11
Bias Current
Bias Current
Bias Current
Bias Current
Procedure:
1. Power up the amp as usual. Allow the amp to warm up for 5 minutes or so
before setting the bias current.
2. Attach the meter as shown in the diagram above. Set the meter to read DC
voltage (not current) in the 0-200mV range.
3. Insert the screwdriver into the hole shown, and make sure that it properly mates
with the adjuster screw of the trimmer pot. Adjust the trimmer until the meter
shows the correct value. This is a 10-turn trimmer pot, so you may have to
rotate it through several turns to arrive at the value desired.
Target Value (as read on the meter connected as shown above):
50mV
Technical Details:
Inside the Majesty, there is a 1-ohm resistor connected between the cathode of the
output tubes and ground. The bias test points on the back panel of the amp allow you
to measure the voltage across this resistor, and compute the combined cathode current
of both output tubes. Although the cathode current is actually the sum of the plate
current and screen current, the screen current is negligible at idle.
Because you are measuring the voltage across the 1-ohm resistor, you know that 1mV
across the resistor indicates 1mA of current through the resistor, and this 1mA of
cathode current. The value measured on the meter shows the combined cathode
current for both tubes.
I prefer to set the bias current to 25mA per tube. This adds up to 50mA total for both
tubes, which is indicated by a voltage reading of 50mV at the bias test points.
