuit for Gm. It might even show distortion
factors if the scope was to be connected vertical to input and
horizontal to output.
http://wa5bxo.shacknet.nu/HAMPICTURES/transconductance_check.gif

What do you think?

John, WA5BXO