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8
PASSION AUDIO KIT ©
BIAS ADJUSTMENT
How it
A bias adjustment of the output tubes is required on a tube amplifier when replacing the output tubes
and it should normally be checked every 500 hours of operation. You would have a DC voltmeter.
Any noticeable change in the bias adjustment may indicate that the tubes are beginning to deteriorate
and that they will have to be replaced.
is done:
The voltage at the cathode resistance terminals of each output tube is measured (10 ohms). The
ohm law (a=v/r whereby 0.45V / 10 ohms=.045A) makes it possible to calculate the current
circulating in each tube. The ideal current value is 45mA. A higher value (50mA) reduces the tube
lifetime, whereas a lower value (40mA) will extend the tube lifetime but with a higher distortion rate.
It is essential to use a high sensitivity voltmeter (20 000 Ohm/volt) to get a precise reading of the
voltage between 0 and 2 volts DC.
Power your amplifier on. It must be connected to its speakers and the volume of the preamplifier
must be completely turned off. Connect the negative lead of the voltmeter to the frame or one of the
black terminals (negative) of the speakers output. Now, the tip of your positive lead on the «TP4»
test point" right behind the Vb4 6550 power tube. The voltage must be 0.45 volts DC. Using a small
screwdriver, adjust the bias knob corresponding to the tube you are adjusting, that is VR4. Avoid
getting burned on the power tubes.
For Vb3, use TP3 and adjust VR3
For Vb4, use TP4 and adjust VR4
For Vb1, use ТР1 and adjust VR1
For Vb2, use TP2 and adjust VR2
Repeat the procedure for each tube. Tip: check again a few times, because there will be some
interaction between each adjustment. Repeat the procedure after 20 minutes. If the voltage remains
constant, the bias adjustment is done.
The PAK-i12 may need an additional adjustment for«the differential input circuit». This adjustment
must be performed by a qualified technician. Do not touch VR6 and VR7, as they are not subject to
a bias adjustment.
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