VAC PA90 - Manual (Page 7)

7
BIASING
Your VAC Power Amplifier has been shipped with output tube bias set for an idle current of 54
milliamperes (mA) per tube. This should be checked when you install your amplifier, and
approximately every month thereafter. It must also be set whenever an output tube is changed.
Refer to Figure 3. Bias levels must be set with accessory PM1 milliammeter. Insert the phone
plug of the PM1 into the socket labelled "METER" on the top front of the PA90. Depress each
black button in turn to read the idle current for each output tube. Follow a pattern of checking
a tube on the left side, then a tube on the right, then the other left, then the other right, etc.
Adjust the corresponding bias potentiometer until a reading of 54 mA is obtained. (Turning the
control counter clockwise increases the current.) Now, recheck each tube and adjust as
necessary. (A change in the setting of one tube may affect the others). Repeat until all tubes
are at the desired idle current. Recheck the settings after 30 minutes and one hour. A change
of several milliamperes during warm up is normal and no cause for concern.
Bias levels should be checked monthly to ensure optimum sound quality. Allow the PA90 to
warm up for at least one hour before checking. It is not unusual for bias current to change with
time, particularly when tubes are new. In fact, the greatest amount of drift occurs during the
first 200 hours of a tube's life. The drift may change direction periodically, such that the bias
control must be increased and then later decreased, or vice versa.
If a PM1 current meter is not available, you may use a high quality VTVM or FET-VM to make
the measurements. Wire a 1/4 inch phone plug to the meter with the tip of the plug connected
to the positive input of the meter. Set the meter to measure direct current on a scale that reads
to approximately 100 milliamperes. If you hear a hum when you try to make the measurement,
STOP - the meter is not suitable for this purpose.
Note that in order to make an accurate bias measurement you must have a meter with a very
low series resistance. This is due to the fact that the meter's resistance appears in the cathode
circuit of the tube being measured, and will induce some cathode (self) bias during the
measurement. The result of this is that the actual idle current will be greater than that which
was measured, possibly by enough to damage the output tubes.
For further information, refer to Tips & Advice: A Word About Bias Levels.