I method I worked out to bias a Marshall EL34 100/100 Dual Monobloc Power amp using nothing more than multimeter i.e. no bias adapter required. I worked this method out not using any kinda adapters or mods from electrical first principles and the circuit diagram, and have since seen it confirmed in an internet article that using the measured current through the output transformer is a perfectly good way.
If you know the max plate dissipation per tube (e.g. 25W) and measure the DC resistance across each half of the output transformer you should be able to calculate the plate (anode) current to a pair of tubes and bias it based on voltage drop.
Comments?