Chuck:

Placing the plate current meter in the B+ lead is risky unless the meter is
well insulated from ground and mounted behind a glass or plastic panel so
the operator can't accidently come in contact with high voltage.

Isolating the B- from chassis ground allows the plate current meter to be
placed between the cathode and the power supply's negative terminal. The
meter will then read plate current but won't have the HV B+ on it. A low
value resistor is used between B- and the chassis to keep the voltage
difference between chassis and B- small.

Is your triode amp configured for grounded grid operation? You can also
place a second meter from cathode to chassis ground to read grid current.

If the B- from the power supply is grounded to the chassis and you have a
single meter between cathode and ground, it will read cathode current which
is the sum of plate and grid currents.

73,

Roger K6XQ