TUBE BIASING
Bias voltage applied to the grids of the power tubes in the output stage should be
checked from time to time to keep these tubes operation at the optimum
operating point. A properly biased power tube will have a longer life.
To check and adjust the bias voltage of the EL34 tubes, simply follow the
following procedures.
Tools Required:
Digital Multimeter
Long-handled slotted screwdriver
Procedures:
1. Making sure that the power switch is in the OFF position.
2. Connect your speaker leads, input cable and power cord to the amplifier.
3. Loosen the screws that secure the top cover of the amplifier to the chassis.
Remove the top cover to get access to the inside of the amplifier.
4. Turn ON the amplifier.
5. Turn on the multimeter. Select the section for measuring up to 20V DC.
Attach and secure the negative test lead to the chassis or to the binding post.
6. Carefully insert the positive test lead into the bias measuring point labeled as
T1 on the main PC board.
7. Carefully insert a long-handled slotted screwdriver into the bias adjustment
pot labeled as VR1 on the PC board. Slowly rotate the screwdriver to adjust
the bias voltage until it reaches 0.35V.
8. Repeat steps 6 and 7 to adjust the bias voltage for the three remaining tubes
in the same fashion as biasing the first one. Use measuring points labeled
as “T2, T3 and T4” for measuring, and bias adjustment pots labeled as “VR2,
VR3 and VR4” for adjusting. You will need to go back and forth a few times
until all four tubes are properly biased.
9. Once you have finished biasing all four tubes, wait a few minutes and recheck
the settings again.
10. Turn the power OFF. Detach the negative test lead from the chassis. Put the
top cover and the screws back on to the amplifier.
11. The CA-770 power amplifier is now ready to use.
