If the plate supply has it's own power supply with a separate power
transformer or winding the meter can be placed in the negative lead of the
power supply. Be sure to place the meter outside of any bleeder resistors so
you don't measure the bleeder current.

Thanks for your suggestion, but in my initial message I had mentioned that my
power supply is structured in such a way not to permit measuring plate current
on the negative lead as it would otherwise be normal to do.

As a matter of fact the low-voltage supply is part of the high-voltage supply;
they are not separate. Therefore the current flowing in the negative lead is the
sum of the currents of the two supplies, and it is then not possible to isolate
the plate current of the final tube.

Unfortunately no alternative to measuring it on the high-voltage lead.

73

Tony I0JX