Paul,
First, on a PP transformer, the windings may not be equal in resistance, so you need to know the resistance of each winding. The voltage may be different even though the currents may be the same.
If you KNOW the resistance of the windings feeding B+ to the output tubes, it IS possible to determine current through the tube by measuring the voltage drop across the winding and dividing by the resistance. It is sometimes difficult to get a good resistance reading off an inductive winding, so the best way is run a known, fixed current through the transformer and measure the voltage drop across (SE) or on each side of the winding (PP), and keeping a record of the measured winding resistance (measured Voltage/known current). However, it is so much easier to install a precision (i.e. 1%) 1 ohm or 10 ohm resistor (which can be unbypassed) at the cathode, and accessible test point(s), so to read current by measuring the drop across the resistor. With a 1 ohm resistor, the measure voltage drop is the same as the current, but may run into issues with multimeter digit display. Better to use a 10 ohm resistor, and divide the reading by 10 to get the current. This scheme works for both fixed bias (where this is the only resistor from cathode to ground) and cathode bias. You could put that resistor at the plate as well if you want, but your meter needs to be rated to the B+ voltage (most are, up to 1000V, but there are some cheap ones that may be rated at 600V), and you will need two test points. Much safer to measure at a point of low voltage in event you come into contact with lead, and with the resistor installed at the bottom of the cathode resistor with one end at ground, you only need a single test point since you can use the speaker ground connection as the other test point.
David
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