Gibson GA5-T crestline B+ low and bias questions

[TimmyP1955]

My check for plate current is the shunt method. Set your meter for milliamps, 100 or 200 mA scale whichever you have. Clip one probe to the OT center tap and the other to an output tube plate. We're assuming the ammeter's resistance is a small fraction of the resistance of the OT winding, otherwise this won't be very accurate, but it often is. The meter will give you the plate current for the tube it's attached to. Then do the same for the other tube.

This would be a short across 1/2 of the OT. One would need to connect the meter in series with the winding, not in parallel with it.

[g1]

That's how shunt method works, you short out half the OT. The current runs through your meter instead of the OT. It's probably the most dangerous bias measurement method.
It works in most cases but may not be accurate with OT's that are very low resistance, or meters that have a high "burden voltage" spec. for their current ranges.