If you measure current from the filament transformer center tap to ground,
you'll have not only cathode current, but plate current, as well. Just
look
over at the grid current meter, subtract that from the plate meter, and
there's
your plate current, AND it's in a nearly ground potential position in the
series circuit.
Return the grid leak resistor and/or PA bias supply to filament xfmr ct.
instead of to ground, and your meter will read true plate current. If it is
a screen grid tube, return the negative lead of the screen supply to
filament ct. as well.
Don k4kyv
