I put my plate current meter from filament transformer midtap to ground. I
return the grid bias supply/grid leak to the filament transformer. That
way, the plate current meter reads plate current only and the grid current
meter reads only grid current. I use a pushpull final with separate grid
and plate current meters, so this requires two separate grid leaks, one for
each tube, a separate protective bias supply for each tube, and of course, a
separate filament transformer for each tube.
For the class-B modulator I put the meter from filament xfmr to ground, so
it reads total cathode current, but that reading is less critical than the
plate/grid current of a class-C final, and under normal operation the grid
current to a class-B modulator is only a tiny fraction of the plate current,
so it makes little difference.
I never liked putting the plate current meter between the hv transformer
midtap and ground, unless the filter caps are returned directly to the
transformer. Otherwise, the charging current of the filter caps shows up on
the meter, and with large enough caps, could actually damage the meters on
startup. The bleeder needs to go to the fil xfmr as well, or else the meter
will read plate current plus bleeder current.
Don k4kyv
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