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An example is as follows:
You might wonder why your DMM is set to millivolts and not milliamps – simply, we have a 1 Ohm resistor placed between your
probe jacks and ground to convert a current reading to a voltage reading. That way, a bias current of 35mA measures as 35mV
on your DMM.
NOTE – Only set your DMM to mV for measuring bias on the Joyful Music! amplifiers. Not doing so may damage your DMM.
Now that you know how to calculate bias, all you need to do is:
1) Follow steps 1-5 on page 11
2) Calculate what bias voltage reading you will set your tubes to (in this case, we will use 35mV)
3) Insert your red probe in the probe jack of the power tube reading somewhere in the middle bias measurement of your
power tubes (variation in bias measurements from tube to tube is to be expected, so selecting one measuring in the
middle will approximate bias for all tubes)
4) Turn the bias potentiometer shaft SLOWLY until your DMM reads 35mV
5) Check the other tubes by plugging into the other red probe jacks and ensure they are within a reasonable range
6) Wait 1 minute
7) Recheck all power tube bias measurements
8) Readjust bias potentiometer shaft if necessary
30��
× 55% × 1000 = ���������� 35 − 40����
455������
13
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