[Boatanchors] about BA amp metering [long]

Brian KA9EGW ka9egw at ka9egw.com
Fri Dec 11 13:41:28 EST 2015

I've been digesting what W8JI's excellent site has to say about metering of
toob amplifiers, particularly with respect to floating the negative rail.
Talking about cathode-driven G-G triode[s] here.  My Hunter Bandit 2000C
[2x3-400Z] has no provision for metering grid current, and a superfluous
meter movement for relative output.  I would have to ask anyone answering my
questions to grab a copy of the schematic from BAMA.
According to W8JI's site, it would seem at first glance that R-8,
100milliohms from the negative supply rail to chassis ground, with the tube
grids directly grounded, it would seem the voltage drop across R8 would vary
as a function of grid current.yet in this amp, according to the schematic,
when set to read PLATE current the meter is directly paralleled with R-8.
Taking a closer look, I observe that on W8JI's metering webpage, the
cathode[s] or filament[s] center tap is returned to the negative supply rail
through a plate current meter shunt whereas on the bandit 2000C the filament
center tap is returned to chassis ground.  As such it appears the voltage
drop across R-8 is the voltage drop caused by *cathode* current [Ik] even
though it's labeled Ip on the schematic.  Meter polarity, positive to
chassis ground when measuring "Ip" would tend to support this, I think...
Setting aside the obvious concerns about shielding, bypassing, feedback
loops, etc, for the sake of conversation, is it accurate to say that moving
the filament center tap from chassis ground to negative supply rail would
cause the meter to read Ig instead of Ik?  
Taking it a step farther is it also accurate to say that interpolating
another 100milliohm 1% resistor between negative supply rail and filament
center tap would allow the second meter [assuming both movements are the
same full scale sensitivity and same internal resistance] would give a
proper place to measure Ip?
Thanks in advance
73, Brian KA9EGW

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